A Guidebook to Mechanism in Organic Chemistry

Addition to Carbon-Oxygen Double Bonds

(tl)

Attack by the nucleophile, ROH, apparently need* to occur from a

direction more or less at right angles to the plane in which the proton­

ated carboxyl group lies and such line of approach is now blocked,

from either side, by a bulky methyl group: no esterification thus takes

O e ^HO HO

R3CO-C-R'=^it RjC^Oi-C-R'^ R,C® + 0=C—R'

1i

H

*°

— H® ®

R,COH±— R,C—OH

3 H

reflecting the tendency of the tertiary alkyl group to form a relatively
stable carbonium ion. Similar alkyl-oxygen cleavage also tends to
occur with esters of secondary alcohols that yield tljemosjt stable
carbonium ions, e.g. Ph 2 CHOH. Attempts at ester-exchange with
esters of such alcohols lead not surprisingly to acid+ether rather
than to the expected new ester:
O HO

l> II© m^1

RjCO-C—R 4 =i R 3 C + 0=C—R

^ROH

R.COR ^=5:, —H® © R3COR'

3 3 H

Acid catalysed Verification or hydrolysis is found to be highly

susceptible to sterjc^hindrance, thus 2,4,6-tri me thy 1-benzoic acid is

insusceptible to estenfication under the normal conditions (cf. p. 24).

This is due to the fact that such relatively bulky ortho substituents force

the initially protonated carboxyl group (LI) out of the plane of the

benzene nucleus:

HO a OH