A Guidebook to Mechanism in Organic Chemistry

Addition to Carbon-Oxygen Double Bonds

carbonyl carbon atom, and p-MeO-C 6 H 4 -CHO is found to react

at only about one-seventh the rate of benzaldehyde.

(d) Perkin reaction: Closely related to the above is the Perkin

reaction for the synthesis of a^-unsaturated acids from aromatic

aldehydes and aliphatic acid anhydrides in the presence of an alkali

metal salt of the corresponding acid, e.g. for cinnamic acid,

PhCH=CHCO,H:

Ph—CY CHjCO

\

H

Me CO

(XXXIV)

OH

(i) Addition |

> Ph—C—CH^

(ii) Protonation | V

H O

/

Me CO

(XXXV)

-h,o

Ph—CH=CH C0 2 H Ph—CH=CH CO

h.o \

+

MeCOjH

O

Me CO

(XXXVI)

In acetic anhydride solution, acetate ion is a sufficiently strong base
to remove a proton from the activated a-position of the anhydride to
yield the carbanion (XXXIV), which adds to the carbonyl group of
the aldehyde, the product, after protonation, being the aldol-like
species (XXXV). Under the conditions of the reaction (ca. 140°),
(XXXV) undergoes dehydration in the presence of acetic anhydride
and the resultant mixed anhydride (XXXVI), on being poured into
water at the end of the reaction, is hydrolysed to cinnamic and acetic
acids. The reaction is a general one, depending only on the presence
of a CHS group in the a-position of the anhydride. That the reaction
follows the above course is confirmed by the fact that aromatic alde­
hydes will, in the presence of Suitable basic catalysts, react with
anhydrides but not with the corresponding acid aniqns, and aldol-like
intermediates such as (XXXV) have in some cases been isolated.

(e) Claisen ester condensation: This too is effectively an aldol type
reaction, e.g. with ethyl acetate: