ubc_1951_a8 g43 i5
DESCRIPTION
jjkjTRANSCRIPT
AN INVESTIGATION OP THE EFFICIENCY OF DIMETHYL
SULPHATE AS A METHYLATING AGENT FOR CARBOHYDRATES
by
DOUGLAS WILLIAM GLENNIE
A THESIS SUBMITTED IN PARTIAL FULFILMENT OF
THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF ARTS
i n the Department
of
Chemistry
We aceept t h i s thesis as conforming to the
standard required from candidates for. the
degree of MASTER OF ARTS
Members of the Department of
Chemistry
THE UNIVERSITY OF BRITISH COLUMBIA
A p r i l , 1951
ABSTRACT
Dimethyl sulphate has been used for carbohydrate raethylation under various conditions of concentration, time and alkalinity i n order to determine optimum reaction conditions* Mesquite gum was used as a representative water-soluble polysaccharide.
ACKNOWLEDGMENT
The author i s indebted to Dr. E.V. White, who suggested this thesis problem and offered valued assistance and guidance throughout the course of work.
- i i i -
TABLE OP CONTENTS
PAGE
INTRODUCTION 1
PURPOSE OP STUDY 2
LITERATURE SURVEY . 2
PLAN OP INVESTIGATION . . . 5
EXPERIMENTAL METHODS AND RESULTS • • 8
Part 1. P u r i f i c a t i o n of crude mesquite gum . . . . 8
Part 2. Comparison of continuous methylation and
stepwise methylation e f f i c i e n c y 10
Part 3. Minimum time of methylation f o r equi
valent substitution under constant conditions . . . 11
Part 4. Relation between the e f f i c i e n c y of
methylation and t o t a l reagent concentration; the
methylation e f f i c i e n c y and a l k a l i concentration . . 14
Part 5. The ef f e c t of methyl alcohol and dimethyl
ether on the methylation e f f i c i e n c y • 17
Part 6. Rate of hydrolysis of dimethyl sulphate
i n aqueous and al k a l i n e media; heat of reaction
during alkaline hydrolysis of dimethyl sulphate
and during methylation. 17
Part 7. S p e c i f i c r o t a t i o n measurements on
p a r t i a l l y methylated mesquite gum samples . . . . . 24
DISCUSSION . . . . . . 25 SUMMARY AND CONCLUSIONS 31
i v BIBLIOGRAPHY APPENDIX •
j LIST OP TABLES
' ' ; " PAGE Table 1. A n a l y t i c a l values f o r mesquite gum
preparations 10
Table 2. Summary of methylation series 1, 2 and
3 (Part 2) • • • 1 2
Table 3> E f f i c i e n c y of reagent i n methylation
series 1, 2 and 3 (Part 2) 13
Table 4. Extent of methoxyl sub s t i t u t i o n with
decrease i n reaction time under constant methylation
conditions (series 1 and 2) . . . . . . . . . . . • 14
Table 5, Summary of r e s u l t s i n Part 4 showing
reagent r a t i o s and methoxyl content of p a r t i a l l y
methylated products . . . . 15
Table 6. Calculated reaction e f f i c i e n c i e s (Part4), • 15
Table 7. Results of methylation with 2, 5, 11,1,
15.5 and 20$ sodium hydroxide (series l ) . Results
at very low a l k a l i concentration (series 2). Results
on the e f f e c t of methyl alcohol and d i e t h y l ether
(series 3) 18
Table 8. Aqueous and a l k a l i n e hydrolysis data for
dimethyl sulphate . . . . . . . . . . . . . . . . • 19
Tables 9-18. Temperature r i s e during hydrolysis of
dimethyl sulphate with sodium hydroxide, ammonium
hydroxide and during methylation 21
Table 19. S p e c i f i c rotation of p a r t i a l l y methylated
mesquite gum products and methoxyl content . . . . . 24
-v-
LIST OF FIGURES
Fig* 1. Variation in methoxyl content with time of methylation for series 1, 2 and 3. Fig. 2. Average efficiency and efficiency for each reaction with progressive methylation. Fig. 3. Methoxyl content of partially methylated products with time of reaction under constant methylation conditions. Fig. 4. Variation in methoxyl content and efficiency with total reagent concentration; comparison with series 3, Part 2. Fig. 5. Methoxyl content and efficiency versus alkali concentration. Fig. 6. Rate of hydrolysis of dimethyl sulphate in water, alkali and salt solutions at various temperatures. Fig. 7. Temperature variation during the alkaline hydrolysis of dimethyl sulphate. Fig. 8. Comparison of the variation in temperature during hydrolysis and methylation. Fig. 9. Specific rotation of partially methylated products versus methoxyl content.
- v i -
AN INVESTIGATION OF THE EFFIGIENGY OF DIMETHYL
SULPHATE AS A METHYLATING AGENT FOR CARBOHYDRATES
by
DOUGLAS WILLIAM GLENNIE
INTRODUCTION
The methylation of carbohydrates i s undoubtedly
one ©f the most important reactions i n the f i e l d of carbo
hydrate chemistry. It i s used extensively i n determining
the structure of n a t u r a l l y occurring carbohydrate substances
as well as those prepared s y n t h e t i c a l l y . In the words of
B e l l ( l ) : " I t i s perhaps i n t e r e s t i n g to r e c a l l that, despite
so many advances i n technique, methylation, o r i g i n a l l y
conceived i n the St. Andrews laboratory of Purdie and Irvine,
remains an es s e n t i a l t o o l f o r a l l fundamental s t r u c t u r a l
investigations". As a research t o o l the methylation reaction
plays an indispensable part i n studies on the nature of
linkages between component uni t s of a carbohydrate compound.
During methylation free hydroxyl groups not involved i n
such linkages are e t h e r i f i e d . Under mild hydrolysis these
linkages may be s p l i t to y i e l d the i n d i v i d u a l component
units containing a free hydroxyl group only wherever linkage
existed. This method of " l a b e l l i n g " free hydroxyl groups
by forming t h e i r methyl ether derivatives finds some analogy
i n the use of traeer technique i n radiochemistry. This same
-a- .
procedure also forms the basis f o r the so-called "end group"
method of determining chain length i n long chain poly
saccharides (2,3). Studies of carbohydrate methylation have
afforded some information on the r e l a t i v e r e a c t i v i t y of
constituent hydroxyl groups(4-9). F i n a l l y , the methylation
reaction has been employed i n industry f o r the preparation
of methyl derivatives such aB methylcellulose which f i n d
use i n the manufacture of films and plastics(10-13)•
PURPOSE OF STUDY
Dimethyl sulphate and a l k a l i , as a methylating
agent, has received extensive use i n carbohydrate studies.
In the methylation of carbohydrates, however, i t i s often
necessary to repeat the methylation treatment several times
before maximum subs t i t u t i o n i s achieved. Even a f t e r repeated
methylation i n some cases s u b s t i t u t i o n i s incomplete. Despite
many modifications of the reaction conditions, there i s
apparently no data available on the e f f i c i e n c y of reaction
at various degrees of substitution or on the r e l a t i v e
e f f i c i e n c y of reaction under varied conditions. These
problems were investigated i n an e f f o r t to f i n d optimum
methylation conditions.
LITERATURE SURVEY ,
The f i r s t method f o r e t a e r i f y i n g sugars,was
introduced by Purdie and Irvine(14), who employed, methyl
iodide and s i l v e r oxide as a methylating agent. In t h i s
method reducing sugars are converted to their,glycosides
•2-
before methylation i n order to prevent oxidation by the
s i l v e r oxide reagent. In the presence of solvents such
as methyl alcohol side reactions (formation of methyl ether)
reduce the reaction e f f i c i e n c y considerably. However, as
the degree of sub s t i t u t i o n increases the s o l u b i l i t y i n
methyl iodide increases and complete substitution i s
f a c i l i t a t e d . For t h i s reason "Purdie's reagents" are
found to be most e f f e c t i v e i n advanced stages of methyl
a t i o n (15-18).
