1937 - Keffler - Homology and Isomerism in Long-chain Compounds. I. A Thermochemical Study of the n -Alkyl Esters Derived from the Monoe

HOLLIOLOGY A S D ISO3IERISII I N LONG-CHAIN
COIIPOUNDS. I
,4 THERMOCHEMICAL
STUDYOF
THE ?Z-ALXTL
ESTERS
DERIVEDFROM
111OS OETHYLENI c 11ON OCARBOXTLI c - ~ C I Ds IN C
THE
L. J. P. KEFFLER
The Thermochemical Laboratory, Inorganic Department, C'nicersity o j Liaerpool,
Liverpool, England
Receioed September 16, 1936
INTRODUCTIOX
-1decade ago a preliminary study was attempted of the relative stability
of oleic and elaidic acids and of their methyl esters from a comparison of
the eneygy contents of the best samples of these substances which were
available a t the time (1).
K i t h regard t o the acids it was found that the heat of combustion of
oleic acid was higher than that of elaidic acid by a much larger amount
than could be accounted for merely by the difference between the states
of aggregation of the two acids before combustion. This observation,
when taken in conjunction xvith Stohmann's rule, led t o the conclusion
that, in agreement with observations derived from other fields, oleic
acid should be regarded as the cis-form and elaidic acid as the trans-form
of the same chemical structure.
With regard to the methyl esters, however, the sole pair to be investigated a t the time, the differences between the heats of combustion were
so small (1 part in 10,000 only) as t o be within the limits of the experimental error for the calorimetric measurements (1 in 10,000 also) and well
within these limits for the extent of the purification process for these
substances. No definite conclusion could therefore be drawn as to the
relative stability of these two esters.
It is with a view t o the removal of this ambiguity t h a t the work t o be
described in the following pages has been attempted. Instead, however,
of limiting the comparison t o the methyl esters only, the ethyl, n-propyl,
and n-butyl esters have been examined as well.
COKTROL O F THE P r R I T Y
zill the esters considered in this paper arc. liquids at ordinary tempcrature; the elaidates remain so down to the region of the nielting point of
715
716
L. J. P. KEFFLER
ice and the oleates down to some 20°C. lower. The melting point and
the setting point are therefore not very useful for testing the purity of
these substances, the more so that the values of these constants are not
known, at lea4 with any degree of exactness, from the literature. The
same remark applies, though to a lesser extent, to oleic acid itself. -4s
for elaidic acid, the International Critical Tables give values as far apart
as 44" and 51°C. Froin our own experience, it is the former of theie
which is in the immediate neighborhood of the true value for the setting
point, but as it changes only slowly with further purification, its usefulness
is extremely limited a i a criterion of purity.
When, however, the elaidic acid is obtained from samples of oleic acid
which have been previously freed from polyethylenic impurities (as was
&own to be possible in a previous paper on the purification of oleic acid
( 2 ) ) , the determination of the iodine value gives a true indication of the
extent of the purification, without any ambiguity. The purification of
elaidic acid was actually carried on until the iodine value was raised to
the theoretical value for that substance and went no further. As an
additional control of the purity, the heat of conibuition of a large number
of different fractions m-as determined; only small variations were found
for that constant, except for the early tail fractions, which were therefore
rejected for the preparation of the esters. The purity of the elaidates
and oleates was also controlled by comparison of the observed with the
theoretical iodine values, under the same conditions of non-ambiguity.
DISCUSSIOS
In order to facilitate the discussion of the results detailed in table 1,
the molecular heats of combustion of the various esters have been calculated and collected in table 2 . The value of the energy increment (represented by ACH, in the table) when passing from one member of the series
t o the next higher homolog has also been calculated. The corresponding
values observed for the series of the alcohols which were used in building
up the two series of esters have been added for the sake of comparison.
(For the difference between the methyl and ethyl alcohols, the latest
value, namely that of Rossiiii (4) has been taken, while for the other
alcohols, the values of Richard.; (3) were selected.) Finally the heat of
transformation for each member of the czs-series to the corresponding
member of the trans-series hai: been reported.
The first striking fact i, the complete parallelism between the series of
energy increments for the elaidates and for the parent alcohols; it indicate.: that the replacement of the hydrogen atom of the alcohol by the
heavy radical CH3(CH2);:(CH2);CO- ha< not deeply altered the structural relationships inside thcl liquid molecule. On the other hand, the
nearly pcrfcict coii*tmCy of thc. increinent betn-een the second and fifth
HOMOLOGY I N LONG-CHAIK
71 7
COMPOUNDS
members of the elaidate series and of the alcohol series provides a strong
argument in favor of the additive character of the molecular heat of combustion of organic liquids whose molecules possess a large number of
carbon atoms, as is the case for the elaidates, or whose terminal groups
are not endowed with strong residual affinities, as is the case for the
alcohols.
