Or\' PHESYLACETALDEHPDE AXD I T S POLYSIERISXTIOX BY JAMES ROBERT POUND
I t is known that phenylacetaldehyde polymerises easily, especially on contact with air and light. Enklaar' kept a specimen in the dark for six months and found it had then polymerised to the extent of nearly one-half; finally he obtained a crystalline solid, melting at 33'. A sample of phenylacetaldehyde was obtained by the writer from L. Givaudan and Co., France. The manufacturers state that storage in a stoppered bottle in a dark place reduces polymerisat ion and that the alcoholic solution keeps almost indefinitely. Our sample of phenylacetaldehyde was a colourless, sweet-smelling and fairly viscous liquid; it was kept corked in a dark cupboard and gradually became more viscous, and after 1 7 months small crystals appeared in the body of the liquid, though some crystals had formed above the liquid some months previously. The refractive index of the liquid increased with age, as well as its density and viscosity; these changes have been studied at room temperatures (7'-20') and at 30'. Some experiments were carried out with phenylacetaldehyde on watchglasses, which were weighed from time to time. Though phenylacetaldehyde boils at about zoo' and has a vapour pressure of j mm at 74.5' (Givaudan), it is sufficiently volatile to lose weight continuously when kept at room temperatures on watch-glasses in air or in closed vessels over sulphuric acid or calcium chloride or water. In all these experiments the decrease in weight falls off with age. When kept over sulphuric acid or calcium chloride, the phenylacetaldehyde vaporised is picked up by these reagents. I n air and over sulphuric acid and calcium chloride, the phenylacetaldehyde becomes in time slightly yellow, covered with a skin or viscous surface-layer, develops crystals, and finally becomes a hard glassy mass: these changes proceed more rapidly over the desiccating agents; but the rate of loss of weight is approximateIy the same in all cases. When kept in a closed vessel over water, the phenylacetaldehyde does not become glassy hard or full of crystals, but it remains somewhat fluid and becomes opalescent with age : the loss of weight however is the same as before. This loss of weight depends on the room temperature and is due to simple evaporation. One sample, kept in the open air at an average temperature of zoo, lost in I j o days 14 per cent of its original weight: minute crystals were present in the liquid at the start of the experiment, were numerous after 30 days, and constituted the bulk of the mass, which was then slightly straw-coloured, after 90 days. Another sample of phenylacetaldehyde was placed in a flask and attached to a Lunge nitrometer containing mercury : after several days no absorption of oxygen was noticed. The flask was opened, a tube of calcium carbide was enclosed, and the whole J. Chem. SOC.Abs. A, 1926, 614.
1175
PHENYLACETALDEHYDE AND I T S POLYMERISATION
system joined up again: after several weeks the volume of the gas in the system was unaltered. Thus no absorption of oxygen and no evolution of water vapour occurred. The acidities of samples of the phenylacetaldehyde were determined by dissolving known weights in alcohol and titrating with carbonate-free alkali, using phenolphthalein as indicator: the end-points were sharp, and the acid was calculated as phenylacetic acid. The stock phenylacetaldehyde showed 1.17~ acid. h sample dissolved in alcohol had air aspirated through it for 2 0 hours at i o and then showed 1.87, acid. Another sample after exposure to air on a watch-glass for 56 days showed 2.37, acid; and that sample exposed to air for I j o days showed finally 2 . 0 7 ~acid: but the residues from the acid'. A sample, other watch-glass experiments (see above) showed lower '7, I11 (see later), after keeping at 30' for 30 days showed 1.3 j c o acid. Thus phenylacetaldehyde does not oxidise appreciably on exposure to the air. The refractive index, n, was determined at 20' by a Hilger-Abbe refractometer, which gave results to the fourth decimal place for nZso. For the stock sample of phenylacetaldehyde, S, kept at room temperature, about I jo, the refractive index increased with age at a diminishing rate; but after crystals had formed, the refractive index of the liquid varied little: with this and other samples n actually diminished a little after crystals had separated, -indicating supersaturation : also just before crystals appeared, the 'line' in the refractometer was blurred, as it is with suspensions,-indicating incipient (liquid?) crystals. The refractive index is the most convenient property to indicate the age of phenylacetaldehyde; and crystals are about to form, polymerisation having progressed thus far, when nZ;' = 1.570 approximately. Some results for the stock sample, S, were:.ige of sample (days) ng