Denham and Woodhouse(l9) introduced dimethyl
sulphate and a l k a l i to the carbohydrate f i e l d i n t h e i r
studies on the methylation of c e l l u l o s e . Haworth(20) l a t e r
extended the use of these reagents to the methylation of
simple sugars. Methylation was carr i e d out by dropwise
addition o f dimethyl sulphate and 30$ sodium hydroxide to
a concentrated aqueous solution of the sugar kept vigorously
s t i r r e d . The rate of addition was adjusted i n such a manner
as to maintain the reaction mixture s l i g h t l y a l k a l i n e as a
precaution against hydrolysis. The heterogeneous nature of
the reaction mixture necessitates vigorous s t i r r i n g and
side reactions necessitate the use of a large excess of
reagents. Owing to the competitive nature of the reaction,
the e f f i c i e n c y i s low. Nevertheless, the Haworth process
i s widely used, e s p e c i a l l y f o r preliminary methylation
(15-18,20-22).
Many modifications of the o r i g i n a l Haworth
procedure have been made i n connection with studies on
the methylation of c e l l u l o s e . One of these, developed i n
-3-
the Haworth laboratory(23), consists of a simultaneous
regeneration and methylation treatment of ce l l u l o s e
acetate dissolved i n acetone. I t was found that there
was no advantage gained by substituting 45% potassium
hydroxide f o r the usual 50% sodium hydroxide. Lithium
hydroxide gave i n f e r i o r y i e l d s with a low degree of sub
s t i t u t i o n . I t has been shown(24) that sodium methyl
sulphate with sodium hydroxide i s e f f e c t i v e i n the prepara
t i o n of low but uniformly substituted methylcellulose. In
a l l these modifications, which have been reviewed(lO), the
physical c h a r a c t e r i s t i c s of c e l l u l o s e play a dominant r o l e
and the methods are not of general a p p l i c a b i l i t y . Haworth
and co-workers have also applied the simultaneous
regeneration-methylation technique to the methylation o f
starch and glycogen(25,26). The methylation of starch
with dimethyl sulphate and other a l k a l i e s , such as ammonium
hydroxide, has been suggested(27).
In some instances carbohydrates containing
resis t a n t hydroxyl groups, p a r t i c u l a r l y primary hydroxyl
groups(22), have been methylated with m e t a l l i c sodium,
potassium or l i t h i u m i n l i q u i d ammonia and other solvents
(21,22,28-32). The e f f i c i e n c y of the reaction i s l i m i t e d
by the low s o l u b i l i t y of the a l k a l i metal derivatives(29)
and there i s some controversy as to whether concurrent
decomposition r e s u l t s i n t h i s reaction(31-33).
Qther methods of carboydrate methylation are
recorded i n the l i t e r a t u r e . These methods are usually
^slight modifications of the Purdie or Haworth methods or
involve, i n some cases by necessity(15,18), a combination
of these treatments. The important ones, from a s t r u c t u r a l
standpoint, are reviewed by B e l l ( l ) .
PLAN OF INVESTIGATION
To i n i t i a t e t h i s study a carbohydrate suitable
for methylation was required. Mesquite gum, a water soluble
gum somewhat res i s t a n t to methylation, was readily available
and judged to be representative. The structure of the gum,
as proposed by White(34-37), i s shown diagramatically. Other
studies by Cuneen and Smith(38) indicate that the structure
i s more complex than that represented. For s i m p l i c i t y the
structure i n t h i s work i s taken to be the free acid hepta-
saccharide of molecular weight 1061 containing one methoxyl
group and 17 hydroxyl groups'available f o r methylation.
-D-GALACTOSE- JL46. -D*-GALACTOSE-I 1*3 I
4^Me0-D-GLTJCURONIC AGID ( s a l t , ester)
1:2 L-ARABINOSE
l i s L-ARABINOSE
1:2 L-ARABINGSE
•ItS L-ARABINOSE
n
To prepare the crude mesquite gum at hand for
characterization and methylation studies a preliminary
i n v e s t i g a t i o n on a method of p u r i f i c a t i o n was i n order.
In t h i s connection the methods of White(34) and Anderson
*and Otis(39) were ref e r r e d to. I t was decided that no
-5-
e f f o r t should be made to prepare the free acid gum by
repeated treatment with mineral a c i d , since there i s a
danger of hydrolysing the l a b i l e pentose u n i t s . Results
expressed on the ashless basis were assumed to take into
account any v a r i a t i o n i n the degree of s a l t formation i n
the gum.
In order to assess the e f f i c i e n c y of methylation
reactions a method of analysis was required. I t was decided
that the percentage dimethyl sulphate actually u t i l i z e d for
methylation could be taken as a c r i t e r i o n of e f f i c i e n c y .
I t was seen that, with t h i s c r i t e r i o n and a knowledge of
the y i e l d of methylated product, the i n i t i a l and f i n a l
methoxyl content, and the amount of dimethyl sulphate used
for reaction, an estimation of the reaction e f f i c i e n c y could
be made. With t h i s i n mind the techniques of methoxyl
analysis were practised on standard v a n i l l i n samples u n t i l
reproducible accuracy was obtained. A l l methoxyl analyses
were c a r r i e d out by the method of Viebock and Schwappach(40)
as modified by Glark(41).
The purpose of the study has already been out
l i n e d . However, s p e c i f i c problems deemed worthy of
consideration may be mentioned. One of these i s the problem
of continuous methylation versus stepwise methylation. In
p a r t i c u l a r , a comparison of the e f f i c i e n c y of repeated
methylation with the e f f i c i e n c y of methylation i n one
continuous operation was desired. I t was also desired to
know the minimum time of methylation for equivalent sub
s t i t u t i o n or time a f t e r which no further substitution occurs.
-6-
Another problem a r i s e s i n consideration of the methylation
e f f i c i e n c y as the t o t a l reagent concentration varies*
Closely connected to t h i s problem i s one concerning the
methylation e f f i c i e n c y with v a r i a t i o n i n a l k a l i concentra
t i o n alone* Linked with the question o f v a r i a t i o n i n t o t a l
reagent concentration and a l k a l i concentration i s the
question of eff e c t of d i f f e r e n t solvents on the methylation
e f f i c i e n c y . S t i l l another pertinent problem i s to know
something of the extent of hydrolysis or saponi f i c a t i o n of
dimethyl sulphate under methylating conditions i n the
presence and absence of carbohydrate material.. Although a
knowledge of the temperature dependence of methylation
e f f i c i e n c y i s important, time did not permit i n v e s t i g a t i o n
of t h i s phase. On the other hand, some insight into the
problem may be gained through a knowledge of the heat of
reaction during methylation. F i n a l l y , i t i s of some intere s t
to know the r e l a t i o n , i f any, which ex i s t s between the
s p e c i f i c r o t a t i o n of p a r t i a l l y methylated products and t h e i r
methoxyl content. I t may be seen that, i f a well defined
r e l a t i o n e x i s t s , the s p e c i f i c r o t a t i o n measurements would
be a convenient supplement to methoxyl analyses.
The foregoing plan i s intended only as a summary
to the experiments described i n d e t a i l i n the next section
and the problems mentioned are discussed more f u l l y under
the "Discussion".
EXPERIMENTAL METHODS AND RESULTS
For convenience description of the techniques
and methods employed throughout the investigations enumerated
i n the previous section i s grouped under seven headings.
These headings are l i s t e d below:
Part 1. P u r i f i c a t i o n of crude mesquite gum.
Part 2, Comparison of continuous methylation and stepwise
methylation e f f i c i e n c y .
Part 3, Minimum time.of methylation for equivalent substi
t u t i o n under constant conditions.
Part 4, Relation between the e f f i c i e n c y of methylation
and t o t a l reagent concentration; the methylation e f f i c i e n c y
and a l k a l i concentration, , ,* ...
Part 5, The e f f e c t of methyl alcohol and dimethyl ether
on the methylation e f f i c i e n c y , „,. > ] ?, ! ; 3. n».
Part,, 6,. Rate of hydrolysis of dimethyl sulphate i n
aqueous and a l k a l i n e media; heat of reaction during
al k a l i n e hydrolysis of dimethyl sulphate and during
methylation.
Part 7, S p e c i f i c r o t a t i o n measurements on p a r t i a l l y
methylated mesquite gum samples.
