A Guidebook to Mechanism in Organic Chemistry

Structure and Reactivity

similar situation obtains for any quantity of acid will convert it to the
®
unreactive species, R-NHa. We should thus expect to find that the
rate of addition shows a maximum at a moderately acid pH, falling
off sharply on each side. This is, indeed, observed in practice, the
curve below representing the pH dependence of the addition of many
different reagents to carbonyl compounds:

Apart from actual protonation, the positive character of the car­

bonyl carbon atom will also be enhanced, albeit to a smaller extent,

on formation of a hydrogen-bonded complex by an acid with the

carbonyl oxygen atom:

:—O. HA

STRUCTURE AND REACTIVITY

As high reactivity will depend on the carbon atom of the carbonyl

group being positive, the introduction of groups having an electron-

donating effect-towards this atom will reduce its reactivity; thus the

following sequence is observed:

V

o o

H—C—H > R-*--C—H > R--C-f-R ^>

O O O

ii o- II r>. II s~\

The electron-withdrawing inductive effect of oxygen and nitrogen,

in esters and amides, respectively, is more than outweighed by the

tendency of thtir unshared electron pairs to interact with the n orbital

of the carbonyl group (mesomeric effect). Reactivity is also reduced

by attaching, to the carbonyl carbon atom, an aromatic nucleus, for

its delocalised w orbitals also act as an electron source: