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CHAPI'ER II
RESULTS AND DISCUSSIONS
The present work can broadly be divided into two
parts, the first dealing with the effects of electrolytes
on the activity coefficient of·the initial and transition
states as·a function of the identity and concentration of
the electrolyte for the hydrolysis of 1-(p-chlorophenyl)
ethyl hydrogen succinate and the other consisting of the
hydrolysis of 1-phenylethyl hydrogen succinate and its
para substituents (cH3
, c2H
5, CH
3o, F, Cl and Br) and
the solvent chosen for the study is water.
Water is unique among liquids because of the
possibility of tetrahedral coordination with four nearest
neighbours. This built in tendency towards a three
dimensional structured conditions has long been
recognised and has been repeatedly emphasised in dis
cussions of water structure (40-45). The unusual
properties resulting from the structural nature of water
are reflected in the aqueous solutions of ions and the
different but quite unusual properties of aqueous solutions
of weakly polar molecules such as alkyl halides. Yet
inspite of the recognised importance of some reactions in
aqueous media, until recently but a limited number of
kinetic studies on the hydrolysis of simple halides and
similar compounds had been made in water. Water is a
notoriously poor solvent for most.organic compounds of
interest to kineticist. Mixed solvents, in which water
17
was frequently one component, were more convenient to use
and at the same time provided a graded series of solvolytic
media in which to explore the relation between rates,
mechanism and effects of changing solvent properties.
Various attempts have been made to treat these solvent
effects in a more quantitative manner. These attempts
succeeded within the limitations of the model and the
questionable value of dielectric constant. Such treatments
had some success where ion solvent interactions were large
but were recognised to have limited analytical value.
Laidler (46) has reviewed briefly the problem of inter
preting kinetic data, and while he recognised the potential
value of such derived functions as the enthalpy and entropy
of activation, he concludes that free energy considerations
provide the soundest basis for an electrostatic model. An
alternative was provided by the correlation equations of
Grunwald and Winstein (47). Many factors favoured water
as a solvolytic medium and it was known to be a very good
ionising medium and hence solvation effects in innogenic
reactions were expected to be large and more easily determined.
18
Salt effect studies can provide considerable information
of theoretical importance as to the complex interactions
of ions and neutral molecules and as to the unique nature
of water as a solvent.
SALT EFFECTS ON THE KINETICS OF HYDROLYSIS OF 1-E-CHLORO
PHENYL ETHYL HYDROGEN SUCCINATE IN WATER
The addition of any solute to water is likely to be
accompanied by changes in one or several of the parameters
which have been utilized as indicators of the degree of
water structure and it is tempting to correlate these changes
with other effects of the salt on the solution. These
structural changes will certainly be included in a complete
theory of salting out. The insertion of a nonpolar molecule
into water may be divided into, first, the pulling apart of
water molecule to make a hole into which the solute can fit
and second, the insertion of the solute into the hole. In
the case of a nonpolar solute which does not interact
strongly with water the greater part of the free-energy
requirements for this process may be ultimately ascribed
to the decrease in the mutual interaction of water molecule
in the first step. This provides the principal reason for
the low solubility of organic molecules in water when the
solute is not sufficiently polar to provide a compensating,
19
favourable interaction with water in the second step. The
addition of any material which will increase the average
cohensive energy of interaction among the water molecule
will make it more difficult to separate the water molecule
and dissolve the solute where as addition of a substance,
such as an alcohol, which will decrease the average mutual
attraction of the water molecules will facilitate these
process. Most salts increase the average strength of
mutual interaction and cohensive energy-density of an
aqueous solution. The salting out of a nonpolar solute
may be regarded qs simply a squeezing out caused by the
electrostriction and increased average strength of the
mutual interaction of the solvent molecule in the presence
of salt. The strong directional interaction of water mole
cules with each other makes it certain that the mutual
orientation of water molecules in liquid water is far
from random, and many of the properties of water can be
described by a model in which the orienting effects of water
molecules on each other are considered in terms of a kind
of structure, perhaps a number of 'flickering clusters' of
water molecules (41,45,48). Such oriented regions of
hydrogen bonded molecules will have some of the properties
of one or another form of ice, although it is unlikely that
they represent regions with the structure of ordinary ice.
The introduction of an ion with its surrounding electric
field, will change this state of affairs, although the
ways in which the changes take place are not simple. A
small highly charged ion will orient the immediately
adjacent water molecules into a firm hydration sphere,
which will remain with the ion for an appreciable time
20
and will have properties which are quite different from
those of the bulk water molecules. These water molecule
constitute the immediate hydration shell of the ion and
define the hydrated radius and hydration number although
these quantities vary over a wide range depending on the
method used for these estimation (49,51). These molecules
may be regarded as more structured than the bulk molecules
of liquid water. Outside this firm hydration shell or
immediately adjacent to the surface of larger ions the
electrical field will not be strong enough to bind water
molecules in a hydration shell, but will be strong enough
to compete effectively with the dipole - dipole forces
which provide some orientation and structure to the mole
cules of liquid water. There are at least two types of
structure forming ions, small ions with a high charge density
that orient surrounding water molecules by a strong electro
static interaction and very large ions which may orient
surrounding water molecules because their electric field
is too weak to have a significant influence on the solvent.
For this reason a number of the parameters which measure
water structure show maximal values for the smallest and
largest ions, surrounding a minimum for the structure
breaking ions of intermediate size. Some of the most
visualized paramters which are thought to be measures of
water structure are viscosity, entropy, rate of self
diffusion and dielectric relaxation time of water.
21
Simple salts such as the alkali halide cause a
decrease in dielectric relaxation time of water (52). This
is one of the most straight forward lines of evidence for
the structure breaking effect of such salts in water. The
decreased relaxation time reflects the increased freedom of
movement of water molecules in the presence of these ions.
However, in presence of small highly charged ion there will
be some water molecules in the first layer of hydration shell
which are so strongly oriented by the intense electric field
of the ion that they are not free to reorient themselves in
·an-applied electric field at all. These water molecules
which may be designated as irrotationally bound will cause
a decrease in the observed dielectric constant of the solu
tion (52,49). Because of this structure forming tendency,
such small ion cause a smaller decrease in dielectric
relaxation time than medium sized ions. The fact that the
dielectric relaxation times passes through a minimum as the
22
salt concentration is increased and eventually reach a
value higher than that for pure water. As the salt con-
centration is increased, there is a decrease in the number
of interstiti�l water molecules which have an increased
freedom and which account for the decreased dielectric
relaxation time. Eventually the salt concentration reaches
a point at which all the water molecules are in the
immediate viscinity of ions and have their freedom of move-
ment restricted by the field of ions (53). If the size of
the ion is increased sufficiently the charge will be
effectively shielded from water and the strength of the
interaction of the ion with water molecule will become
even less than that of another water molecule. For such
ions the electrostriction and even the structure breaking
effect associated with most ions disappear and the
behaviour of the ions approaches that of a nonpolar solute.