Part 1, P u r i f i c a t i o n of crude mesquite gum,
A 25 g, sample of crude mesquite gum was placed
i n a beaker with 50 ml. of d i s t i l l e d water and allowed to
stand overnight. The syrup obtained was s t i r r e d with an
addi t i o n a l 200 ml, of water to y i e l d a t h i n syrup of
-8-
approximately 9% solution. This t h i n syrup was f i l t e r e d
through kieselguhr to remove bark p a r t i c l e s and other
impurities. The f i l t r a t e , a pale yellow t h i n syrup, was
dialyzed i n cellophane against cold tap water for 24 hours
to y i e l d a f i n a l volume of 330 ml. This volume was
reduced to 130 ml. by evaporation under reduced pressure
at 30°. An equal volume of 95% ethanol was added and the
soluti o n poured i n a f i n e stream into 1500 ml. of 95%
ethanol made B/5 with sulphuric acid. The milky suspension
obtained was allowed to s e t t l e and was f i l t e r e d and washed
with 95% ethanol, dry ethyl ether, and f i n a l l y with l i g h t
petroleum ether. The product was pulverized and dried to
y i e l d a white amorphous powder, r e a d i l y soluble i n water,
aci d and d i l u t e a l k a l i but insoluble i n organic solvents;
y i e l d 20 g.
A second preparation from 100 g. of crude
mesquite yielded 85 g. of p u r i f i e d product.
In a t h i r d preparation 300 g. of crude gum were
dissolved i n water, f i l t e r e d and s t i r r e d with 60 ml. of
concentrated hydrochloric acid for one h a l f hour. A f t e r
d i a l y s i s the p u r i f i e d product was i s o l a t e d and dried as
before; y i e l d 210 g.
A summary of the y i e l d s and a n a l y t i c a l values
fo r each product, numbered i n order of preparation, i s
given i n Table 1. Included i n the table are values f o r
mesquite gum published by Otis and Anderson(38) and values
calculated from the structure of mesquite gum proposed by
White(34-37). The calculated values are based on the
structure i l l u s t r a t e d on page 5.
Table 1. A n a l y t i c a l values f o r mesquite gum preparations.
Value Sample 1 Sample 2 Sample 3 (38) (.34-37)
% Recovery 81.0 85.0 70.0 Methoxyl 2.43 2.40 2.44 2.86 2.92 Pentosan 47.02 56.6 Carbon dioxide 3.86 3.55 4.15 Carbon 45*46 44.10 Hydrogen 6.39 6.08 Ash 2.14 2*52 0.85 0.07 0.00 Mol. wt. 1222 1061 c°or 68.3' 70.8°
Part 2. Comparison of continuous methylation with stepwise
methylation e f f i c i e n c y .
In this, i n v e s t i g a t i o n 5 g. samples of p u r i f i e d
mesquite gum were used throughout. Samples were dissolved
i n the required amount of d i s t i l l e d water i n stoppered fl a s k s
and shaken with dimethyl sulphate and 30$ sodium hydroxide on
a mechanical shaker f o r i n t e r v a l s of one hour. The temperature
of methylation was maintained over the range 20-25° by cooling
when necessary. P a r t i a l l y methylated gum solutions were
dialyzed to remove spent reagents and evaporated to dryness
under reduced pressure at 30°. Samples of approximately 3 g.
were removed from re a c t i o n vessels and dried f o r analysis.
In the f i r s t methylation series samples of p u r i f i e d gum were
methylated for one, two, three and four hours with d i r e c t
addition of reagents to the reaction mixtures at the end of
-10-
each one hour period. In a second methylation series step
wise methylation was c a r r i e d out on samples for one, two,
three, and four hour periods. Reaction mixtures were d i a -
lyzed, evaporated to dryness and the methylation treatment
repeated once, twice and t h r i c e . In a t h i r d methylation
s e r i e s samples were methylated continuously i n the same manner
described f o r the f i r s t series except time i n t e r v a l s of 15
minutes were used throughout. The r e s u l t s are summarized i n
Table 2. In Pig. 1. a p l o t of methoxyl content with time of
methylation f o r each series i s shown. Where duplicate runs
were made the average methoxyl values are graphed. F i g . 2
i l l u s t r a t e s the v a r i a t i o n of methylation e f f i c i e n c y with
number of methylations f o r each seri e s . Sample calculations
of reaction e f f i c i e n c y are shown i n the Appendix, page i and
the e f f i c i e n c y values are l i s t e d i n Table 3.
BarjL-£. Minimum time of methylation for equivalent s u b s t i
t u t i o n under constant conditions.
A study was made of the extent of su b s t i t u t i o n
effected under constant methylating conditions when the
reaction time i s systematically reduced. In the f i r s t s e r i e s ,
series 1, 5 g, samples of p u r i f i e d gum were dissolved i n
10 ml. of d i s t i l l e d water and shaken with 5.0 ml. of dimethyl
sulphate and 5.83 ml. of 30$ sodium hydroxide i n stoppered
f l a s k s on a mechanical shaker. A f t e r shaking for the desired
time reaction mixtures were dialyzed immediately and the
p a r t i a l l y methylated gum products i s o l a t e d by evaporation
and drying as described i n Part 1. In a second s e r i e s ,
-11-
Table 2. Summary of methylation series 1, 2 and 3. 30$ NaOH
Series & sroduct
Starting material Water
Dimethyl sulphate
1-1 1-2 1-3 1-4 1-5 1-6
1- 7
2- 1 2-2 2-3 2- 4 3- 1
3-2
3-3
3-4
Sample 1 n Sample 2
tt
Sample 2
Sample 3
Sample 3
10 ml, it ii M
1©
10 ml)
• J 10 ml.
II
10 ml.
5,0 ml, it
5,0 ml, ti
5,83 ml.
- i a -
Yield % Yield MeO
% Ash
%MeO (Ashless)
4,5 g. 4,85 3,80
90 97 76
14.0 15.8 23.3
2.66 4.08 2.65
14.4 16.5 24.0
5,0 100 23.0 2.75 23.6 5.0 100 28.7 2.04 29.3 4,6 92 29.6 2.11 30.2
4,5 90 30.3 2. 36 31.1
5,0 100 25.5 2.16 26.1 5.0 100 26.5 2.32 27.1 4*2 84 32.8 2.62 33.7
4.5 4-5 g.
90 90
32.3 16.6
2. 35 1.91
33.1 16.9
4-5 90 24.4 1.92 24.9
4-5 90 28.7 2.06 29.3
4-5 90 29.3 1.98 29.9
Table 3. Efficiency of reagent,methylation series 1, 2 and 3.
Series and
Product
~ 1st methylat ion
" 2nd methylat Lon
3rd methylati on
4th methylation
Overall methylation Series
and Product
MeO gps» introd. E f f i c .
MeO gps. introd. E f f i c .
MeO gps. introd. E f f i c .
MeO gps. introd. E f f i c .
MeO gps. introd. E f f i c .
1-1,2 1-3,4 1-5,6 1-7 2- 1,2 2- 3,4 3- 1 3-2 3-3 3-4
4*60 II
II
ii ii . .. it
5.16 ii it ii
41.0 n II
ii II . --
it 46.1 it
ii ii
3.4 it M
4.6 ti
3.32 II
n
30.4 it it
41.0 ti
29.6 ti it
2.60 it
3.10
1.92 it
23.2 II
27.6 CM
17.1 it
1.6 mm
0.3
5.35
2.68
4.60 8.0
10.6 11.2
~ 9.2 12.3 5.16 8.48
10.4 10.7
41.0 35.7 31.5 25.0 41.0 36.8 46.1 37.8 30.9 23.9
-13-
Figure-1» V a r i a t i o n i n methoxyl content (ashless)
with time of methylation for series 1, 2 and 3.
M a x i m u m fsAeO (calculated)
series I
series 3
NaOH _ t i 0
MeiS04~
M e , S 0 4 = 1 ' i Z
Gum
i 3 A-Number of methyl at ion 5
Figure 2, Average e f f i c i e n c y and e f f i c i e n c y
f o r each reaction with progressive methylation,
Number o*f metfty/oti'ons
s e r i e s 2, samples were methylated i n the same manner but
reaction mixtures were neutralized with hydrochloric acid
before d i a l y s i s . The re s u l t s are shown i n Table 4 and
graphically i n Pig, 3.