ox I
jj64
I
145 5631
I
343 5700
I
364 jj06
I
413' j709
I
563% 5703
I
692 5730
Kotes:"On first opening the stock bottle, 4 months aftw arrival in Ballarat. 'After small crystals had appeared. "From only a thin layer of liquid above a paste of crystals. Between dates y and z, n lay between these values. Part of the stock sample was distilled under reduced pressure, the B.P. rising somewhat throughout: the distillate, sample I, was kept at room temperature, say about IS', for 85 days and the refractive index was determined from time to time, see Table and Graph. The refractive index increased throughout, and this sample never reached crystal-formation. The refractive index, like the density and the viscosity, changes somewhat abnormally with the fresh sample for the first few hours. Another part of the stock sample was distilled under 79 mm pressure: a first fraction was discarded, and then was collected the main sample, 111,
I 1;6
JAXES ROBERT POUND
boiling from 125' to 141'. This sample was divided into three lots which were kept at 30' for eighteen days and more. One lot was kept in a specificgravity bottle, in which its density was determined at intervals, the bottle being filled up as required from the third lot. The second lot was kept in an Ostwald viscometer, closed with calcium chloride tubes, and the time of flow was found at intervals; from the contemporaneous values of the density the viscosities were calculated. After eighteen days the density of the liquid from the viscometer was 1.085 j, while that in the specific gravity bottle was 1.0912 (both at 30'). The increases in density of the two lots were therefore
FIG.I Graph showing variation of properties of phenylacetaldehyde with lime
not quite equal; but as the viscosity increases in this time to over twelve times its initial value, the errors introduced by taking the densities of the viscometer liquid as equal to those of the specific-gravity bottle liquid are negligible. For these reasons however the densities are only quoted to four decimal places and the viscosities to the third significant figure. From the third lot of the liquid the specific gravity bottle was filled up from time to time and samples were withdrawn for the refractive index determinations. In these samples kept a t 30' crystals appeared in thirty days. That part of samples I and I11 which remained after the experiments therewith, was redistilled and gave a sample IT' of similar boiling point to 111. Sample IT' was kept a t 30°, in two lots, for eighteen days, the density and refractive index being determined at intervals as before. The results obtained with the samples I, I11 and IV are given in the table and are plotted in the graph.
1177
PHEXTLACETALDEHTDE AXD ITS POLTMERISATIOS
TABLE I Table showing the Change of Properties of Phenylacetaldehyde with Time Age of Sample
Sample I (kept a t room temps)
Sample IV (kept at 30’)
Sample I11 (kept at 30’)
nzOQ
D30‘
n200
40
D
D
hour j hours I day 3 days
1.5370
I ,5272
-
I ,5282
-
-
I , 5291
I . 5402
4 6 9
”
1,5411
”
1,5422
I2
”
IS I7
”
18
”
1,5444 I ,5466 I ,5481 I . 5488 I ,5492
1.5310 I . j319 I ,5329 1.5351 I . 5390
,0274 1.0357 I ,0398 I ,0481 1.0597 1.0698 I . 0783
20
”
I .os02
I
”
”
I
,5440
I
,5482
-
1.5j1o I ,5651 I . 5663
I
,0242
I
,0912 1.0969 I
D30°
n20Q D
40
-
1,5295 I ,5296
I
,5275
1,5339
I
,0347
1.5431 1.5519 I ,5578 I ,5618 I ,5649 1.5654 I . 5670 1,5713 I ,5716
1.0419 1.0488 I ,0586 I ,0716 I ,0792
-
-
-
-
1.5540 I . I 160‘ ,5543 I ,5678’ I .5722 ” 1.5550 * I . 5689’ ” 1.5575 1,5736 * I . 5692‘ I ,5593 1.5743= I . 5762’ I . 5658 *I ,5723’ )’ 85 ” I . 5667 I .5767= x Crvstals had formed here. * Ti& part of sample I11 was kept a t room temperatures, say noo, from the 3nnd day.