Studies of the near infrared absorption spectra of water
show that the spectra are influenced characteristically by
temperature changes and dissolved salts (54). The order
of increased structure breaking power deduced from spectral
studies is
Cl-< N03 - <.._ Br- L..._ I- � Cl04-
Na + L._ K+ z... Cs
+ z. ( CH3) 4 N+.
and Li+ <
Before discussing the present data it might be of
23
advantage to give a brief account of salt effects on the
stability of the initial and transition state of hydrolysis.
Taft and coworkers have studied the salt effects on the
hydrolysis of t-butyl chloride in water (24). The speci
ficity of salt effects on the initial and transition state was
studied for a number of salts. The finding was that for all
electrolytes both 1-1 and 2-1 electrolyte - the total effect
of the salt contained a specific part, the magnitude of
which depended on the identity of the electrolyte. This
was true of both initial and transition state. However
with simple inorganic salts, the specific effect was
essentially the same in both the initial and transition
state, so that they behaved as though they were true
Debye-Huckel electrolytes. On the other hand, with other
electrolytes, particularly those which showrnicelle forma
tion, the specific effect were different in the initial and
transition state. It was confirmed that the difference of
the total salt effects of simple inorganic electrolytes
between the initial and transition state was nonspecific
in nature.
Kohnstam studied the problem by considering the
effects of salts on the stability of the initial and tran
sition state for the SN1 hydrolysis of 4-nitro-4'-phenyl
diphenyl methyl chloride in 70% v/v aqueous acetone (55).
All salts stabilized the transition state, the magnitude of
stabilization depending on the identity of the salt. Regard
ing the effect on the initial state tetramethyl ammonium-
fluoride, sodium chloride, sodium benzene sulphonate
decreased the activity coefficient while �odium nitrate,
sodium borofluoride, sodium bromide and sodium perchlorate
had the opposite effect. It was suggested that the effect
on the activity coefficient of the initial state could be
understood on the basis of the operation of a salt induced
medium effect which depends on the capacity of the salt to
dry the solvent. It was also suggested that the known
effect of solvent shanges on the stability of the initial
and transition state in this reaction requires that the
increasing values of t1/�0 where t1 and �o are rates in
the presence and absence of the electrolytes, should arise
mainly from an increase in r/r0
and only to a relatively
small extent from a redu6tion in r*/r0+(55). This conclusion,
that the stability of the transition state is often insen-
sitive to the nature of electrolyte as was originally
suggested by Ingold (56) appears to be derived from the
circumstances that the spread in the values of the activity
ratio of the transition state is smaller than that in the
initial state. The study also disclosed that both the
initial and transition state were stabilized by the per-
chlorate and benzene sulphonates. This is ascribed to
25
specific short range interaction with the substrate (57,58).
The conclusion that the initial state is stabilized by
perchlorate not withstanding an increase in the ratio of
the activity coefficient, appears to be based on the idea
that this electrolyte should have given much larger value
for the activity coefficient ratio.
Bockris and Egan had measured the solubility of
benzoic acid in ethanol water mixture both with and without
added sodium chloride (1M) (59). The benzoic acid is salted
out by the electrolyte in pure water but as the organic
cosolvent is gradually added the salting out changes.to a
salting in and with further increase in the concentration
of the organic cosolvent the electrolyte again salts out
the solute. For methanol water system, the salting out
initially decreases with increase in methanol content and
reaches a minimum at about 58% w/w methanol and then
slightly increases. Although a salting in does not occur
here, the pattern of the result is similar to that of
ethanol water system. In dioxan water, the maximum dioxan
concentration investigated was 24.4% w/w. In this region,
the decrease in the salting out was noticed. It is possible
that a further increase in the dioxan concentration would
bring about the existence of a minimum as with the other two
solvent system. Grunwald has measured the solubility of
nap'}halene and 1-naphthoic acid in 50% w/w dioxan water
26
system, without and with added salts (60). For both organic
solutes, there was a well defined salting out order. Of
the simple inorganic salts examined, the following results
are observed. In the.naphthalene system, sodium perchlorate$,
potassium perchlorate, potassium iodide and potassium
bromide showed a salting out effect, their magnitude
decreasing in the order, potassium chloride and sodium
chloride showed a salting in, the magnitude being about
the same. For naphthoic acid, the salting out decreased
in the order Nac104 :;> KCl04 > KI. The salting in effect
increased in the order KBr< NaCl < KCl. The salting
order in aqueous organic solvent is almost exactly the
reverse of the order in water. In discussing the results,
the authors refer to the model proposed by Mcdevit and
Long (61) applicable to aqueous solutions. This model
implies that when a salt is added to an aqueous solution
of the non-electrolyte, the increase in the initial
pressure resulting from ion solvent interaction squeezes
out the non-electrolyte molecule. Since this model cannot
account for the data in dioxan water system, they proposed
an additional· mechanism for the salt induced effect in
aqueous organic solvents which depends on the various
electrolytes to salt out the organic cosolvent. This
reversal of the salting out order was rationalised by the
authors by a theoretical treatment which showed that one
27
of the factors determining the salting out or salting in
of a particular electrolyte is d 1��
x where xis the
ratio of the thermodynamic activities of the components of
the solvent and z is the mole fraction of water in the
solution without taking into account the mass of the solute.
In water, since z is one and therefore, since d log x/dz
occurs in the denominator, the term containing the factor
vanishes. The authors concede that while the change in
solvent composition would also affect the other terms
contributing to the overall salt effect; the variation in
this term is the most important. The importance of this
work lies in their proposal for a general understanding of
the problem of salt effects on solubility in water and in
aqueous organic solvents.
We believe that much of the difficulty associated
in accounting for salt effect can be understood on the model
proposed by Grunwald, particularly with reference to the
effect of the salt on the stability of the initial state.
For a given substrate, a particular electrolyte will pro
bably show salting out in pure water thus increasing its
activity coefficient. Regarding the identity of solute, the
following observation is relevant. In solvent water Long
and Mcdevit have shown that for a polar non-electrolyte the
salting out increases with the molar volume of the solute (61).
