Organic Reactions Volume 4 - Sciencemadness Dot Org

280 ORGANIC REACTIONS
from acetic acid, in which the benzoins are sparingly soluble, is advisable
when catalytic reduction is to follow. 40 ' 76
The catalyst for the condensation is the cyanide ion. Only alkali
salts of hydrogen cyanide have proved successful. A large number of
possible catalysts for the benzoin reaction have been explored, but in the
absence of cyanide ion little or no benzoin has been obtained.
Both the aldehyde and the alkali cyanide may contain substances that
inhibit the reaction. Aldehydes sometimes contain the corresponding
acid, hydroquinone, or quinone; and alkali cyanides may contain such
inhibitors as alkali halides. Other inhibitors are iodine, hydrogen sulfide,
sulfur, and carbon disulfide. It has been found that a quantitative relationship
sometimes exists between a specified amount of inhibitor and a
definite quantity of potassium cyanide; 0.3 g. of hydroquinone renders
inactive 0.7 g. of potassium cyanide. Inhibitors can be removed from
the aldehyde by washing with aqueous sodium carbonate, by formation
of the bisulfite compound, or by treatment with solid potassium cyanide
overnight at room temperature in an atmosphere of nitrogen. 28,34,75 The
effect of the impurities is decreased by an increase of the proportion of
water and by the addition of an excess of cyanide.
Experimental Procedures
The detailed description of the preparation of benzoin is given in
Organic Syntheses. 75
4-Methoxybenzoin, 77 To 50 g. of potassium cyanide dissolved in 350
ml. of water in a 3-1. flask is added 272 g. (2 moles) of 4-methoxybenzaldehyde,
212 g. (2 moles) of benzaldehyde, and 700 ml. of 95% ethanol.
The mixture forms a solution at the boiling temperature and is refluxed
for one and one-half hours. Steam is then passed through the solution
until all the ethanol and nearly all the unchanged aldehydes are removed.
The water formed by condensation in the reaction flask is decanted from
the product, which crystallizes on chilling. The product is then pressed
as free as possible from oily material on a suction funnel and washed
with cold ethanol. In this way about 250 g. (52% yield) of crude product
is obtained. After recrystallization from ethanol the 4-methoxybenzoin
melts at 106°.
2'-Chloro-3,4-dimethoxybenzoin. 76 A solution of 8.3 g. (0.05 mole) of
3,4-dimethoxybenzaldehyde (veratraldehyde), 7.0 g. (0.05 mole) of
2-chlorobenzaldehyde, and 4.0 g. of potassium cyanide in 75 ml. of 50%
ethanol is refluxed for two and one-half hours. The product crystallizes
76 Buck and Ide, ./. Am. Chem. Soc, 52, 4107 (1930).
77 Kinney, J. Am. Chem. Soc, 51, 1592 (1929).