Table Of ContentThe rennsylvanis. State Co 1 lege
The Graduate School
Department of ohemistry
The molecular State of Acetic Acid Vapor
A Dissertion
toy
Howard Lester Ritter
oubmitted in partial fulfillment
of the requirements
for the degree of
Doctor of Philosophy
October, 194s
Approved:
October oP 194b - / -J>/. . \ « of Chemistry
uctober __%_ 194b - H ead of the Depto
AC ivi'J 0'v» LKJDGMEi'iT
The v/riter wishes to acknowledge the continual
encouragement and counsel offered by Doctor
<josei)h n. uimons during the progress of this
\York, and to express here his thanks therefore
COWTEHTb
History of the Problem Page 1
xhe Experiments! Procedure
Theory of the method 6
The Apparatus 8
Preparation of Material 21
Density of liquid acetic acid 26
Description of procedure 31
Primary data (Table 2 ) 32
Calculation of molecular weight 34
nesults and Discussion
Monomer-Dimer Equilibrium 38
The Isotherm at 120° 47
jvionomer-Trimer Equilibrium 50
Monomer Tetrsmer Equilibriurii 53
o onclusion
ihe Molecular state of Acetic Acid 60
Summary 62
Bibliography 63
Appendix
-L Calibration xable for .Burettes Bi
ll Bulb Characteristics 65
III Deviation curves for '-thermocouples 6b
IV Calibration Table for Dilatopycnometer 67
V The Liquid Density of Acetic Acid 68
INTRODUCTION
It lias long been known that molecules containing an
hydroxyl group exhibit a mutual attraction in considerable
excess of what can be attributed to van der Y<aals' forces.
In the liquid phase, the effect is manifested, for example,
in the high boiling point, relative to the molecular weight,
of water and the carboxylic acids. There is also a devi
ation from normal behavior of the heat of vaporization, so
that with the high boiling point, these substances possess
abnormal values of irouton's constant. In the vapor phase,
the most striking result is the discrepancy between the
mass of the grain-molecular volume and the formular weight.
Thus at the normal boiling ooint, the effective molecular
v/eight of formic acid is 73 (l); of acetic acid is 1©1 (2,3,4)
and of propionic acid is 11C (5). The same phenomenon is
displayed to a lesser extent by molecules in which a hydrogen
atom is attached to a nitrogen atom (as in amines); to a
sulfur atom (as in thiols;; and to a fluorine atom (as in
hydrogen fluoride); as well as in such hydroxyl compounds as
water, alcohols, and carboxylic acids.
The classical explanation has been that these substances
form "associated" molecules, with no further explanation as
to the kind of association involved. The juxtaposition of a
hydrogen atom and another atom possessing unshared electron
pairs, however, led Latimer and 11016111311 to hypothesize that
the hydrogen atom may act as a bond between two oxygen
atoms (6). Considerable evidence has been adduced to con
firm this possibility. The shift of the infra-red frequency
associated with the hydroxyl bond when hydrogen bonding takes
dace is characteristic, and a comparison of the intensities
of the normal and shifted lines in the spectrum of acetic
acid (7) have given excellent agreement with the data of
MacDougall (B) on the concentrations of monomer and dimer
in acetic acid vapor. Electron diffraction measurements on
formic acid vapor (9) have shown the close proximity of the
oxygen atoms in the associated molecular doubles, a strong
indication of hydrogen bonding, although the positions of
the hydrogen atoms are, of course, not revealed.
Prom this evidence and some imagination, this structure
of the dimer of a carboxylic acid ha3 been proposed (22);
H O x
R — < C - R 1
0~ H O
Pauling and Brockway give the distance between the hydrogen-
o
bonded oxygen atoms as 2.67 A (9), which is less than the
si.am of the atomic radii of two oxygen atoms. From this,
Hammet (10 ) makes the startling conclusion that the hydrogen
occupies no space at all1. More likely is thec onclusion
that the O-H-O groun is not linear as shown in the above
configiiration, and that this bond, like many others, does
not follow the rules of semi-bond additivity. In this con
figuration the two hydroxyl hydrogens are symmetrically
employed by each acid nucleus, and the carbonyl groups have
disanpeared. 'the six-rnembered ring thus formed (omitting
the hydrogens) is cited as additional favorable evidence for
this structure, although any theory on the basis of a mini
mum hayer strain must dispose of the hydrogen atoms in a
manner more satisfactory than ignoring them.