29 30 32 40 47 79
” ”
I
J J
The density increases linearly with the time and the specific-volume decreases linearly with the time; the density of sample IV increases less rapidly than that of sample 111; and the mean density at time zero, D$’, is 1.026, and the density seems to rise less rapidly for the first few hours than it does afterwards. The fluidity-time curve is linear for the first few days, indicating f o = 50.15 or q o = 0.01994, and then the curve becomes convex to the timeaxis. The viscosity-time curve is convex to the time-axis and becomes steeper with time. The (logarithm of the viscosity)-time curve is nearly linear; it is slightly convex to the time-axis, and when the time is large the 9 approaches a constant value. I n general the refractive index-time ratio log time curve is linear. However the refractive index, n, tends to rise abnormally rapidly at the start, that is just after the distillation of the sample. And the various n/time curves a t last become concave to the time axis or the rate of increase of n slackens with the age of the specimen and becomes much less after crystals have formed. With sample I11 crystals appeared before the thirty-second day, when the n was 1.5678. The average rate of increase of
1178
JAMES ROBERT POUND
n is O . O O I ~ at 30' and 0.00043 at 15'. I t may also be concluded that the specific refractive power, (n- I ) Id, does not vary much with the age of the sample. The above results seem to indicate that the rate of formation of the polymerised phenylacetaldehyde is constant at the one temperature, and finally the crystalline polymer separates from the liquid. For the specific volumes of simple liquid mixtures are additive; and the refractive index is also an additive property. And many observers have concluded that the logarithm of the viscosity is an additive property,-see Arrhenius (1887) and Kendall and Monroe (1917). The rate of formation of the polymer is approximately three to four times faster at 30' than at IS', which is about the normal result for a chemical change. But by distillation the unpolymerised liquid is again obtained. The rate of formation of the polymer doubtless is influenced too by the impurities in the liquid, such as traces of moisture and of acid and of other substances which were present to some extent in our samples, as well as by exposure to the light. In conclusion there seems little reason to doubt that the rate of formation of the polymer is constant under constant conditions. The crystals of the polymerised phenylacetaldehyde were much less soluble in alcohol than the unchanged liquid. Thus the stock sample of phenylacetaldehyde, containing much polymer, was treated with alcohol and the residual crystals were washed with alcohol on a glass filter until they were free of the viscous mother-liquid, and then the crystals were dried in a desiccator over sulphuric acid. Two separate lots of the polymer, a white opaque, granular powder, were thus prepared: they proved to have identical properties. This polymer melted sharply at 104': no water vapour was evolved on melting, and the fused mass cooled to a glassy hard solid. This polymer was soluble in hot alcohol or in excess of cold alcohol, insoluble in cold or boiling water, and was more or less soluble in petrol (gasoline; B.P. = 82"-98"), carbon tetrachloride, chlorobenzene, benzene, bromoform, ethylene chloride, ethylene bromide, acetophenone and benzophenone. From the carbon tetrachloride and the benzene (and possibly from ot.her solvents) the solid polymer was obtained again as the solvent evaporated. Combustion analyses of this polymer gave for lot ( I ) , 7cH = 6.67 and ycC = 79.75, and for lot (2), ycH = 6.71 and 7cC = 80.0; while the calculated values from the formula (CsH80), are 7cH = 6.72 and 7cC = 80.0. The molecular weight of the polymer was found from cryoscopic measurements. In solution in bromoform the depression of the freezing-point for a solution of I gram of polymer in IOO grams of solvent was 0.1895', and the molecular weight = 144j0.1895 = 760; whence n is 6. (The molecular weight is 1zo.n) But comDounds like (CsH80, CHBr3)2,M.W. = j46, of (C~HSO), are possible. In carefully purified benzene the depression of freezing point for a solution of one gram of polymer in IOO grams of solvent was 0.142'; whence the molecular weight was 51i0.142 = 359; and thus n is 3. (Raoult's cryoscopic constant for benzene is quoted from 49 to 5 2 , and we suppose that similar or larger errors are probable for the 'Raoult's constant' given for the
PHENYLACETALDEHYDE AND ITS POLYMERISATION
1179
other liquids used.) In ethylene dibromide (K = 118) the molecular weight of the polymer was 383; in benzophenone (M.P. = 4 7 O and K = 98) the molecular weight was 400 approximately; in acetophenone (K = 56.5) the molecular weight was 349; in cyclohexanol [K = 383) the molecular weight was 330; and in paraldehyde (K = 70.5) the molecular weight was 403. Thus in the last six solvents the polymer dissolved as (CsHsO)$ and in bromoform as (CsHs0)e. The crystal unit of the solid polymer is therefore
(CSH8O)b.
Summnrv Phenylacetaldehyde polymerises with time. Any sample becomes with age more viscous, denser, and of higher 2. refractive index, until finally there separate crystals of the polymer. The density and the refractive index increase, in general, linearly with the time, but the viscosity increases much more rapidly. 3. The refractive index is a very convenient property by which to determine the age of a specimen of phenylacetaldehyde. 4. Phenylacetaldehyde does not oxidise appreciably in air a t ordinary temperatures. 5 . I n most solvents the polymer dissolves as (C8HsO)&but in bromoform as (CsHs0)e. The author wishes to thank the Council for Scientific and Industrial Research, Australia, for a grant towards the expenses of the investigation. I.
The School of Mines, BaUard, Auatralia.