In the work on the solvolysis of 1-phenylneopentyl chloride
28
in 80% aqueous acetone the enthalpy of activation has been
observed to be 24.9 k.cal/mole (62), while in water, the
enthalpy of activation is 17.8 k.cal/mole (63). In general
the enthalpy of activation of SN1 reactions of alkyl
chloride is increased while going from aqueous organic sol-
vents to water. For example the enthalpy of activation of
t-butyl chloride in 80% aqueous acetone is 22 k.cal/mole
(64), while in water it is 23.8 k.cal/mole (65). In this
particular case the reverse trend observed must be ascribed
to the fact that the larger molar volume of the organic
solute precludes any significant solvent-solute interaction,
thus destabilising the initial state. Grunwald suggests
that in the presence of salts, 'tight' salvation shells
are formed around the ions whose composition differs from
the bulk of the solvent. Because of this, there is a com-
pensating change in the average composition outside the
salvation shell which would lead to increased solubility.
Oakenfull studied the kinetics of hydrolysis of
acetic anhydride in presence of various electrolytes (66).
There are three possibilities for salt effect in the
hydrolysis of acetic anhydride (i) The specific interaction
between the ions and acetic anhydride (ii) Specific effect
resulting from changes in the medium, ie the dielectric
constant and (iii) The structure of water. Most ionsinter
act strongly with water and modify its structure (67). The
29
viscosity B. Coefficient from Jone-Dole equation seems to
be a useful 'intuitive' measure of the effect of salts on
the structure of water. These B values show that the order
of the structure making effect is Bu4NC1 � MgC12 > LiCl>
Me4NC1 > NaCl > NaBr > NaI ;> RbCl > CsCl with the last
two being structure breaking. There is obviously a rough
correlation and it is tempting to conclude that salting out
occurs when increased water structure makes it more difficult
to insert the solute. It has been pointed out however that
this kind of argument is unsound. One can equally well
argue that the increased water structure caused for ex�ple
by the large tetraalkyl ammonium ions would make it easier
to form a shell of structured water around the solute.
In the water catalysed neutral hydrolysis of �-nitro
phenyl and p-methoxyphenyl dichloroacetate in water Engbersen
and Engbert studied the effects of t-BuOH, 1l Bu4 NBr, KBr
and Nac104 (67). Except for aqueous Nac104 extrema in AH*
and 6.S are observed as a result of large compensatory
changes in these quantites of activation as a function of
solvent composition. The s-pecific pattern of the .6...H* and 6S*
relationship clearly depends on the nature and concentration
of the additive and serves to indicate the overwhelming
importance of solvation factors. The combined evidence
strongly suggests that effects due to changes in the diffu-
sionally averaged water structure may provide a rationale for
30
understanding these phenomena. Water is the ubiquitous
solvent for fundamental chemical reactions involved in
life processes. There is abundent evidence that chemical
reactivity in aqueous media is profoundly influenced by
the three dimensional hydrogen bonded structure of liquid
water (68). For instance, one of the.consequences of this
structural property is the unique propensity of water mole-•
cules to participate in intermolecular proton transfer
process (69,70), which constitute such an important feature
of enzyme catalysed reaction. Since it has been recognised
that.diffusionally averaged water structure may be either
appreciably decreased or increased compared with pure water··
around the active site of enzyme (71 ,72) it would be of
great interest to investigate the effect of perturbration
of water structure on the rate and energetics of proton
transfer reactions in water.
In an effort to probe into the effects of electrolyte
on the dynamic basicity of water Menninga and Engberts have
studied the kinetic salt effects on the water catalysed hydro-
lysis of two covalent arylsulphonyl methyl perchlorates in
water (73). The finding that the hydrolysis rates are
retarded by cations and enhanced by anions cannot be
explained by extended Debye-Huckel or Bronsted theories for molecule-
molecule reactions. They proposed that the salt effects
31
originate predominantly from electrostatic ion-water inter-
actions and specifically reflect the nature of the dipolar
transition state for deprotonation. The structure breaking
effect of cations and anions is in the increasing order of . ( ) + / 2+ < 2+ < + effectiveness 74,75 nBu4
N '- Mg Ca Me4N �
Li+ ( Na+ < K+ < Cs+ and so4
2-< Cl-< Br-< c104- where
+ 2+ 2+ + . + + nBu4N, Mg , Ca , Me4N, Li and Na are structure
makers. The salt effect on the hydrolysis of p-nitrophenyl
sulphonylmethyl perchlorates follow the sequence HC104 >
MgC12
> CaC12
> NaClO 4 )> HCl > NaCl L/""\ Li Cl '-../"\NaBr V'IKBr >
CsCl > Me4NBr > nBu4NBr > Na2so
4 and for the hydrolysis
of £�methyl phenyl sulphonylmethyl perchlorates Nac104 \.../J
NaBr / NaCl> LiCl > Me4NC1 > nBu4NBr. If water structure
effects or salting in and salting out parameters would
dominate the sign and magnitude of the salt effectJCsCl
would cause a stronger rate decrease than LiCl which is
in contradiction with the experiment. The observed salt
effects may be rationalised by assuming that the magnitude
of the salt effect is primarily determined by the charge
type and charge density of the distinct ions. The salt
effect of £-nitro and £-methylphenyl sulphonylmethyl perchlorates
follow the sequence Mg2+> Ca2+> H+ > Li \J, Na\_..,-, K+
> Cs+>
Me4N+
> nBu4
N+
and c104 > Br-> c1·> so4 2- corresponding
with the order of charge density. The results suggest the
32
existence of an electrostatic interaction between the dipolar
transition state and electrostatic field of the ions
operating via polarised water molecules between the ion
and the partially broken C-H bond (73). The same authors
present another paper of the water catalysed hydrolysis of
two covalent arylsulphonyl methyl perchlorate in dioxan
water, t-BuOH-H2
o and CH3CN-H
20 (76).
Hibbert and Long studied detritiation of malono-
nitrile in mixed aqueous organic solvents and in salt
solutions (77). Addition of tetraammonium halides and
of organic solute cause a strong decrease in the values
of the activity for the transition state (i.e.) a negative
free energy of trans£er (salting in) whereas addition of
alkali halide cause a strong increase in the activity or
a salting out of the transition state. Any solute which
increases the stability of water clusters is said to be
structure making, nonpolar molecules and large ion salts
are structure makers. Additives which increase the water
structure lead to an inereased rate of detritiation,the
reverse is true for structure breakers. Addition of a
structure maker such as tetraethyl ammonium bromide should
decrease the required further structure making and hence
cause a rate acceleration.
The kinetics of salt effect on the hydrolysis depends
on various factors (i) the dielectric constant and the
polarity of the medium (ii) the size and shape of the
substrate (iii) the nature and properties of the electro
lyte (i.e) the structure breaking and making abilities of
the electrolytesand (iv) the charge type and charge
densities of the distinct ions. The results for the
hydrolysis of 1-£-chlorophenylethyl hydrogen succinate in
water with various electrolytes are set out in Table 1 and
2. One significant feature of the result is that the
electrolytes have practically no effect on the activity of
the initial state except at high concentrations of the
electrolyte5as can be evident from the solubility data.
