8/17/2019 Puntos de Ebullición Alcanos - On the Boiling Points of the Alkyl Halides

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Edited by

R LPH B~RDWHISTELL

Universityof West Florida

Pensscoia. FL 32504

On the Boiling Points of the lkyl Halides

John

orreia

Saint Mary's College, Moraga,

CA

94575

Most introductory organic chemistry texthooks devote

some time to the discussion of the ~hv si ca l roperties of

organic compounds and

n

the way in which the-se properties

aredetermined hv the natureof the functional groups within

-

the molecules.A; excellent opportunity is thus provided for

the presentation of a variety of concepts, particularly those

-

that are of value in explaining attract ive forces between

molecules.

Amone several different kinds of nronerties and com-

& .

pounds, a number of the texts specifically examine the hoil-

ing points of the alkyl halides. All of them note the general

increase in hoiling points of the halides relative to the al-

kanes from which they are derived, hut t he reasons given for

this t rend are varied and, in some cases, not convincing. For

example, some of the texts (I , 2) attr ibut e the increase sim-

ply to the increase in molecular weight that takes place when

halogen is suhstituted for carbon or hydrogen. Others (3-6)

-

emphasize dipole-dipole interactions as major contributors

to the higher hoiling points (particularly for fluorides and

chlorides). and there are those

4 - 7 )

tha t assien consider-

ahle impo;tance to the surface area of the halogens and/or

their nolarizahilities. Which of these factors is reallv of ma-

jor influence here? Is there perhaps a complex interplay

amone them tha t chanees from halogen to halogen?

~e f; lr e ddressing ti e matter of the halides, let us look

closelvat the wavin which the properties of molecules deter-

mine boiling Many em$r;cal relationships exist he-

tween molecular weights of homologous compounds and

their twiling points, h;t theessential question iswhether the

mass of a molecule is important in this regard as opposed to

other properties that are related to molecular weight. The

simple kinetic theory of evaporation predicts that, other

things being equal, the vapor pressure ot a liquid should he

proportional to the inverhe square of the mass of its mole-

cules 81; owever, the experimental data do not bear this

out.

Comparison of nonpolar compounds that differ in molecu-

lar weight, yet have a shield of outer atoms with identical

polarizahilities, shows th at their hoiling points vary less than

expected (9).For example, (CzHdaC (MW = 128) hoils a t

146 OC while (CsH6)aSi (MW

=

144) boils a t 153 OC;

Mo(CO)S (MW = 264) has a hoiling point of 156 "C and

W(C0)6 (MW = 352) hoils at 175 C;and so on.

When care is taken to compare compounds tha t not only

have outer atoms of identical polarizahilities hut that also

have identical surface areas, the surprising result is that

their hoiling points are not merely similar hut often the

lower molecular weight molecule hoils at a higher tempera-

ture

10).

Consider, for example, NhFs (MW

=

188) hoiling

at 235 "C and TaFn (MW = 276) boiling a t 229 "C. This

phenomenon is also known t o exist in certain isotopic mole-

cules, and i t has been attrihuted to an increased translation-

al entropy for the heavier molecule in the vapor state and,

hence, a greater probability of it vaporizing (11).

The fundamental effect of molecular weight on volatility

in complex organic molecules is undoubtedly not a simple

62

Journal of Chemical Education

one. However, it is safe to say that its effect is small and that

it will he outweighed by such things as surface area and

polarizahility of molecules as well as other molecular proper-

ties that influence intermolecular attractions.

The focus of attention then should be on the cohesive

forces that exist in liquids. For polar, non-hydropen-bonded

molecules, these loosely termed van der ~ a a l sorces are

conventionally divided into three parts: (1) dipole-dipole

interactions (orientation attractions). (2) dinole-induced di-

, , , .

pole interactions (induction at trac tions), and (3) induced

dipole-induced dipole interact ions (dispersion attractions)

1 7 2 ) ~

\-- .

