In the presence of sodium benzylate two molecules of benzaldehyde
react with the alcoholate to form an addition product.
When the reaction mixture is overheated an important side reaction
may occur, as follows:
/ OCH2C6H5
C6H5C -- OCH2C6H5 --> C6H5CO2Na + C6H5CH2OCH2C6H5
\ ONa
Dibenzyl ether no doubt forms the chief impurity in benzyl benzoate.
Since the boiling-point of the former lies near that of the ester,
it is not removed during the process of purification by distillation.
The causes of variations in yield by the use of the older methods
can now be explained. When benzaldehyde is added TO THE ALCOHOLATE,
and especially when the latter is still warm, local overheating results;
in fact, the temperature may rise far above 100'0 with the result
that benzyl ether is formed. Simultaneously, the sodium benzylate
is converted into sodium benzoate, which is of no value for inducing
the desired reaction, and consequently very little benzyl benzoate
is obtained. The same side reactions explain the failure of this
experiment when the benzyl alcohol used in preparing the catalyst
(sodium benzylate) is contaminated with benzaldehyde.
The benzyl alcohol used in this preparation must be free
from impurities, especially aldehyde. One cc. dissolved in 50 cc.
of water and treated with a freshly prepared clear solution of
phenylhydrazine acetate should give no appreciable precipitate.
If it is not pure, it must first be treated with alkali
as described below.
The benzaldehyde should be titrated in order to determine its acidity.
If it is found to contain sufficient benzoic acid to react
with a considerable proportion of the sodium alcoholate, a poor
yield of ester will be obtained. Less than 1 per cent of benzoic
acid will not interfere seriously with the yields obtained,
but the presence of larger quantities of acid will be found to be
detrimental and must be removed by washing the benzaldehyde with
a sodium carbonate solution and redistilling with the precautions
necessary to prevent too free an access of air to the distillate.
The order of mixing the reagents and the temperature of the ingredients
at the time of mixing are the most important factors in the experiment.
The temperature at which the reaction mixture is maintained
after mixing, provided that it is held below 100'0, is less important
from the standpoint of purity.
The reaction mixture is not treated with acetic acid, as usually
recommended, for the reason that such a procedure yields a final
product contaminated with benzoic acid, unless an alkaline wash
is applied subsequently.
The recovered benzyl alcohol can be used for the preparation
of a second lot of benzyl benzoate only after it has been boiled
with strong sodium hydroxide to remove all traces of benzaldehyde.