As the sodium chloride concentration increases from Oto
0.05M the rate of reaction also increases. After when the
electrolyte concentration is increased there is a slight
decrease in the rate and then the rate become almost steady
as the salt concentration is increased. Sodium nitrate and
sodium bromide show similar results but the rate increase
is less. For all these electrolytes the changes in the
rate constant on addition of the electrolyte are the addition
properties of the related anions and cations. The results
can be considered to result from the differences in the
ionic strength effect which is independent of the nature of
the electrolytes and the structure breaking or making effect
of the electrolyte. For anions the structure breaking
34
effect increases in the order Cl-< N03 < Br-< c104 and
for cations Li+< Na+ <. K
+ ..( (CH3
)3N (74,75) and this
order agrees with the experimental observation.
For lithium perchlorate the result is slightly
different. The nature of the interaction of perchlorate
has now to be considered. Besides the ionic strength and
structure breaking effect some other effect is also operat-
ing. Attention has been drawn to Grunwald's view that
perchlorate ion complexes with dioxan (57). Waind has
shown that the solubility of ethyl acetate in water is
increased by the addition of perchlorate which is not
generally true of other salts ( 61 ) • For 'i' -butyrolac�tone
the salt effect on solubility decreases in the order NaCl>
KCl >NaBr > NaI > Nac104·;, KI, the last three salts showing
a salting in. It is interesting to note that there appears
to be a relation between the anion size and salt effect.
Similar results have been obtained by Taft (24). All these
suggest a rationale for understanding the peculiar effect
of perchlorate ion on the rate of reaction. The assumption
is that perchlorate ion will interact with an organic so lute,
the greater its size. Possibly a nearly covalent type of,
linkage is involved. This factor is .particularly important
in understanding the effect of perchlorate ion on the
transition state of the reaction.
TABLE 1
EFFECT OF ADDED ELF.cTROLYTE IN THE RATE OF HYDROLYSIS OF 1 -p-CHLOROPHENYLEI'HYL HYDROGEN
SUCCINATE IN WATER
(est er) -� 0. 008M Temp : 1 00 ° C
---------------------------------------------------------------------------------------------------------·
Electrolyte
0.00)'1 0.005M 0.011'4 0 .025JvJ 0.05M . 0 .1 OJ'<J ------ ------------ ------------- ------------ ------------ ------------
4 4 % age 4 10 !f
1 1 0 �1 va�i- 10 t1at ion
% age 4 % age 4 % age 4 % age vari- 1 O t
1vari- 1 0 t
1 vari- 1 0 !f
1 vari
�ioo �ioo �ioo �ioo
0. 1 51'<'�
4 % age
1 0 !f1
variation
0. 251YJ
4 % agE1 0 !f
1 vari· ati01
---------------------------------------------------------------------------------------------------------·
NaCl 0.1 70 0.202 18.8
KCl
NaN03 0.1 92 12.9
NaBr
Na2so4 0.122 -28,2 0.127 -25.3
Li2SO,:,:
0.154 -9.4
LiC104 o.1so 5.9
0.1 97 15 .9 0.213 25.3
o.182 7.0
00209 22.9 0.1 97 1 5 .9
0.157 -7.6 0.185 8.8
0.1 26 -2509 O. 11 0 "'.35 • 3
0.1 48 -12.9 0.151 -11 .2
Q.170 0
0.1 95 14.7 0.185 8.8 0.1 93 13.5
o.187 1 0.0 0.195 14.7
0.1 93 13.5 o.185 8.8
0.125 -26.5 0.128 -24 • ..,
0 • 1 5 6 -8 • 2 O • 1 5 3 _q O • 0 O • 1 46 .q 4 • 1
0 • 1 88 1 0 • 6 0 • 1 87 1 0 • 1)
----------------------------------------------------------------------------------------------------------
\>I V1
36
TABLE 2
EFFECT OF ADDED ELECTROLYTES IN THE SOLUBILITY OF 1-(£-CHLORO-
PHENYL)ETI-IYL HYDROGEN SUCCINATE IN WATER AT 35° c
---------------------
-------------------------------------------
OM 1 .607
0.01M 1.587 1 .511 10554 1.643 1 .604
0.025M 1.582 1.543 1.572 1 .631
0.05M 1.550 1.533 1.548 1.606 10545 1.660
0.10M 1.592 10500 1 .623 1 .Lt-66 1.510
0.15M 1.560 1 .514 1.488 1.594 1.443
0.25M 1 .472 1.567 1.493 1.239 1.289
--------------------
--------------------------------------------
37
Bunton has examined the acid hydrolysis of ethyl
and t-butyl acetate in water and finds that for ethyl acetate,
chloride ion has a larger effect than perchlorate ion while
the reverse is true for t-butyl acetate (78). This result
has been ascribed to an interaction between carbonium-ion
like transi tion state and the perchlorate ion. He has also
shown the effect of salts upon the stability of the tri-£
anisylmethyl cation relative to that of Q-nitroanilinium
ion to be in the order Nac104 ·> LiCl04) MeS03
Na > NaBr >
NaN03
:> LiCl. The implication is that the magnitude of
the interaction will increase not only with the size of
the organic moiety but also with the increase in the
delocalisation of the cationic charge.
Further support comes from the work of Diamond (79).
He has found that for large unhydrated univalent ions, a
tighting of the surrounding water structure is a dominant
feature of their aqueous solution behaviour. As represented
by their activity and osmotic coefficients, this corresponds
to a rise in the coefficient above the Debye-HUckel limiting
law and the increase is greater, the larger the ion. But
if both the cation and anion are such larger hydrophobic ion,
the hydrogen bonded water structure forces them together to
maximise the water - water interaction and minimise the
disturbance to itself. This water structure enforced ion-
38
pairing is very different from the more usual Bjerrum type
of ion pairing. This is the view accepted and exploited l't
by Bu,ton (78). He has accounted for the large decrease
in the activity coefficient of the transition state in
the solvolysis of isobornyl chloride in both aqueous
methanol and aqueous acetone by invoking this type of
ion pairing between the carbonium ion like transition
state and perchlorate ion. If we postulate this type of
ion pairing in water we can account for the slight increase
in rate when the lithium perchlorate concentration is
increased.