The approximate expressions for the potential energies of

the interactions between identical molecules are V(orienta-

tion) = -2p4/3kTR6, V(induction) = -2ap2/R6, and V(dis-

persion)

=

-3a21/4R6, where p is the dipole moment,

k

is the

Boltzmann constant, T is the absolute temperature, R is the

distance between the interacting portions of the molecules,a

is the polarizahility of the molecule, and is its ionization

potential. I t should be noted tha t only the orientation energy

has a dependency on temperature, and i t is such tha t higher

temoeratures will decrease the average attractive enerev of

twohipoles. Thus, the relative contri ktion of dipole-dipole

interactions to the total enerw will he less at hieher than a t

.

lower temperatures.

Furthermore, in those common organic molecules whose

polarity is due to the presence of a small polar functional

group amid several methyl and methylene groups, the dis-

.

tance between interarting dipoles in neighboring molecules

should he relatively largeand dependent upon the sizeof the

molecule. Since the van der wails energies show an inverse

dependency on the 6th power of R one would expect the

orientation enerw then to fall off ra~idlvs the molecule

.

increases in size.

The induction enerw should also decrease with size in

these kinds of molec& since the dipole of one molecule

must effectivelv "contact" a neiehhorine molecule so tha t a

dipole can he iiduced. ow ever ;^ the interaction can occur

anywhere on the neighboring molecule (as opposed to di-

pole-dipole interact ions), induction energies should not de-

crease as drastically as the orientation energies in larger

molecules.

By study of heats of vaporization, Meyer (13a, b ) has

developed a method for estimating the percent contrihution

of each of the three types of interactions t o the total cohesive

enerev for certain oreanic liauids (snecificallv. ketones. ni-

trileGand chlorides)."~riefl~;e finds t hat thk'contribution

of the dispersion enerev is dominant in all cases, the induc-

tion energy makes asmall hut significant and persistent

contrihution as the molecules aet laraer, and the orientation

energy sta rts out small then r>pidl;he'comes insignificant.

For example, the contribution of dispersion, induction, and

orientation to the total energy of attraction is calculated to

he 73%, 21% and 6%, respectively, for methyl chloride; 8490,

1270, and 470, respectively, for 1-chloropropane; 91%, 9%,and

070,

respectively, for 1-chlorohexane; and 9570, 5'3, and

096

8/17/2019 Puntos de Ebullición Alcanos - On the Boiling Points of the Alkyl Halides

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respectively, for l-chlorododecane

13b ) .

t should be men-

tioned tha ithese calrulations are done at different tempera-

tures lor the different chlorides, paralleling roughly the de-

creased volatilitv with molecular size.

With all uf the above ideas in mind, let us return to the

consideration of the boiline points of the halides. The fi rst

question to ask is, to whaishould the boiling points be

comoared? Because molecular weight effects are small and

dispersion effects paramount, it seems appropriate to make

comoarisons between halides and alkanes of similar size and

shape rather than between those of similar molecular weight

as is often done. Th e surface areas of the halide groups can

be estimated from their van der Waals radii, and, when this

is done, one finds that the chloro group is 19%smaller than

the methyl group, the hromo group is only 5% smaller, and

the iodo group 16% larger. On thi s basis, i t would seem

helpful to compare the boiline points of these halides (R-X)

to those of R - ~ H ~ .

On the other hand. the small fluoro group has less than

half of the surface area of a methyl

bu t i t is only 25%

larger than hydrogen. In addition, fluorine and hydrogen

have similar group polarizabilities (14) so it is more informa-

tive here to compare the boiling points of R-F to those of

R-H.

The hoiling points of the normal alkanes and the l-haloal-

kanesare given in Table 1 The figure illustrates their appro-

priate differences (i.e., R X ompared to R-CH3 for

X

=

CI,

Br. and I: and R-F comoared to R-H) as the number of

carbons in th e halide increases.

The chlorides can be looked at first since Mever's data ar e

directly applicable here. T o a good approximation, the cohe-

sive energy of a liquid is equal to its energy of vaporization,

which is directly proportional to the hoiling point in absolute

temperature. Assuming that the percent contribution of

each interaction calcul&d by ~ e ~ e rs approximately valid

at the normal boilinn point, then, in a sense, the percent of

the hoiling point du e t o that interaction can be estimated.

Thus, for l-chlorohexane boiling at 134 C (407

K

nd

having 91 dis)~ersion nergy and 9% induction energy at 56

O C

the percent contribut ionulinduction to the boilingpoint

w d d he 3:   C.