Tab_le_4, The extent of methoxyl sub s t i t u t i o n with decrease
i n reaction time under constant methylation conditions
(series 1 & 2),
Series and
Product Sta r t i n g material Time Y i e l d Y i e l d
' % MeO Ash
% Methoxyl (ashless!
1-1 Sample 2 30 m. 4.5 g. 90 15.9 2.56 16.3 1-2 ii 15 4-5 80-90 16.0 2.01 16.3 1-3 H 7.5 4-5 80-90 15.5 2.23 15.9 1-4 ti 3.75 4-5 80-90 15.7 2.09 16.0 2-1 Sample 3 15. m. 4.5 g. 90 16.3 1.50 16.6 2-2 ii 7.5 4-5 80-90 16.5 1.93 16.8 2-3 it 3.75 4-5 80-90 13.6 1.87 13.9
Part 4. Relation between the e f f i c i e n c y of methylation and
t o t a l reagent concentration; the methylation e f f i c i e n c y and
a l k a l i concentration.
For the purpose of the f i r s t study, a number of
samples of p u r i f i e d gum were methylated i n the same manner
described i n Part 1 and Part 2 with the exception that, while
the r a t i o of 30$ sodium Hydroxide to dimethyl sulphate was
maintained at 1.1, the r a t i o of dimethyl sulphate and a l k a l i
to gum was varied. In each case a 5 g. sample of p u r i f i e d
gum (sample 3) was dissolved i n 10 ml. of d i s t i l l e d water
and methylated f o r 15 minutes and the reaction mixture imme
di a t e l y dialyzed. The f i n a l products were i s o l a t e d for
analysis i n the usual manner. The res u l t s are summarized i n
-JL4-
Figure 3 . Methoxyl content of p a r t i a l l y methylated products with time of reaction under constant methylation conditions.
25
V
—c
T i
8
10
ime in m
-© o — series 1 0— series Z
NjiPJi . i.io M e , S 0 »
. 1.12
Irxvt
J5 es
20
Table 5 and shown graphically i n Pig, 4. The curve from Series 3, Fig, 1 i s included i n Fig, 4 for comparison. Calculated efficiencies are li s t e d i n Table 6 and a plot of the efficiencies i s also shown i n Fig, 4, The curve from Series 3, Fig. 2 of the overall efficiency is again included for comparison.
Table 5, Summary of results i n Part 4 showing reagent ratios and methoxyl content of partially methylated products.
Product KaOH Me£0 4
MeSOi Gum
^ —
Yield % ~ MeO Ash
% MeO (ashless)
1 11.7 ml. 10 ml. 2.24 90 23.6 1.92 24.1 2 17.5 15.0 3.36 90 26.1 2.08 26,7 3 23.3 20.0 4.48 90 27.1 1.71 27.6
Table 6. Calculated reaction efficiencies (Part 4).
Product MeSO. Gum
Me£0* avail. OH
MeO gps. introduced Efficienegr
3-l(Pt.l) 1.12 0.70 5.16 46*1 % 1 2.24 1.40 8.13 36.3 2 3. 36 2.10 9.20 27.3 3 4.48 2.80 9.60 21.4
For the purpose of the second study, samples of purified mesquite gum were methylated i n the usual manner but the alk a l i concentration was varied in an effort to determine the optimum concentration with^respect to time of reaction, extent of substitution and reagent- efficiency. In a series of experiments 5 g. samples of gum were dissolved i n d i s t i l l e d water, a l k a l i added and the solution shaken in stoppered flasks with 5 ml, of dimethyl sulphate for 15
-15-
Figure 4. V a r i a t i o n i n methoxyl content and
e f f i c i e n c y with t o t a l reagent concentration
and comparison between series 3, Part 2.
\
I I I L.
Raiio of Me*50 4 -to gum
minutes. In every ease the molar ratio of sodium hydroxide to dimethyl sulphate was kept constant at 1.1 Results with 2,5,11.1,15.5, and 20$ sodium hydroxide are li s t e d i n Table 7.
In two separate experiments the methylation e f f i ciency was investigated at very low alkal i concentrations. In the f i r s t experiment a 5 g. sample of purified gum was dissolved i n 10 ml. of d i s t i l l e d water and shaken with 5 ml. of dimethyl sulphate. A total of 5.83 ml.(10$ excess) of 30$ sodium hydroxide was added dropwise (about 1 drop per minute) such as to maintain the pH between 7 and 9. The reaction required 2.5 hours for completion as evidenced by constant pH of the reaction solution. Results from this experiment are included i n Table 7 under experiment 2-1.
In the second experiment a 5 g. sample of purified gum was dissolved i n a buffer solution consisting of 30 ml. of d i s t i l l e d water containing 20.8 g. of Na^HP04.12H20. During the reaction 0.3 ml..of 30$ sodium hydroxide was added dropwise over a total shaking period of 45 minutes such as to maintain the pH between 7 and 8. The product (2-2) was isolated for analysis i n the usual manner and results are l i s t e d i n Table 7.
In a third separate experiment the effect of replacing sodium hydroxide with ammonium hydroxide on the reaction efficiency was investigated. A 5 g. sample of purified gum was dissolved i n 10 ml. of d i s t i l l e d water and shaken with 5.83 ml. of 18.8$ ammonium hydroxide (equivalent to 5.83 ml. of 30$ sodium hydroxide) and 5 ml. of dimethyl sulphate. The product (2-3) was isolated for analysis as
-16-
before and results are l i s t e d i n Table 7,
Part 5. The effeet of methyl alcohol and diethyl ether on the methylation efficiency.
In this investigation the reagent efficiency with water as solvent was compared with the efficiency with methyl alcohol-water (1:3 by volume) and diethyl ether-water (1:3) as solvent. In the f i r s t experiment 5 g, of purified gum were dissolved i n 15 ml, of methyl alcohol-water (1:2) and the solution shaken in the usual manner with 5 ml, of d i methyl sulphate for 15 minutes. The second experiment was a repetition of the f i r s t with 15 ml, of diethyl ether-water (1:3) replacing the methyl alcohol-water solvent. Results are included i n Table 7 under product 3-1 and 3-2respectively,
Part 6, Rate of hydrolysis of dimethyl sulphate i n aqueous and alkaline media; heat of reaction during alkaline hydrol y s i s of dimethyl sulphate and during methylation.
To study the hydrolysis of dimethyl sulphate 5 ml, portions of dimethyl sulphate were shaken with d i s t i l l e d water and also with sodium hydroxide solution for various time intervals i n the same manner i n which methylations were carried out. Aqueous hydrolysis was studied by shaking 40$ aqueous solutions of dimethyl sulphate for a given time and ti t r a t i n g the acidity with standard sodium hydroxide solution. In two series of alkaline hydrolysis dimethyl sulphate was shaken with 2$ and 11,1$ sodium hydroxide solution (10$ i n excess of Me SO for given periods and unneutralized sodium
Table 7. -Results of methylation with 2,5,11.1,15.5, and 20$ sodium hydroxide (series l ) .
Results at very low a l k a l i concentration (series 2). Results on the effect of methyl alcohol
and diethyl ether (series 3).
Series & Product Water
30$ NaOH MetS0A Time
$ Yield
$ MeO
$ Ash
$ MeO (ashless)
MeO gps. introd. E f f i c .
1-1 81.8 ml. 5.83 ml. 5 ml. 15 min. 80-90 10.0 1.94 10.2 2.60 24. 6$ 1-1 29.3 10.0 5.5 P~ 9
ti II n it 13.1 1.88 13.4 3.82 36.2 1-3 1-4 i —^
29.3 10.0 5.5 P~ 9
it « tt tt 16.6 1.91 16.9 5.16 46.1 1-3 1-4 i —^
29.3 10.0 5.5 P~ 9
ii ii ti ti 18.3 1.92 18.7 5.89 55.7 1-3 1-4 i —^
29.3 10.0 5.5 P~ 9 ii ti ti ti 19.6 2.01 20.0 6.41 60.2
A —O 2-1 10.0 ti ft 25 hrso ii 18.4 1.87 18.8 5.94 56.2
2-2 u (dropwise) dropwise)
ft 45 min. ii 2.6 2.04 2.66 -0.1 0
(buffer ) 2.82 -0.04 2-3 tt 5.83 ti 15 min. tt 2.80 0.64 2.82 -0.04 0
3-1 3-2
MeOH-Et,0-
(NHA0H ) 5.83
n ti it
ti it
it tt 12.6
18.8 1.91 1.93
13.0 19.2
3.67 6.10
34.7 57.7
-18-
hydroxide t i t r a t e d with standard hydrochloric acid. During
a l k a l i n e hydrolysis heat of reaction was allowed to proceed
without compensatory cooling. During aqueous hydrolysis the
temperature of the reaction mixture remained constant at the
I n i t i a l temperature of 24°. Results are shown i n Table 8 and
are plo t t e d i n Pig. 6. Dotted curves i n Pig. 6, taken from
the work of Lewis, Mason and Morgan(42), are included f o r
comparison.
Table~8. Aqueous and alkaline hydrolysis data f o r ;dimethyl
sulphate.
D i s t i l l e d water
30 % NaOH
Net [NaOH] Time
% Meg>S,Q reaetea
10 ml. Ormn. 0.32 ti — — 5 0.90 it — - 10 1.10 it - - 15 1.73 II — — 20 2.04 ii _ _ 30 2.62
81.8 5.83 2 % 5 31.8 it it it 10 41.0 II it it 15 45.0 II II it 20 47.0 fi it ii 30 48.5
10.0 5.83 11.1 5 45.7 ii ti it 10 51.4 n it it 15 53.2 •i it ii 20 54.9 ii it ii 30 55.0
To study the v a r i a t i o n i n reaction heat during
a l k a l i n e hydrolysis of dimethyl sulphate, 5 ml. portions of
dimethyl sulphate were shaken with sodium hydroxide solutions
f o r various time i n t e r v a l s i n the same manner i n which methyl-
ations were performed, with the exception that the heat of
reaction was not compensated f o r by cooling. At the end of
each time i n t e r v a l the temperature of the reaction mixtures
( i n stoppered flasks) was recorded. In a l l cases the molar
Figure 5. Methoxyl content and
e f f i c i e n c y versus a l k a l i concentration.
r a t i o of sodium hydroxide to dimethyl sulphate was maintained
at 1,1. Results are l i s t e d i n Tables 9 to 18 and the various
p l o t s of temperature with reaction time are shown i n Figs. 7
and 8. Pl o t s H and U 2 , F i g . 8, show the r i s e i n temperature
when 5 g. of p u r i f i e d mesquite are methylated with 5 ml, of
dimethyl sulphate and 11.1$ sodium hydroxide and the r i s e a f t e r
further addition of an equal quantity of reagents.
-20-
Figure 6 . Rate of hydrolysis of dimethyl sulphate i n water, a l k a l i and s a l t solutions at various temperatures.
9o rf
80
J9£l MeiS04 (aqueous)
i i
- (95°)
2 # N o O H (24-34" ;
z o 3 0 4 0
" T i m e i n m i n u t e s
Table 9. Temperature change during hydrolysis of 5 ml.
dimethyl sulphate with 87.6 ml. of 2% NaOH.
Time Temp. Time Temp. Time Temp.
0 m. 1 2 3 4 5
24.0° 26.0 28.3 30.2 32.2 33.2
6 m. 7 8 9
10 11
33.8° 33.8 33.7 33.6 33.3 33.1
12 m. 13 14 15 20
32.8° 32.5 32.1 31.9 30.3
Table 10. Temperature change during hydrolysis of 5 ml.
of dimethyl sulphate with 35.1 ml. of 5$ NaOH.
Time Temp. Time Temp. Time Temp.
0 m. 25.0° 5 m. 44.8° 10 ra. 36.0° 1 29.0 6 43.8 15 31.0 2 33.0 7 40.7 20 28.4 3 39.3 8 39.2 — —
4 45.6 9 37.5 -
Table 11. Tenroerature chance during hydrolysis of 5 ml.
of dimethyl sulphate with 15.8 ml. of 11.1$ NaOH.
Time Temp. Time Temp. Time Temp.
0 m. 24.0° 6 m. 49.0° 12 m. 34.4° 1 26.3 7 45.5 13 33.0 2 29.0 8 42.2 14 31.9 3 32.8 9 40.0 15 31.1 4 39.6 10 37.8 20 28.0 5 53.5 11 35.9 — —
-21-
Table 12. -Temperature change during hydrolysis of 5 ml,
dimethyl sulphate with 11,3 ml. of 15$ NaOH,
Time Temp, Time Temp. Time Temp.
0 m. 24,0° 6 m. 42.0° 12 m. 34. 3 c
1 24.5 7 56.2 13 32.3 2 26,0 8 49.5 14 30.8 3 27.9 9 44.3 15 29.5 4 31.0 10 40.1 20 26.0 5 35.0 11 36.9 -
Table 13. Temperature change during hydrolysis of 5 ml.
of dimethyl sulphate with 8.7 ml. of 20$ NaOH.
Time Temp. Time Temp. Time Temp.
0 m. 25.0° 6 m. 29.8° 12 m. 46.0° 1 25.8 7 31.5 13 41.5 2 26.1 8 34.2 14 38.0 3 26.9 9 40.5 15 35.2 4 27.8 10 52.2 20 27.7 5 28.6 11 52.0 - —
Table 14. Temperature change during hydrolysis of 5 ml.
©f dimethyl sulphate with 15.8 ml. of 18.8$ NH.OH
Time Temp. Time Temp. Time Temp.
0 m. 25.0° 6 m. 43.3° 12 ra. 31. 0 C
1 71.0 7 40.2 13 29*9 2 66.0 8 37.6 14 29.1 3 58.2 9 35.5 15 28,6 4 52.2 10 33.6 20 26.5 5 47.4 11 32.1 — —
,-22-
Table 15. Temperature change during hydrolysis of 5 ml. of dimethyl sulphate with 15.8 ml. of 11.1$ NaOH and 5 ml. of MeOH. Time Temp. Time Temp. Time Temp. 0 m. 1
24.0° 34 .0
1.5 2.0
55° -explosion
Table 16. Temperature change during hydrolysis of 5 ml. of dimethyl sulphate with 15.8 ml. of 11.1$ NaOH and 5 ml. of ether. Time Temp. Time Temp. Time Temp.
0 m. 24.0° 4 31.3° 8 m. 35.2° 1 26.7 5 32*8 9 34.5 2 28.2 6 34»2 10 33.2 3 30.0 7 35.2 -
Table 17. Temperature change during methylation of 5 g. gum with 5.0 ml. dimethyl sulphate 15.8 ml. of 11.1$ NaOH. Time Temp. Time Temp. Time Temp.
0 m. 25.0° 6 m. 46.0° 12 m. 33*6° 1 28.0 7 43.0 13 32.1 2 36.8 8 40.3 14 31.3 3 53.2 9 38.3 15 30.3 4 53.0 10 36.5 20 27.8 5 49.8 11 35.0 - -
Table 18. Temperature change during 2nd methylation following addition of 5 ml. of Me2SG-4& 5.83 ml. of 30$ NaOH.
Time Temp. Time Temp. Time Temp. 0 m. 25.0° 6 m. 41.3° 12 m. 33.5° 1 30.0 7 39.6 13 32.8 2 39.2 8 38.1 14 31.9 3 47.0 9 36.8 15 31.4 4 45.3 10 35.6 20 30.0 5 43.1 11 34.7 — —
-23-
Figure 7, Temperature v a r i a t i o n during
a l k a l i n e hydrolysis of dimethyl sulphate.
f l - NaOH IL- 51 » BT-J5.51 «
20^ Y - NM4OH S I - J I . l l NaOH
( M e O r O m - i l . l l N a O H
T i 10
m e fn If /JO
minutes IS
Figure 8. Comparison of the v a r i a t i o n i n
temperature during hydrolysis and methylation.