The result with sodium sulphate and lithium sulphate
appear to be difficult to account for. The results are in
Table 1 and 2. There is negative salt effect for both
sodium and lithium sulphate. The decrease in rate for
sodium sulphate is greater than that of lithium sulphate
which is in agreement with the structure breaking effect
of the anions. Taft observed rate increase for t-butyl
chloride in water (24) for sodium sulphate. Ramaswamy Iyer
studied the effect of lithium sulphate on the hydrolysis of
t-butyl, t-amyl, diethyl methyl, triethyl chloride in 60%
aqµeous acetone (80). At lower concentration, the rate :nc�e�se
increases as the series is ascended, but the reverse order
is noticed at the higher salt concentration. Since sulphate
39
ion is doubly charged, greater salt induced medium effect and
consequently greater stabilisation of the initial state is
expected. This appears to be the dominant effect at the
higher salt concentration because the trend in the values
is closely similar to that with sodium chloride. On the
other hand the results at low salt concentration appear
to need the postulation of a small contribution due to the
ion pairing effect. Menninga and Engberts showed that in.
the hydrolysis of aryl sulphonyl methyl perchlorates in
water sodium sulphate increases the rate of hydrolysis (73).
Sivaramakrishnan observed negative salt effect for the
hydrolysis of acid phthalates of a number of aliphatic
alcohols with sodium and lithium sulphate in water (81).
The results are set out in Table 3 and 4 and the work in
this field is progressing. At this stage we are not in a
position to predict the exact cause of this negative salt
effect but in the hydrolysis of acid esters of dicarboxylic
acid in water there is negative salt effect both for sodium
sulphate and lithium sulphate. Clearly a great deal more
needs to be learnt about salt effects on rate of reaction in
water.
KINETICS OF THE HYDROLYSIS OF SOME ACID SUCCINATES IN WATER
In this section we consider the kinetic data for
the hydrolysis of acid succinates of 1-phenylethyl alcohol
TABLE 3
EFFECT OF LITHIUM SULPHATE IN THE HYDROLYSIS OF THE HYDROGEN PHTHALATESOF ALIPHATIC
ALCOHOLS IN WATER AT 65° C
(Li2so4
)= O.OOM 0.05:M 0.10M 0.20M 0.30M 0.50M -�------ ------------- -------------- ------------- ------------- ---------------
Ester 104k 104k % age vari--1 -1 at ion
n-Amyl hydro-gen phthalate 0.0549 0.0283 -48.5
Sec.Amyl hydro-0. 01 86 -25 • 9gen phthalat e O .0251
n-Butyl hydro-gen phthalate 0.0662 0.0474 -28.4
iso-Butyl hydrogen 0.0551 0.0408 -29.9 phthalate
t-Butylhydrogen 00815 0.545 -33.1 phthalate
104k % age
4 varia- 10 �1
% age vari--1 tion at ion
0.0239 -56.5 0.0187 -65.9
0.0163 -35.1 0.0132 -47.4
O .0423 -36.1 0.023 -57.7
0.0364 -33.9 0.0318 -42.3
0.520 -36.2 0.522 -35.9
104k % age
104k ;lo age
vari- vari--1 at ion -1 at ion
0 .0142 -74.1 0.0108 -80.3
0.0106 -57.8 0.0104 -58.(
0.0216 -67.4 0.0202 -69.5-
0.0235 -57.4 0.0239 -56.6
0.525 -35.6 0.565 -30.7
-----------------------------------------�-------------------------------------------------------
.p,.
C)
TABLE 4
EFFECT OF SODIUM SULPHATE IN THE :HYDROLYSIS OF THE HYDROGEN PHTHALATES OF
ALIPHATIC &b-�fil ALCOHOLS IN WATER AT 65 ° C
(Na2
so4
)= O.OOM 0.05M 0 .1 OM 0.20M 0.30M 0.50M ------- -------------- -------------- ------------- ------------- --------------·-
Ester
n-Amyl hydrogen phthalate
Sec.Amyl hydrogen phthalte
n-Butylhydrogenphthalate
Iso-butyl hydrogen phthalate
t-Butylhydrogenohthalate
104
k 104
k% age
104k
% age 104
k% age
104k
% age 104k
7b agE vari- vari- vari- vari- vari-
-1 -1 ati.on-1 at ion
-1at ion
-1 at ion -1 at ion
0.0549 0.0242 -55.9 0.0239 -56.5 0.0212 -61 .4 0.0137 -75.1 0.0130 -76.3
0.0251 0.020 -20.3 0.0204 -18.7 0.0129 -48.6 0.0113 -55.0 0.0104 -58.6
0.0662 0.0451 -31.9 0.0448 �32.3 0.0258 -61.0 0.0244 -63.1 · 0.0204 -69.2
0.0551 0.0418 -24.1 0.0364 -33.9 0.0362 -34.5 0.0266 -51.7 0.0218 -60.4
0.815 0.583 -28.5 0.533 -34.6 o.470 -42.3 o.423 -48.1 o.368 -54.s
-------------------------------------------------------------------------------------------------
Ap
42
and its para substituents (CH3
, C2H5, CH3
o, F, Cl aid Br)
in water.
Table 5 contains the first order rate constants
for the hydrolysis of the acid succinate�-
TABLE 5
FIRST ORDER RATE CONSTANTS IFOR THE HYDROLYSIS IN WATER OF
THE ACID SUCCINATES OF 1-PHENYLETHYL ALCOHOL AND ITS PARA
SUBSTITUTED DERIVATIVES
1. 1-Phenylethyl hydrogen succinate
Temp oc 1 o4!!1 (sec-1 )
85 0.0811
90 0.139
95 0.216
110 1.03
2. 1.:.P-Tolylethyl hydrogen succinate
65 0.319
70 0.532
73 0.774
80 1 .51
85 2.52
90 4.00
3. 1.:,p-ethylphenylethyl hydrogen succinate
75
80
85
90
00807
1.36
2.01
3.45
4. 1-p-meth0xyphenylethyl hydrogen succinate
20
28
33
40
45
o.457
1 .16
2.05
4.47
8.18
5. 1=,p-fluorophenylethyl hydrogen succinate
90
95
100
110
6. 1-p-chlorophenylethyl
90
95
97
1·00
110
115
0.29
0.534
0.819
2.05
hydrogen succinate
0.0781
0.114
0.142
0.170
0.384
0.598
7. 1=:P-bromophenylethyl hydrogen succinate
97
100
110
115
0.106
0.136
0.245
00355
The first point to be noted is that the rate of
44
hydrolysis of the esters follow the Baker and Nathan order
(82). In a careful review, Berliner has listed two possible
factors, the operation of which, either alone or in con-
junction, could explain the Baker-Nathan order without
recourse to hyper•conjugation, although he himself has
expressed his preference for the latter alternative (83).