It

is found that the difference between the

~~ ~~

boiling points of l-chlorohexane and n-heptane is 36 OC. I t

seems that. in this case. the boiline point increase of the

halide is di e essentially'to th e induciion interactions tha t

are present in the halide and not in the alkane.

Table 1. Normal Bolllng Polnts

of

l-Haloalkanes CJi2,,X)*

Boiling Poi m. C

n

x

=

H

F Ci Br I

Taken horn Oreisbaa.

R. R.

mysIcalP1opertiasofcbmica lcompwnm: Amarican

chemical

society

washington.

DC

1959: VOI 2:

and 1967

v o ~

.

~ o i ~ i n goints are

rounded to th nearest

degree.

The argument holds only insofar as the dispersion forces

in the alkane are the same as those in the chloride. This

appears to be reasonable. The surface area of l-chlorohex-

ane issliehtlvsmaller than that oin- hept ane,but the moler-

ular pol&z&ility, as calculated from the refractive indices

via the Lorentz equation (15), is slightly larger for the chlo-

ride (1.377

X

mL) than for n-heptane (1.370 X 10W3

mL). Furthermore, the ionization potentials of R-CH3 and

R-CI should he quite similar (e.g., they ar e 11.28 eV and

11.65 eV for CH3CI and CH3CH3, respectively (16)) so we

expect tha t the dispersion forces of the two compounds will

indeed not be very different.

Use of Meyer's data for other alkyl chlorides with similar

calculations shows that their higher hoiling points are also

due essentiallv to induction interactions except for the

smaller compounds where the orientation interartion makrs

a small contrihution. Even for methyl chloridr; hou,erer, it

appears t o be responsible for only abbut 20% of th e increase.

The other halides can be examined only in a qualitative

sense. Even though R-Br and R-I do not have appreciably

different dipole moments from R-C1(17), their polarizabili-

ties are much greater (14). The induction energies in the

bromides and iodides will be greater than in the chlorides

but so also will he the dispersion energies. And since the

dispersion energy depends on the square of the polarizabili-

tv. its percent contribution to the total cohesive enerav is

. .

greater a s wellas itsabsolutevalue. l hus, the large increases

in boiling oointsuf It-Hrand

R-I

nredue toacombinatiunuf

an induciion contrihution (a very significant one by analogy

to the chlorides) and larger dispersion interactions. In this

sense, the high polarizabilities of Br and I can be said to he

responsible for the high boiling points of the bromides and

iodides.

5 10

15

2 0

CARBONS N HALIDES

Bailing

point

elevationsof 1-haloalkanes i.e.. R-X compared

to

R-CHS for

X =

CI, Br. and

I;

and R-F compared to

R-H)asa

function

of

me umber of carbons

in

the

halides.

Volume 65 Number

1

January

1988

63

8/17/2019 Puntos de Ebullición Alcanos - On the Boiling Points of the Alkyl Halides

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The alkyl fluorides show the most dramatic changes in

boiling point relative to their parent alkanes; they s tar t out

boiling much higher tha n R-H and eventually boil a t a lower

temperature. I t is reasonsable t o suppose th at the high boil-

ing point of CH3F relative to CH4 is probably due to several

factors-the larger size of F, the usual induction contribu-

tion, and a significant orientation energy. As the carbon

number increases, we expect the orientation contribution to

decrease as in R-Cl and the size differential to become less

important. Th e low polarizability of fluorine (somewhat less

than th at of hydrogen) then becomes the key determinant of

the fluorides' behavior. This results in small induction ener-

gies and, eventually, smaller dispersion energies for R-F

than for R-H so th at their boiling points become more alike.

It is important to note also that the low boiling point

increases of R-F provide evidence for th e lack of dipole-

dipole interactions in the medium and large size alkyl ha-

lides. If, for example, these interactions were primarily re-

sponsible for the elevated boiling points of R-C1, then one

would certainly expect them to exert th is ef fect for the fluo-

rides and lead t o comparable boiling point elevations.