Part 7. Specific rotation measurements on pa r t i a l l y methylated" mesquite gum samples.
Partially methylated products prepared i n previous experiments were dissolved i n d i s t i l l e d water and the optical rotation measured i n an effort to determine any relation between methoxyl content and specific rotation. It was necessary to employ dilute solutions i n order to obtain s u f f i cient light penetration through the solutions. Samples were weighed directly into volumetric flasks and allowed to dissolve slowly i n water overnight i n order to avoid froth formation which accompanies agitation of the solutions. He-suits are l i s t e d i n Table 19 and a plot of specific rotation against methoxyl content i s shown i n Fig, 9,
Table 19* Specific rotation of partially methylated mesquite gum products and methoxyl content.
Part-Series - Product
Methoxyl content M : Part-Series
- Product Methoxyl content
1-0-2 2,40 +59.3° 2-3-1 +16.9 +56.3 ° 1-0-3 2.46 68.3 4-1-1 24.1 58.4 6-1-1 10.2 56.2 2-3-2 24.9 51.0 ,6-1-2 13.4 55.4 2-2-1 26.1 42.4 3-2-3 13.9 57.0 4-1-3 27.6 58.8 3-1-3 15.9 37.5 2-3-3 29.3 50.5 3-1-4 16.0 56.3 2-3-4 29.9 57.8 2-2-2 16.5 57.7. 2-2-3 33.7 44.3
-24-
Figure 9. S p e c i f i c r o t a t i o n of p a r t i a l l y
methylated products vs. methoxyl content
'DISCUSSION
The r e s u l t s of studies i n Part 2 show that methyl
ation i n steps i s more e f f i c i e n t than continuous methylation,
p a r t i c u l a r l y i n the advanced stages of substitution. I t may
be seen i n Pig, 1, that a f t e r a methoxyl content of about 25$
i s reached the rate/of s u b s t i t u t i o n for continuous methylation
decreases and at about 30$ methoxyl content the rate i s almost
zero. On the otiaer hand, the rate of stepwise su b s t i t u t i o n
decreases to a l e s s e r extent and remains f i n i t e above a meth
oxyl content of 35$, An examination of the e f f i c i e n c i e s
(Table 3) shows that a f t e r 2 additions of reagents the over
a l l e f f i c i e n c y f o r continuous re a c t i o n has dropped by 10 to
15$ while a f t e r 2 repeated reactions the e f f i c i e n c y has
f a l l e n by only 4,2$, The res u l t s show also that as the degree
of s u b s t i t u t i o n increases or the available hydroxyl concentra
t i o n decreases the rate of sub s t i t u t i o n decreases s i g n i f
i c a n t l y f o r both methylation processes.
In the f i r s t methylation series of Part 3 no
s i g n i f i c a n t decrease i n the extent of s u b s t i t u t i o n was ob
served for reaction times of from 3,75 to 30 minutes. The
methoxyl contents of the four products were a l l within 0,2$,
In the second methylation series a s i g n i f i c a n t
decrease i n methoxyl content was found when the reaction
time was reduced to 5 minutes or l e s s (Fig, 3),
The r e s u l t s show that i n the f i r s t series methyl
a t i o n p e r s i s t e d during d i a l y s i s and, as a r e s u l t , no mini
mum time i n t e r v a l was observed. On the other hand, i n the
second seri e s wherein the reaction was stopped before
d i a l y s i s , a minimum time i n t e r v a l was observed. Taken to
gether these r e s u l t s show two things. One i s that the
methylation reaction may be stopped or considerably reduced
by n e u t r a l i z i n g the a l k a l i present. The second i s that,
under the conditions used, at room temperature, no gain i n
extent s u b s t i t u t i o n r e s u l t s a f t e r about the f i r s t f i v e
minutes of reaction.
Results on the e f f i c i e n c y of methylation with
v a r i a t i o n i n t o t a l reagent concentration (Part 4), i l l u s t
rated graphically i n Pig. 4, show that by increasing the
i n i t i a l t o t a l reagent concentration the extent of s u b s t i
tution i s Increased. On the other hand, the extent of sub
s t i t u t i o n i s not as great f o r the single operation process
as i t i s f o r the continuous process (Part 2). A comparison
of the e f f i c i e n c i e s f o r the two processes brings out the
same r e l a t i o n . The e f f i c i e n c y of the continuous process i n
which a t o t a l o f 2.8 times the th e o r e t i c a l amount of d i
methyl sulphate was added equally a f t e r three 15 minute i n
t e r v a l s i s 2.5$ higher than f o r the single stage process i n
which the same t o t a l amount of reagent was added i n i t i a l l y .
Since the difference i n e f f i c i e n c y i s not great, i t follows
that almost the same extent of s u b s t i t u t i o n may be reached
i n shorter time by employing a large excess of reagent. I t
i s possible that dropwise addition of reagents i n a con
tinuous methylation process would show a greater difference
i n e f f i c i e n c y over the one stage process.
-26-
The eurve i n Pig. 5 "plotting the extent of sub
s t i t u t i o n against sodium hydroxide concentration i s almost
l i n e a r f o r the most part. This shows c l e a r l y that the extent
of s u b s t i t u t i o n i s very nearly proportional to the a l k a l i
concentration. I t may be seen from the p l o t of e f f i c i e n c y
against sodium hydroxide concentration (Fig. 5) that above
a concentration of about 5$ sodium hydroxide the e f f i c i e n c y
of reaction increases almost l i n e a r l y with a l k a l i concentra
t i o n . Beyond a concentration of about 20$ sodium hydroxide
the gum does not form a f l u i d s o l u t i o n and intimate mixing
of the gum with reagents i s hindered. I t i s to be expected,
therefore, that the optimum concentration of sodium hy
droxide i s close to 20$.
The a n a l y t i c a l r e s u l t s on a sample methylated by
dropwise addition of sodium hydroxide show that t h i s process
i s more e f f i c i e n t than the one stage process. In f a c t the
e f f i c i e n c y i s comparable to methylation with 15$ sodium hy
droxide, although i t i s about 5$ l e s s e f f i c i e n t than the one
stage process with 20$ sodium hydroxide. Also i t may be seen
(Table 7) that a much longer time of reaction i s required f o r
complete reaction where dropwise addition of a l k a l i i s made.
Results of the second separate experiment wherein
a buffer s o l u t i o n was employed show that no substitution can
be effected under these conditions. A l l the dimethyl s u l
phate was u t i l i z e d i n Bide reactions so that the e f f i c i e n c y
of the reaction was 0$. I t may be i n f e r r e d from t h i s r e s u l t
that a s l i g h t degree of a l k a l i n i t y i s required or that at
l e a s t a c e r t a i n amount of a l k a l i must be present i n order f o r
-27-
methylation to proceed.
The e f f e c t of substituting ammonium hydroxide f o r
sodium hydroxide was to reduce the reaction e f f i c i e n c y to
0%, This r e s u l t was unexpected i n view of the fact that
Lolkema(27) has suggested the use of ammonium hydroxide with
d i a l k y l sulphates f o r the methylation of starch. Prom a
study of the heat of reaction of dimethyl sulphate with
ammonium hydroxide (Fig, 7), however, i t would seem that
the reason that no subs t i t u t i o n i s obtained i s because the
hydrolysis side reaction occurs too quickly. I t i s possible
that at lower temperatures (less than 20°) hydrolysis i s
slower and methylation can occur.
Results on the e f f e c t of methyl alcohol and d i
ethyl ether on methylation e f f i c i e n c y (Table 7) show that
methyl alcohol reduces the methylation e f f i c i e n c y consider
ably. This reduction may be attributed to the side reaction
between dimethyl sulphate and the methyl alcohol, since i t
was observed that considerable pressure from dimethyl ether
resulted during reaction. This side reaction i s probably
involved even i n aqueous solvents to a large extent, since
considerable pressure, presumably due to dimethyl ether,
r e s u l t s during methylation.
With d i e t h y l ether-water (1:2.) as solvent, on the
other hand, the e f f i c i e n c y of reaction was higher by about
10$, I t may be that the ether-water solvent f a c i l i t a t e s
contact of the water insoluble dimethyl sulphate reducing
the heterogeneous nature of the reaction mixture, thereby
f a c i l i t a t i n g s u b s t itution. In t h i s l i n e of investigation
-28-
i t would be i n t e r e s t i n g to determine the reaction e f f i c i e n c y
i n other solvent p a i r s and the optimum solvent r a t i o .
The curves shown i n Pig, 6, bring out several
i n t e r e s t i n g facts. For one thing i t may be seen that hy
d r o l y s i s of dimethyl sulphate by water alone at 24° i s very
slow. On the other hand, hydrolysis under the same condi
tions at 95° i s very rapid and i s more rapid the higher the
concentration of dimethyl sulphate(42), I t may be seen also
that dimethyl sulphate i n 2$ sodium hydroxide solu t i o n over
the range 24-34° loses only one methyl group by hydrolysis.
With 11.1$ a l k a l i over the range 25-54° almost 10$ of the
second methyl group i s l o s t i n 20 minutes. At 95° , even i n
d i l u t e sodium hydroxide, about 70$ of the dimethyl sulphate
i s hydrolysed i n 15 minutes. The lower two curves show the
influence of s a l t s on the rate of hydrolysis. Prom an exam
ina t i o n of a l l these curves i t would seem that the methyl
ation e f f i c i e n c y w i l l be aided by low temperatures (0-20°)
i n the i n i t i a l stages of reaction, since at higher tem
peratures the i n i t i a l reaction i s probably too rapid f o r
mehtylatlon to occur. On the other hand, i f use i s to be
made of the second methyl group i n dimethyl sulphate, higher
temperatures are required. I t follows also that the higher
the concentration of a l k a l i employed f o r reaction the lower
the temperature required f o r removal of the second methyl
group, although the two are not inversely proportional. The
detrimental e f f e c t of the presence of excess s a l t i n the -
reaction mixture suggests that removal of spent reagents from
time to time i s advisable. This i s i n agreement with the
-29-
findings i n Part 2 wherein the stepwise methylation process,
involving removal of reagents a f t e r each step, was shown to
be more e f f i c i e n t than continuous methylation, a process i n
which the spent reagents are allowed to accumulate.
The curves i n Pig, 7 and Pig, 8 showing the v a r i a
t i o n i n temperature of reaction mixtures with reaction time
bring out several f a c t s . In the f i r s t place i t may seen that
there i s a good c o r r e l a t i o n between heats of reaction and
rate of hydrolysis. With 2% sodium hydroxide the maximum
temperature reached i s about 34°, while with 20$ a l k a l i the
maximum i s about 62 , I t may be seen also that the more
concentrated the a l k a l i s o l u t i o n the greater i s the l a g i n
reaction. This suggests that with concentrated a l k a l i solu
tions a longer reaction time may be required or the reaction
may require i n i t i a t i o n by heating. The fac t that the ammon
ium hydroxide curve reaches a maximum of 71° i n about one
minute supports the conclusion reached i n Part 5, that side
reaction occurs too quiekly f o r methylation to take place.
Pig, 8 shows that the heat of reaction i s just as pronounced
i n the presence of carbohydrate material as i n i t s absence.
The scattered points i n Pig. 9 indicate that there
i s l i t t l e c o r r e l a t i o n between the s p e c i f i c r o t a t i o n of par
t i a l l y mesquite gum products and the methoxyl content. I t
would seem that there i s some decrease i n s p e c i f i c rotation
with increasing methoxyl content but the r e l a t i o n i s not well
defined. This i s not i n agreement with the findings of
Haworth and Percival(26) who state that "the determination of
-30.
s p e c i f i c r o t a t i o n of methylated polysaccharides, as well as
being more convenient, i s more sensitive as a control of
methoxyl content than Z e l s e l estimations". The lack of
c o r r e l a t i o n may re s u l t from two causes. One i s that measure
ments were made i n d i l u t e s o l u t i o n (2%) with a short tube
(0.5 dm.) to allow s u f f i c i e n t l i g h t penetration and, as a
r e s u l t , small errors i n measurement are greatly magnified
(100 times). The second i s that p a r t i a l l y methylated pro
ducts were probably not homogeneous with respect to meth
oxyl content. I t i s possible that s p e c i f i c r o t a t i o n measure
ments on methylated gum samples i n other solvents, such as
chloroform, may be more s a t i s f a c t o r y . However, i t may be
that f r a c t i o n a t i o n of the gum i s necessary f o r t h i s type of
measurement,
SUMMARY AND CONCLUSIONS It was found that stepwise methylation of mesquite
gumcsamples was more e f f i c i e n t than continuous methylation.
In both processes, as the degree of sub s t i t u t i o n increases
to about 30$ methoxyl content, the rate of s u b s t i t u t i o n de
creases considerably.
The minimum time of methylation f o r equivalent
s u b s t i t u t i o n was variable depending upon the reaction condi
tions but appears to be short, i n the v i c i n i t y of 5 minutes.
I t was found that, with the same t o t a l quantities
of reagents, the e f f i c i e n c y of reaction f o r continuous
methylation i s a l i t t l e higher than that f o r a single stage
methylation process. This implies that, i n continuous
methylation, the time required to reach a given degree of
s u b s t i t u t i o n i s roughly inversely proportional to the amount
of reagents used.
For a f i x e d r a t i o of dimethyl sulphate to mesquite
gum (l.S3:l) and a f i x e d molar r a t i o of sodium hydroxide to
dimethyl sulphate ( l . l : l ) , i t was found that the degree of
s u b s t i t u t i o n i s p r a c t i c a l l y proportional to the a l k a l i con
centration with a probable optimum close to 20% sodium hy
droxide,
Dropwise addition of 30$ sodium hydroxide to mes
quite gum solu t i o n i n the presence of dimethyl sulphate
proved to be more e f f i c i e n t than the method of periodic addi
t i o n employed i n continuous methylation.
From the r e s u l t s of methylation i n a buffer
solut i o n i t i s concluded that a s l i g h t degree of a l k a l i n i t y
or at l e a s t the presence of a l k a l i i s required f o r methyl
ation to take place.
At room temperature ammonium hydroxide i s an un
s a t i s f a c t o r y substitute f o r sodium hydroxide, since side re
actions with dimethyl sulphate occur exclusively.
For the above reason methyl alcohol-water proved
to be an unsatisfactory solvent f o r the methylation reaction.
I t i s suggested that the formation of methyl alcohol by side
reaction during methylation i s a chief f a c t o r contributing
to low methylation e f f i c i e n c i e s .
The use of d i e t h y l ether-water (1:2) as a solvent
f o r methylation was found to increase the reaction e f f i c i e n c y
s i g n i f i c a n t l y . The increase i s attributed to a gain i n gum
to solvent to dimethyl sulphate compatibility.
At room temperature side reaction by aqueous
hydrolysis of dimethyl sulphate i s not appreciable. At
temperatures i n the range 20-50° a l k a l i hydrolysis i s re- ,
s t r i c t e d l a r g e l y to attack on only one methyl group i n d i
methyl sulphate. At 95° i n i t i a l aqueous and a l k a l i n e hy
d r o l y s i s i s very rapid and both methyl groups are hydrolysed.
The deleterious e f f e c t of large quantities of s a l t (Na 2S0 4)
on hydrolysis suggests that removal of spent reagents at
suitable i n t e r v a l s during methylation i s advantageous.
The saponification of dimethyl sulphate i s an
exothermic reaction causing a considerable evolution of heat
during methylation. Heat of reaction was found to increase
with a l k a l i concentration and p a r a l l e l s the rate of hy
d r o l y s i s . I t i s suggested that too rapid a rate of hy
d r o l y s i s with a proportionate l i b e r a t i o n of heat lowers the
methylation e f f i c i e n c y by allowing i n s u f f i c i e n t time f o r re
action with s t a r t i n g material.
No d i s t i n c t c o r r e l a t i o n between s p e c i f i c ro
t a t i o n and methoxyl content of p a r t i a l l y methylated mesquite
gum products was found. Lack of c o r r e l a t i o n i s attributed
to the c o l l o i d a l nature of the methylated products i n water,
making measurement of s p e c i f i c rotation uncertain, and to the
inhomogeneity of samples with respect to methoxyl content.