I n the case of the oleates, however, such regularities are not apparent,
though they possess the same number of carbon atoms as the corresponding elaidates. This difference in behavior might be explained by one
of the folloning alternatives:
( 1 ) . While the successive elaidates mere all prepared from the same
pure sample of elaidic acid (iodine number = 89.9), the various oleates
were derived from different samples of oleic acid (iodine number about
0.3 to 0.4 unit lower than the theoretical), none of which was quite as
pure as the elaidic acid, and amongst which there niay therefore have
been slight discrepancies between the energy contents. I n order t o investigate that possibility, a new series of oleates was built up, starting now
each time from the same sample of oleic acid (iodine number = 88.9).
Though this new sample nas, like the earlier ones, quite free from polyethenoid acids, it contained a slightly higher proportion of saturated
impurities, so that the values obtained for the heats of combustion of the
derived esters cannot be strictly compared with those of table 2; but the
value for the energy increment per CH, is naturally more accurate in this
than in the earlier case. The values of the successive increments actually
observed with these new samples of esters were as follows:
Methyl
oleate
ACHz in kg-cal
as against
and
. .
.
.
L
_
-
.
152.5
153 0
153 2
Ethyl
oleate
--
n-Propyl
oleate
n-Butyl
oleate
P
155 5
155 8
155 8
155 1 for oleates
156 0 for alcohols
155 9 for elaidates
The agreement is therefore much better than before.
( 2 ) . Another reason that may be put forward in order to explain the
differences between the energy increments of the various esters depends
on their mode of preparation. Indeed, while the members of the first
series of oleates were all obtained by fractional distillation, on the contrary not a single one of the second series of oleates or of the elaidates was
submitted to such treatment. As a consequence, the older oleates niay
have been obtained in a slightly different thermodynamic state than the
later ones, this difference being made apparent through the irregularities
observed in the values for the energy increment.
( 3 ) . d third possibility that should not be overlooked is that the
state of association of the earlier samples of ethyl oleate (which had been
distilled and burnt directly afterwards) may not have been the same as
718
L. J. P. K E F F L E R
N
N
c,
8
I,
0
r
00
8
W
0
719
HOMOLOGY I S LONG-CHAIS C O M P O C S D S
'
l
w
i
e
0
10
z
1
0
0
wB
9
f0
Q,
w
k
s
720
L. J. P. KEFFLER
for the corresponding ethyl elaidate, which had not been distilled at all!
One should indeed remember in this connection that there has often
been observed a striking anomaly between the methyl and the ethyl
members of homologous series, and that such anomalies may possibly be
of quite different degree for the elaidates and for the oleates, as is emphasized further here by the change of sign of the heat of transformation
TABLE 2
Isothermal molecular heat of combustion (in vacuo) under constant volume and at the
constant temperature of 20°C.
MOLECGLAR
ESTER
Mgz:y
'
~
MOLECULAR
ACHa
COMBUETION
Methyl oleate.. . . . . . . . . . . . . . . .
H E A T OF
TRANBFORMATION
+15
151 0
Ethyl oleate . . . . . . . . . . . . . . . . . . .
-0 7
n-Propyl oleate.. . . . . . . . . . . . . .
+15
I
156 1
I
'
n-Butyl oleate. . . . . . . . . . . . . . . .
n-Amyl oleate.. . . . . . . . . . . . . . . .
338 33
352,36
Methyl elaidate.. . . . . . . . . . . . . .
296.28
2835 . 7
Ethyl elaidate.. . . . . . . . . . . . . . . .
310.30
2988.9
n-Propyl elaidate. . . . . . . . . . . . . .
324.31
3144.7
n-Butyl elaidate. . . . . . . . . . . . . .
338.33
3300.6
n-Amyl elaidate.. . . . . . . . . . . . . .
352 36
3302.3
+17
153.2
155.8
155.9
I
155.9
3456 5
Methyl alcohol.. . . . . . . . . . . . . . . .
153.0
Ethyl alcohol.. . . . . . . . . . . . . .
155.8
n-Propyl alcohol.. . . . . . . . . .
n-Butyl alcohol.. . . . . . . . . . . . . . .
~
,
n-Amyl alcohol. . . . . . . . . . . . . . . . .
I
I
156.0
156.1
~
between the ethyl members of the two ester series. For the other pairs
of esters, the heat of transformation has not only the same sign, but it
seems t o have very nearly the same value, namely 1.6 f 0.1; this shows
a very striking constancy, considering the great difficulty of obtaining
pure samples and the fact that the samples of oleic acid used for the building of the first series of oleates were all different.
A411the rcgulariticq exhibited in table 2 4iould br considered a, -upplying a ytroiig, though indirect, proof of the amoiint of t r u + t nort liinc+
that may IJP accor~ledto the v a l u ~ stabulated.