Another way of demonstrating the absence of major orien-

tation effects is to examine the boiling point differences of

isomeric halides and their parent alkanes. Table

2

shows

these differences for the isomeric chloropentanes and their

constancy is striking. The or ientation energy in, for example,

(CH3)2CC1CH2CH3would be expected to be significantly

less than that in CH3(CH2)3CH2CI because the dipoles in

neighboring molecules of the tertiary compound must he

farther apar t as a result of steric interferences. Since even a

10

ncrease in the average distance between dipoles would

nearly halve the orientation energy, large variations in the

boiling point elevations would be anticipated if dipole-di-

pole interactions played an important role in these com-

pounds. The da ta do show small steric effects, and these are

consistent with either induction or small orientation contri -

butions. A survey of other isomeric series of chlorides and

bromides (where their boiling points are available) shows

tha t their behavior is similar to the chloropentanes.

In conclusion, the higher boiling points of alkyl halides are

best explained in terms of induction and, for the bromides

and iodides, increased dispersion also. The often neglected

induction interactions play a major role here and undouht-

edly in other polar organic molecules. They may he responsi-

ble for some of the effects uncritically at tributed to dipole-

Table 2. Normal Bolling Points of the Isomeric Chloropentanes

and Hexanesa

Boiling Points, 'C

Boiling

Polnt

Structure

X =

CH3

CI

Elevation

a ee

onnote s

n Table 1.

aTaken

from We . R C. CRC m d b & ofChemiswand P h p b 85lh ed ;CRC:

BDFBRaton. FL.

1984.

dipole interactions and they should be given wider exposure

in organic chemistry texts.

Acknowledgmeni

The author wishes to thank Kenneth Brown of Saint

Mary's College for reading and commenting on the manu-

script.

Literature Cited

1. Fessenden,R J.: Fpssenden.J

S

Organic

Chemistry, 2nd ed.;Grant

Boston,

1982;p

163.

2. Morrison,

R.

T ;Boyd,

R N Organic Chemistry,

4th

4.;

llyn

and

Bacon: Boston,

1983:

p 198.

3. Carey,

F.

AOrgonie

Chemistry.

MeGrsw-Hill: New York, 1987;p97.

4.

Loudon.G.

M. Organic

Chemislry;Addison-Wesley:Reading,

MA,

1984;

pp

297-298.

5. Vollhardt, K.

P.

C. Organic Chemistry:

W.

H.

Floeman:

Ssn

Randnm, 1987;

p 190.

6 Wade, L.

G.

Orgonic Chsmlslry; Prentiee-Hsll:Englewmd Cliffs. NJ, 1987;pp 177-

178.

7. strcitrieser,

A ;

Hel

millan: New York

~thmek, . H. Introduecion

to 0rg.nie

ChmLltry, 3rd sd.;Mac-

,1985:p 112.

: Aduoncod Tmotlse

on

Physieol Chemistry, longman's, Green:. Partington, J. R. An

New York. 1951:

Val.

2. p 291

9 Rich,

R.

J

Chom.

Edue. 1962,39,45&

10. Bradley,

D. C.Natura

1054.174.323.

11.

Bradley,

D. CNature 1954.173,26 261.

12. See,

far sxample.

Y d e r , C

H. J Cham. Edur. 1977, 54,

402-408, or

any standsrd

physieslehemietry

text,

suehssRramberg.

J.

P.

PhysicolChrmisliy. 2nd

ed ;

Allyn

and Bamn: Baston, 1981'nn RSbRS7

13. (el

Meye..

E. F.; Wlyner,

-- ~~

I E.

J

Phys. Cham. 1966,70,3162 3168:bl Moyer E. F.;

>er T

J Phva

Chem.

1971.75.1

e n r ~ ~ ~

5 4 2 4 8

14. Ferguson . NOrganic

Molecular

~ 1 m ~ t ~ r ~ ; G r a n f :aston, 1975;p 55.

15. See. for rramole.

Brombere.

J. P. Phwical Chemialrv, 2nd ed.: Allvn and Bacon:

Eioston. 198k:

i p

85m51:

16. W a t a n a k , K. J ChamPhya. 1957.26.542517.

17.

Ingold, C. K.

Structure

ond Mechanism

in

O~ganic

Chemialry,

2nd

ed.; Cornell

University:

Ithsea, NY,

1969;p

123.

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64

Journal of Chemical Education