F i n a l l y , i t may be said that, from the re s u l t s
of studies on the dimethyl sulphate-alkali methylation of
mesquite gum, c e r t a i n guiding p r i n c i p l e s conducive to e f f i
cient methylation of water-soluble carbohydrates are apparent.
-33-
In the f i r s t place, a high concentration of a l k a l i i s des i r a b l e . I f reagents are added dropwise, then an amount of water just s u f f i c i e n t f o r so l u t i o n i s desirable. In the second place, i t i s advantageous to employ an excess of r e agents amounting to 2 or 3 times the th e o r e t i c a l quantities, the
In t h i r d place, removal of reagents and repeated methylation
i s desirable when the methylation reaches an advanced degree
of substitution. These p r i n c i p l e s set f o r t h are not new but
serve to support those which were adopted i n the numerous
methylation studies c a r r i e d out by Haworth(20). Further
study of the methylation r e a c t i o n at various temperatures
and i n the presence of various solvents may serve to define
more clos e l y the optimum conditions f o r e f f i c i e n t dimethyl
sulphate methylation.
-34-
BIBLIOGRAPHY
lo B e l l , D.J. . • • . . Annual Rev. Biochem., 18,87(1949). 2. Haworth, W.N. and Machemer • . . . . J. Chem. Soc., 134,
2,270,2,372(1932), 3. Jeanloz, R Helv. Chim. Acta 27,1509(1944). 4. Spurlin, H.M . J . Am. Ghem. Soc, 61,2222(1939). 5. Bolliger, H.R., and Prins « . . . .Helv. Chim. Acta 28, 465(1945). 6. Timell, T Svensk Papperstidn. 51,52(1948). 7. Timell, T i b i d . 51,199(1948). 8. Timell, T ibid. 51,509(1948). 9. Timell, T. . . . . . Svensk Kem. Tid. 61,49(1949).
10. Haskins, J.F Advances i n Carbohydrate Chem., vol. 2, 1946.
11. De Bucear, M Papeterie 69,202(1947). 12. Sonnerskog, S Tek. Tid. 77,133(1940). 13. Radley, J.A Paint Manuf. 17,83(1947). 14. Purdie, T. and Irvine J. Chem. Soc, 1021(1903). 15. Irvine, J.C., Pringsheim and Skinner 1 I. I. 1 ~i. Ber.6SB,
2372(1929). 16. Smith, F. I • • • • J. Ghem. Soc, 510(1944). 17. Gakhokidze, A.M. . . . . • J. Appl. Chem. (U. S. S.R. )19,
1197(1946). 18. Schlmbach, H.H., and Huchting. . . . . Ann. 561(1949). Id. Denham, W.S. and Woodhouse . . . . .J. Ghem. Soc 105,
2357(1914). 20. Haworth, W.N J. Chem. Soc 107,8(1915). 21. Hendricks, B.C. and Rundle . . . . . J. Am. Chem. Soc
60, 2563(1938). 22. Pacsu, E. and Trister J. Am. Chem. Soc 61,
2442(1939).
23. Haworth, W.N. , H i r s t and Thomas . . . . . J , Ghem. Soc. 821(1931).
24. Maxwell, S.W. . . . . .U.S. Pat. 2,101,263(1937).
25. Haworth, W.N., H i r s t and Webb i l . . . J . Ghem. Soc.
2681(1928).
26. Haworth, W.N. and P e r c i v a l i b i d . 2277(1932).
27. Lolkema, J U.S. Pat. 2,459,108(1949).
28. Preudenberg, K. and Hixon Ber. 56,2119(1923).
29. Muskat, I.E. . . . . . J . Am. Ghem. Soc. 56,2449(1934).
30. Preudenberg, K. and Boppel Ber. 73,609(1940).
51. Hess, K., Schulze and Krajnc Ber. 73,1069
32. Meyer, K.H. and Gurtler . . . . . Helv. Ghim. Acta 31, 100(1948). 33. Schoriginia, N
34. White, E.V. i.
35. White, E.V. .
36. White, E.V. .
37. White, E.V. .
N . . . . J . Gen. Ghem. (U.S.S.R.) 14, 825(1944).
• J . Am. Chem. Soc. 68,272(1946).
. i b i d . 69,622(1947).
. i b i d . 69,2264(1947).
. i b i d . 70,367,(1948).
38. Cunneen, J . I . and Smith J . Chem. Soc. 1141(1948).
39. Anderson, E. and Otis . . . . . J . Am. Chem. Soc. 52,4461, (1930).
40. Viebock, P. and Schwappach . . . . . Ber. 63B, 2818(1930).
41. Clark, E.P J . Assoc. O f f i c i a l Agr. Ghem. 15, 136(1932).
42. Lewis, H.P., Mason and Morgan Ind. Eng. Chem. 16,811(1924).
APPENDIX
Sample calculations of methylation e f f i c i e n c y :
Example 1. The average e f f i c i e n c y of reaction f o r products
1 and 2 i n series 1 i s computed.
Average methoxyl content a f t e r methylation = 15.45
Methoxyl content before = 2.48
Increase i n methoxyl content = 13.0
Molecular weight of s t a r t i n g material = 1061
Volume of dimethyl sulphate used =5.0 ml.
I f x methoxyl groups are present i n the product, i t
follows that x-1 methoxyl groups were introduced, since one
group i s present i n the s t a r t i n g material.
The expression f o r the methoxyl content of the
product becomes:
31.03 x = 15.45 1061 + (x-1} 14.02 100
Solving, x = 5.6, x-1 =4.6 groups
Thus the new molecular weight = 1125 and the
t h e o r e t i c a l y i e l d = 5.3 g. or 106$
Assuming t h e o r e t i c a l y i e l d , since 4.6 methoxyl groups
were introduced per mole of gum, i t follows that
4.6 x 31.03 x 5.0 grams were introduced. 1061
- i -
Assuming that one mole of dimethyl sulphate i s re
quired for the introduction of one mole of methoxyl, then
the amount of dimethyl sulphate u t i l i z e d i s given by
4.6 x 31.03 x 5.0 x 126.13= 2.75 g. = 2.05 ml. 1061 31.03
Thus the e f f i c i e n c y of the reaction expressed as a
percentage i s
2.05 x 100 = 41.0$ 5.0
Example 2. The average e f f i c i e n c y of reaction f o r products
3 and 4 i n series 1.
Prom Tables 1 and 2 the average increase i n methoxyl
content i s 21.32$
The expression f o r the methoxyl content of the product
i s given by
31.03 x = 25.8 1061 + (x-1) 14.02 100
from which x =9.0, x-1 = 8.0 groups
This corresponds to 4.76 g. = 3.57 ml. Me 2S0 4
Since a t o t a l of 10 ml. of dimethyl sulphate were used,
the o v e r a l l e f f i c i e n c y i s
3.57 x 100 = 35.7$ 10.0
I t was shown i n the previous sample c a l c u l a t i o n that
the reagent e f f i c i e n c y during the f i r s t hour of methylation
i s 41.0$. I t follows then that the e f f i c i e n c y during the
second hour i s given by
41.0 + x = 35.7 2
from which x = 30.4$
Example 3. Cal c u l a t i o n of the maximum methoxyl content of
completely methylated mesquite gum.
I f the structure page 5 i s assumed, then i t may be
seen that 17 hydroxyl groups, including the ga l a c t o s i d i c hy-
droxyl group, are available for methylation. I f these are
methylated then there are 18 methoxyl groups per mole present.
The equation f o r the maximum methoxyl content becomes
x = 31.03 x 18 100 1061 + (17 x 14.02)
from which x - 43.0$ methoxyi
Example 4. Ca l c u l a t i o n of dimethyl sulphate necessary f o r
complete methylation of 5 g. mesquite gum.
I f i t i s assumed that one mole of dimethyl sulphate
i s required f o r one mole of hydroxyl, then we require
17' x 5 moles of dimethyl sulphate s 17 x 5 x 126.1 grams 1061 1061
s 17 x 5 x 126.1 = 7.58 ml. 1061 1.332
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