It has been assumed that the structure represented above
is saturated with respect to hydrogen bonding, and Hainmet has
mentioned the failure of such structures to polymerise fur
ther. Iviacijougall (ll), on the other hand, has cited evidence
that nropionic acid vapor contains at least one oolymer of
higher order than the second; and, although he does not
mention it, his earlier paper on acetic acid (8 ) carries the
same evidence.
The fact that the experimental evidence at the present
tine nermits such conflicting statements is ample apology for
the projection of the present research. The simple fact is
that there are neither sufficiently extensive nor suffici-
entl;v precise data on the vapor-phase molecular weight of
acetic acid to determine reliably whether or not carboxylic
acids form higher polymers than the second order; and what
the identity of such oolymers may be if they do exist. Of
the data that are available, those of iViachougall (8 ) cover
the pressure range from three to thirty millimeters of
of mercury at temperatures from 25° to 40° G. with satis
factory precision. The investigation of Ramsay and loung
(?) covers a very wide range of temperature and pressure
(from 20° to 230° G. and up to 24,000 mm of mercury;, but
the data are too sparse and not precise enough to allow any
reliable conclusions regarding the molecular state of the
acid.
This research, then, consists of an attempt to obtain
suitable data on the molecular weight of acetic acid vapor
for a definitive determination of the molecular species
present and their concentrations; and to determine the
equilibrium constants of the appropriate reactions of the
tyre :
Ac OH -- ► I (Ac OH)
ir n
with the corresponding heats of reaction. The most fruitful
attack would seem to be to determine the effective molecular
weight of the vapor as a function of temperature and pressure
and this, in turn, is essentially a question of vapor density
Vve have extended the range of temperatures and pressures in
which the vapor density of acetic acid is known to the normal
boiling point and atmospheric pressure.
Of the many methods for determining vapor density, most
are unsatisfactory on the basis of tediousness (when the
relatively large number of determinations to be made is
considered), lack of precision, wastefulness of material, or
dif f lenity of man.:!putlation . Y,e have devised a highly flex-
ih]t' nrtho'i c f ••.ioterinininp: the vf;ror ions' tv at nr*;/ temper-
."inrc a bnv'; the f.ond'ms.' tier, r-oi nt and at. r ny pressure do-
tvioer. tno verr’r pressure at 1,ne nr lot <• • nu trie crit-
i ■■ I rr-1 nt , of a cond ensahle vapor. It rouuires less than
five °a of r]t t-ria 1 ; j n c: si 1;/ ■ nd r»- pidl./ manipulated, and
;* a ca ratio ah' hi vh pre*'* i si on. An er.ensition oi' this nethcn
f r.1 ] ,-v-rs in, t,ho next sootion .
THE APPARATUS
Consider s. bulb attached cc a long
and thin vertical tube (burette) such as
in the accompanying figure. Let the en
tire system be evacuated, a charge of
liquid be introduced, and the system
sealed off. Now lot the bulb and burette
be individually thermostated, the burette
at a lower temperature than the bulb.
Any liquid in the bulb will distill into
the burette, since the vapor pressure is lower there, and
at equilibrium, all the liquid phase will be there.
Keolecting the connecting tubes, the original charge will be
distributed in three localized phases:
(.1) The liquid phase in the burette
(b) The vapor phase in the bulb
(3) A condensed, adsorbed phase on the inner surface o
the bulb.
Lotting the volume of the bulb be V and the vapor density d;
the inner surface of the bulb be S and the adsorbed phase
surface density, C" 5 the volume 01 tne liquid in the burette
be v and the liquid density, p, we may write:
Vd + S + vp = m' (1)
where rn! is the mass of the original charge.
If now the value of the liquid density is known, the
quantity vp may be computed. Letting m = m'-vp be tlie mass
of material in the combined vapor and adsorbed phase,
eanetion (l) is simplified:
Vd + £>«-= m (£)
Nov/ let two systems of this type be operated independ
ently and in parallel, so that the bulb temperatures are
eoual and burette temperatures are equal. Designating the
two systems tor subscripts 1 and 2 , we may write:
(3)
V d + Sr,<r = m,.
/C
and the tv/o simultaneous equations resulting, may be solved
providing the determinant of the coefficients,
b. — V 3 -V S ^ 0. Since the Vs, Ss and ms are known, all
1 2 2 1
data necessary for the determination of d (and incidentally
of ) are known. The solution will give the value of the
vapor density at the temperature of the bulb and under a
pressure equal to the vapor pressure at the temperature of
the burette.
(It may be noted that by operating three such systems
In parallel, It would be possible to determine the vapor
density without prior knowledge of the liquid density.
Because of the simple means available for determining■liquid
density, however, and because of the simplification of the
calculations by solving two instead of three simultaneous