The first factor is based on the idea that there are no
essential difference in ground state solvation of the sub-
strate is not necessarily valid. For example, it has been
suggested that the Baker-Nathan order observed in the
methanolysis of £-alkylbenzyl chloride is due to this cause (84).
Arnett's calorimetric measurements have served to establish
the reality of a.ch differences (85)o Brown has also
supported this possibility (86). Winstein and Fainberg (87)
as also Hyne (88) have come to similar conclusions. Heat
capacity of activation values by Robertson and his associates
have been adduced in favour of the above hypothesis (89).
Other workers have also argued in support (90-94). Here
45
also, the view is taken that the results can be accounted
for by C-H hyperconjugative interaction. It is also
assumed that the hyperconjugative effect has both a
polarisation and polarisability component. It will be
recalled that it was this idea which led Hughes, Ingold
and Taher to examine the kinetics of para alkyl diphenyl-
methyl chloride to obtain clear cut evidence for C-H
hyperconjugation ( 95). They argued that a strong'1lectron
demanding reaction, such as the SN1* reaction was most
likely to invoke a sufficiently strong polarizability
hyperconjugative effect.
The most commonly encountered and widely studied
influence of a substituent in the reactions of organic
molecule, next to its polar and steric effect is perhaps
the phenomenon of neighbouring group participation where
the substituent influences a reaction velocity by stabiliz-
ing the transition state or an intermediate by becoming
bonded or partially bonded to the reaction site. More often
such participation leads to rate enhancement and is then
turned as intramolecular catalysis. Ester hydrolysis is
perhaps the one single reaction where this phenomenon has
been more frequently met with. Prototropic groupssuch as
* The case for regarding the BAL1 reaction as an SN1reaction has been strongly put by Bender (106)���Vl. �-
46
the carboxyl and hydroxyl group may act either as general
acids, as general bases or as nucleophiles in intramolecular
reactions. Bender (96) and Jencks (97) intensively treat pH
dependence in intramolecular catalysis.
Gaetjens and Morawetz studied the role of intra-
molecul'ar carboxylate attack on ester group by following
the rate of hydrolysis of substituted phenyl acid succinates
and phenyl acid glutarates (98). The anions of phenyl acid
succinates and glutarates are hydrolysed by a unimolecular
mechanism involving an attack of the neighbouring carboxylate
on the ester function. The reaction is very fast compared
to the acetate ion catalysed hydrolysis of phenyl esters.
They determined rates of reaction of 20 substituted deri-
vatives. The rate was found to be unusually sensitive to
electron withdrawing para substituents. The E_-nitrophenyl
glutcl:l.rates reacted 540 times as fast as the phenyl glutarates.
The observations are interpreted by assuming that the inter
molecular carboxylate attack on phenyl esters leads to tetra-
hedrally bonded reaction intermediate, while the intramole-
cular reaction involves a direct displacement of the phenoxide
by attacking carboxylate. Tom,:)Maugh and I{uice studied the role
of intramolecular bifunctional catalysis of hydrogen glutarates
and hydrogen succinates in water (99). In all cases, however,
only one functional group is found to participate directly
47
in the hydrolytic reaction. The hydrolysis of 8-quinolyl
hydrogen succinates and 6-quinolyl hydrogen glutarates have
bell shaped pH rate profile. Succinic anhydride is the
intermediate in the hydrolysis of succinic ester and
glutaric anhydride is probably the intermediate in the
hydrolysis of glutarate esters as at the optimum pH. These
reactions are much faster than the hydrolysis of the
corresponding quinolyl acetate. It seems likely that the
decrease in the rate that occurs not from the loss of an
intramolecular acid catalyst but from the change in inductive
and resonance effect. A similar conclusion was reached con-
cerning the hydrolysis of 2-carboxyphenyl succtntate.
There is no existing evidence for concerbed intramolecular
general acid nucleophilic catalysis for hydrolysis of the
esters in water. We fully agree with the f;indings of
Bruice and believe that there is no bifunctional catalysis
in the hydrolysis of hydrogen succinate in water.
The entropy and enthalpy of activation are higher
for the 1-phenylethyl hydrogen phthalate and its para sub-
stituted derivatives which exhibits intramolecular acid
catalysis than for the terephthalate ester¢ where there is
no such catalysis (100). The values are given in Table 6.
One generally accepted procedure for ascertaining whether
a neighbouring group participates in a reaction involving
a carbonium intermediate is to alter the structure of the
48
compound in such a way that for its reaction the free
energy of activation is sufficiently reduced to make the
need for participation negligible. This principle first
stated by Winstein and extensively used by him in his
brilliant researches in the field of neighbouring group
participation assumes that the more stable the carbonium
ion centre becomes, the less demand that centre will make
upon neighbouring groups for additional stabilisation
through participation (101). Intramolecular hydrogen
bonding as indicated in the transition state structure
of the phthalates would diminish the need for solvation of
the developing negative charge on oxygen, and thereby make
the entropy of the transition state greater than it would
be if solvated to the extent required for the terephthal�c
ester. The decrease in solvation of the phthalate tran-
sition state is achieved at the expense of an increase in
enthalpy. Higher enthalpy and entropy are characteristic
of intramolecular and intermolecular catalysis (102). In
general the enthalpy of activation of SN1 reactions of
alkyl chloride is decreased while going from water to
aqueous organic solvents. For example, the enthalpy of
activation for t-butyl chloride in 80% aqueous acetone is
22.0 k.cal/mole (103) while in water it is 23.8 k.cal/mole
(27). If the same trend. follow in the case of ester
TABLE 6
ENTROPIES AND ENTHALPIES OF ACTIVATION OF HYDROGEN PHTHALATE
AND TEREPHTHALATE OF 1-PHENYLETHYL ALCOHOL AND 1-l2,-ALKYL
PHENYLETHYL ALCOHOLS
Solvent - 75% v/v acetone water
Ester
1-Phenylethyl hydrogen phthalate
1 -12-To·lylethyl hydrogen phthalate
1-12-Ethylphenylethyl hydrogen phthalate
1.-12-Methoxyphenylethyl hydrogen phthalate
1-Phenylethyl hydrogen ter�phthalate
1-12-Tolylethyl hydrogen terephthalate
1-12-Ethylphenylethyl hydrogenterephthalate
1-12-methoxyphenylethyl hydrogenterephthalate
DH+
k.cal/mole
28.7
28.6
28.8
25.5
28.1
26.9
26.9
25.4
6s* e.u.
