The acetophenone should be as pure as possible (m. p.
20'0). Commercial acetophenone contains variable quantities of impurities
which reduce the yield. By distilling commercial acetophenone with
the help of a good still-head (preferably under diminished pressure)
and using only the fraction which boils at 201-202'0 (76-77'0/10 mm.)
greater quantities of benzalacetophenone can be obtained than by using
the entire sample.
Commercial benzaldehyde can be used in place of the purer product,
but the amount used must be increased to make up for the impurities
which are present.
If the temperature is too low, or the stirring too slow, the product
separates as an oil, which later solidifies in large lumps.
If the temperature is allowed to rise above 30'0, secondary
reactions diminish both the yield and the purity of the product.
The most favorable temperature is 25'0.
In recrystallizing benzalacetophenone, the alcohol should be saturated
at 50'0. If the solution is saturated above this temperature,
the benzalacetophenone tends to separate as an oil. The solution
should be allowed to cool gradually, and should finally be chilled
in a freezing mixture. 3. Other Methods of Preparation
The methods for producing benzalacetophenone are: the action of acids
on a mixture of benzaldehyde and acetophenone or on a solution
of these substances in glacial acetic acid;[1] the condensation
of benzaldehyde and acetophenone with a 30 per cent solution of sodium
methylate at low temperatures;[2] the action of sodium hydroxide
on an alcoholic solution of benzaldehyde and acetophenone.[3]
The methods based on the use of acids as condensing agents were
not considered, because Claisen, who devised them, abandoned them
after he found that alkaline condensing agents gave better results.
The preliminary experiments showed that condensation with sodium
methylate takes a long time and gives a product which it is difficult
to handle in large quantities. The method devised by Kostanecki
and Rossbach[3] has therefore been developed.
[1] Ber. 14, 2463 (1881).
[2] Ber. 20, 657 (1887).
[3] Ber. 29, 1492 (1896).