A Guidebook to Mechanism in Organic Chemistry

Addition to Carbon-Oxygen Double Bonds

« ^)J-H etc.

Thus benzaldehyde is less reactive than aliphatic aldehydes. This
effect is heightened by the presence, in the benzene nucleus, of
electron-donating substituents (e.g. OH) and lessened by electron-
withdrawing substituents (e.g. NOg). This is naturally also observed
with aliphatic aldehydes, e.g.:

O O

II II

0,N—«- CH,—«- C—H > Cl—«-CH,—«-C—H >

O

II

CH,-»-C—H > CH,-»-CH,->-C—H

?

Part of the loss of reactivity in aromatic aldehydes and ketones is also

due to the relatively large nucleus inhibiting attack on )>C=0

sterically. Similarly, the bulkier the alkyl groups in aliphatic carbonyl

compounds the less the reactivity, due to the crowding that results

on adding nucleopOMfs to the carbonyl carbon atom, e.g.:

Me—C—Me > Me—C—CMe, > Me,C—C—CMe,

Comparison of the relative reactivity of aldehydes and ketones is

complicated by the fact that, in aqueous solution, they are hydrated to

varying degrees (see below), so that it is difficult to discover the pro­

portion that is actually available in the reactive carbonyl form in any

particular case. A group of characteristic addition reactions will now

be studied in more detail.

ADDITION REACTIONS

(i) Hydration

Many carbonyl compounds form Hydrates in solution:

OH

R—C—H + H,0 r± R—C—H

OH