-7.5
-2 .. 3
-2.1
-0.2
-16.7
-13 .9
-14.2
-6.8
TABLE 7
ENTHALPIF.s AND-ENTROPIES OF ACTIVATION OF. THE HYDROGEN
SUCCINATES OF 1-PHENYLETHYL ALCOHOL AND ITS PARA
SUBSTITUTED DERIVATIVES
Solvent - water
1-Phenylethyl hydrogen succinate 27.2 -7
1-E-Tolylethyl hydrogen succinate 24.1 -9
1-E�Ethylphenylethyl hydrogen·succinate 23.4 -11
1-E�Methoxyphenylethyl hydrogen20.9 -8succinate
49
hydrolysis then the enthalpy for the succinic ester
hydrolysis should decrease, when the solvent water is
changed to aqueous organic solvent which is very much less
than that of the phthalic ester. This is another evidence
that there is no neighbouring group participation in the
hydrolysis of hydrogen succinate in water.
Next we can consider the mechanism involved in the
reaction. The mechanism which are plausible are t�o namely
BAL1 and BAC2. Long amplj:fying a suggestion of Taft and
coworkers (104) have proposed the use of entropy as a
criterion of the mechanism of hydrolysis of reactions. The
enthalpy and entropy and free energy are set out in table 8.
TABLE 8
ENTHALPIES, ENTROPIES AND FREE ENERGIES OF ACTIVATION FOR THE
HYDROLYSIS OF 1-PHENYLETHYL HYDROGEN SUCCINATES AND ITS
PARA SUBSTITUTED DERIVATIVES IN WATER
.6H+
L\s+
.6F*-1 -1k.cal.mole e.u. k.cal.mole
1 -Phenylethyl hydrogen succinate 27.2 -7 29.3
1-2-Tolylethyl hyerogensuccinate 24.1 -9 26.8
1-E-Ethylphenylethyl23.4 hydrogen succinate -11 26.7
1-E-Methoxyphenylethylhydrogen sugcinate 20.9 - 8 23.3
_4.6
_4.4
_4.2
.. 4.0
-3.6
-3.2
_3,Q.._ ________ ..._ ________ ..._ ________ ..i_ ________ ..i... ________ -'-________ �
2.70 2,7S 2.as
(1/'r) 103
2.90 2.95 3.00
FIG.1. PLOT OF 1/T VS LOC �I FOR THE HYDROLYSIS OF 1-(P--ALKYLPHENYL) ETHYL HYDROGEN SUCCtNATE IN WATER.
0 I. e,. TOLYL ETHYL HYDROGEN SUCCINATE. /A I -(e.ETHYLP�ENYL) ETHYL HYDROGEN SUCCINATE.
· .4.5
, .4.3
.4.1
-3.9
9 -3.7
-1.S
.. 3.1
-�.9.,._�----.._��"'"""'�-----i.....---------------�------�-------------3.12 '!,16 ),20 3.24 3.28
(1/T) 103
3.32 ),36 3.40
FIG,2 PLOT OF 1/r VS LOG �i FO� THE HYO�OLYSIS OF 1-(e- METHOXYPHENYL) �THYLHYDROCEN SUCCINArE IN WATER.
3.44
.. 5.1
.4.7
.. 4.S
_4.3
�·
_4-1
· -3 .9
-3.7
/ -3.S
-3.3
-3.1--����...1-����-'-����--����--�����---����-
2.so 2.55 2.60 2.65
(1/T) I03
2,70 2.75 2.�o
FIC.3 PLOT OF 1/T VS LOG k_1 FOR THE HYDROLYSIS Of 1- PHENYL AND 1-(P..-HALOCENOPHENYL) ETHYL H YDROGEN SUCCINATE IN WATER.
+ 1-( P... BROMOPHENYL) ETHYL HYDROGEN SUCCINATE.C:l 1-( 2-CHLOROPHENYL:) ETHYL HYDROGEN SUCCINAT E. 0 I- PHENYL ETHYL HYDROGEN SUCClNATE.A 1-(P-- FLOURO�_HENYL) ETHYL HYDROGEN SUCCINATE.
50
1-£-Fluorophenylethyl hydrogen succinate 25.8 -9 28.5
1-£-Chlorophenylethyl hydrogen succinate . 22.2 -22 28.8
1-£-Bromophenylethyl hydrogen succinate 17. 7 -34 27.8
Valuable discussions have been giv�n by Long on
the values of the entropies of activation in organic
reactions (105) and by Bender (106) on the enthalpies of
activation in ester hydrolysis.
An important principle due to Long (105,107) on the
values to be expected for the entropy of activation can be
summarised as follows. When in a bimolecular reaction, two
initial state particles join together to make transition
state particle, the translational and rotational entropies
of two particles become reduced to those of one, there is a
small additional entropy of vibration but not nearly enough
to compensate for the loss of entropy. These changes con
stitute a negative contribution to the entropy of activation.
There will be other factors also which will affect the magni-
tude of the entropy of activation. But Long proposed that, on
account of the first factor, namely the molecularity, SN2
reactions should generally possess smaller positive or
greater negative entropies of activation than the most nearly
analogous SN1 reactions. Long amplifying a suggestion of
51
Taft and coworkers (104) have proposed the use of entropy
as a criterion of the mechanism of hydrolysis of reactions.
These reactions are usually classified as unimolecular or
bimolecular. In the former case a water molecule does not
participate in the rate determining step, while a water
molecule is usually considered to be bound in the activated
complex in the latter. It seems reasonable that the loss
of trans·1ational and rotational freedom of a water molecule
associated with the bimolecular process should lead to a
lower entropy of activation relative to the unimolecular
case. This prediction is amply borne out by entropies of
activation for unimolecular and bimolecular ester hydrolysis,
typical values of 63 + being O to 10 e.u. for unimolecular
reactions and -15 to -30 e.u. for bimolecular reactions.
Long and Stafford studied the entropies of activation
and mechanism for the acid catalysed hydrolysis of ethylene,
propylene, isobutylene and trimethylene oxides ( 107). The
close similarity in the entropy of activation values strongly
suggests that all of the oxides hydrolyse by the same A-1
mechanism. In a discussion of acid catalysed hydration of
olefins Ta� and coworkers (108) suggested that reaction
by the A-1 mechanism should be characterised by a relatively
more positive entropy of activation than reaction by the
A-2 mechanism since the latter involves a relative increase
52
of constraint on the reaction system in the transition
state due to the orientation and reaction of a specific
water molecule from the solvent. The work of Stimons (109)
on the rates of acid catalysed hydrolysis of t-butyl
esters is a good example. The hydrolysis of t-butyl benzo
ates and t-butyl formates in acidified 60% aqueous acetone
gives .6S* value of -9.5 e.u. and -23.7 e.u. suggesting that
the former hydrolysis proceeds by an A-1 mechanism and the
latter by an A-2 mechanism.
Two examples of hydrolysis of esters following the
BAL 1 mechanism has been provided by Kohnstam (110). This
relates to the hydrolysis of the :12.-nitrobenzoates of
diphenyl methanol and its para-methoxy derivatives in 70%
aqu$us acetone. The entropy values are respectively -5.9
e.u. and -8 e.u. If we accept the argument that resonance
stabilisation of a carbonium ion increases the entropy of
activation of a reaction involving a carbonium ion as
intermediate - the reason is that resonance results in
delocalisation of the positive charge and reduces the
need for solvation - then we should expect a greater entropy
of activation for the esters of the diphenyl methanols than
for the 1-phenylethyl esters in aqueous acetone. For the
hydrolysis of 1-phenylethyl hydrogen succinates in water
the entropy of activation should be greater than for the
53
hydrolysis of the ester in aqueous organic solvent and is
approximately equal to the diphenyl methanol system.
Two recent examples of hydrolysis of esters follow
ing BAL1 mechanism has been provided by Hawkins (111).
This relates to the hydrolysis of hydrogen phthalates of
diphenyl methanol and its para methoxy derivative in 20%
v/v dioxan. The entropy value for diphenyl methyl hydrogen
phthalate is -5 e.u. Radhakrishnan Nair studied the
hydrolysis of acid phthalic and acid terephthalic esters
of 1-phenylethyl alcohol and its para-alkyl and para methoxy
derivatives in aqueous acetone proceeding by the BAL1
mechanism. The entropy and enthalpy values are set out
in table 9 for comparison. He found that optically active
monophthalate of 1-phenylethyl and 1 -12.-t-butylphenylethyl
alcohols yielded racemic alcohols on hydrolysis (100).
Before applying these principles to the present
data, it might be well to remember Long's warning that his
criterion must be used with circumspection because within
the same mechanism a great scatter of values of entropies
of activation is found. But we believe that Long's
postulate might be safely and profitably applied in the
present case. First we note that the trend in values within
series is the same for the succinic, phthalic and tere-
phthalic esters which is also shown by other series follow-
ing Baker-Nathan order. The entropy values for pH, p-CH3
,
54
p-C2H5 and P-CH3o are essentially similar to acid phthalicester suggesting that the hydrolysis proceeds by BAL1
mech�ism. For p-Cl and p-Br derivatives the entropy
values are highly negative -22 e.u. and -34 e.u.
respectively which is typical for a bimolecular reaction
i.e. BAC2 mechanism.
The Okomoto-Brown equation has been applied to the
present data. It is known that in system showing Baker-
Nathan order the Hammett eqµation in its original form is
not applicable (112). The plot is linear one for all
except for Cl and Br derivatives, the correlation coeffi-
ient being 0.997. The suggestion is made that the first
five compounds Q-H, Q-CH3, Q-C2H5, Q-CH3o, Q-F show the
behaviour which is characteristic of the BAL1 mechanism
while the other two follow what is generally observed as
the BAc2 mechanism.
The rate ratios k/k8 for the hydrolysis of succinic
ester in water, phthalic ester in aqueous acetone (100),
benzhydryl chloride in 70% aqueous acetone (113) and benzyl
chloride in 50% aqueous acetone (114) are set out in
table 11.
55
TABLE 9
ENTI-IALPIES AND ENTROPIES OF ACTIVATION FOR THE HYDROLYSIS
OF PHTI-IALATE AND TEREPHTI-IALATES OF 1-PHENYLETHYL ALCOHOL,
1-(p-ALKYLPHENYL) ETHYL ALCOHOIS, 1-(p-METHOXYPHENYL)
ETHYL ALCOHOL
Solvent 75% v/v acetone - water
Ester
1 -Phenylethyl hydrogen phtha;iate
1-2-Tolylethyl hydrogen phthalate
1-(2-ethylphenyl)ethyl hydrogen phthalate
1-(2-iso-propylphenyl)ethyl hydrogen phthalate
1-(2-tert-butylphenyl)ethyl hydrogen phthalate
1-(2-methoxyphenyl)ethyl hydrogen phthalate
1-Phenylethyl hydrogent erephthalat e
1-(2-tolylethyl) hydrogen t erephthalat e
1-(2-ethylphenyl)ethyl hydrogen terephthalate
1-(2-tert-butylphenyl)ethyl hydrogen terephthalate
1-(2-methoxyphenyl)ethyl hydrogen terephthalate
* _1-6.S e .u.
k.cal mole
28o7 -7.5
28.6 -2.3
28.8 -2.1
29�1 -1 .4
29.3 -1 .5
25.5 -0.2
28.1 -16. 7
26.9 -13.9
26.9 -13.9
27 .1 -14.4
·25.4 - 6.8
:r: .x• ...;.,.,
« a: �·
0 . 9.
4
3
2
0
_, -0·9
FIG.4 PLOT OF
-0,7 -0-5 -0,3 -0-1 0,1 0-3
� VS LOG �P., R/�ij FOR THE HYDROLYSIS OF HYDROGEN S!:)CC lNATES lN WATER .
TABLE 10
OKOMOTO-BFOWN � VALUES AND LOG k/k FOR THE HYDROLYSIS. 0
OF 1-PHENYLETHYL HYDROGEN SUCCINATF.1 AND ITS PARA SUB-
STITUENTS IN WATER
Substituent log k/k0
OCH3
3.6034 -0.778
CH31 .4594 -0.311
Et 1 .3993 -0.295
H 0 0
F 0.3265 -0.073I
Cl -0.2510 0.11§
Br -0.3314 0.150
56
57
TABLE 11
RATE RATIOS .KJ'kH FOR THE HYDROLYSIS OF ACID SUCCINIC ESTER,H
ACID PHTHALIC ESTER, BENZJDRYL CHLORIDE AND BENZYL CHLORIDE
Acid succinic ester 29 25 2.1 0.56 o.47
Acid phthalic ester 16.4 14.2
Benzhydryl chloride 29.6 22.2 1.9 0.32 0.25
Benzylchloride 9 1.7 0.59 o.47
Reactions 2 and 3 are unimolecular substitution
reaction (100,113) while reaction 4 though mechanistically
marginal is predominantly bimolecular (115). Kohnstam (116)
Robertson (117) and their coworkers have provided adequate
confirmation. The entropy values for benzyl chloride
clearly points to the conclusion that the reaction is
largely SN2. The same conclusion has been reached by
Kohnstam from consideration of the heat capacity data (116,
118). It can be clearly seen that the rate ratios differ
markedly for reactions belonging to the two categories.
From the results we can infer £-H, £-CH3, £-C2H5, £-F
substituents hydrolyse by the SN1 mechanism while £-Cl
and Q-Br by SN2 mechanism.