A Guidebook to Mechanism in Organic Chemistry

The Strengths of Acids and Bases

MeCH 2 CH 2 C0 2 H Me CH 2 CHC1 C0 2 H

4-82 2-84

MeCHClCH 2 COaH CH 2 C1 CH 2 CHa C0 2 H

4 06 4-52

Other electron-withdrawing groups, e.g. R 3 N-, ^CN, -N0 2 ,

^CO, -C0 2 R increase the strength of fatty acids, as also do hydroxyl

and methoxyl groups; the unshared electrons on the oxygen atoms

of the last two groups are not able to exert a mesomeric effect in

the opposite direction to their inductive effect owing to the inter­

vening saturated carbon atom. ThesS effects are seen*in the pJTa

values:

0„N-«-CHa-*-COaH EtOsC-^-CH 2 -*-C0 2 H

1-68 • 3-35

Me 3 N-*-CHa-<-C0 3 H MeCO-^-CHa-«-C0 2 K

1-83 3-58

NC-*-CHa--<-COaH Me6-*-CHa-^-C0 2 H

2-47 # 3-53

HO-*-CHa-e-COaH

3-83

(vi) Phenols

Analogous effects can be observed with substituted phenols, the
presence of electron-withdrawing groups in the nucleus increasing
their acidity. In the case of a nitro-group, the inductive effect will fall
off with distance as we go o- ->•/«- ->/>-nitrophenol but there will be
an electron-withdrawing mesomeric effect when the nitro-group is in
the o- or />-, but not in the /n-position, and this, too, will promote
ionisation by stabilisation of the resultant anion. We might therefore
expect o- and />-nitropheno1s-to be more acidic than the m-compound
which is, in fact, found to be the case:

C,HS OH

o-OjNCjH.OH

m-0 2 NC 6 H 4 OH

P-OANCEH 4 OH

2:4-(0 2 N) 2 C,H 3 OH

2:4:6-(OAN) 3 • C„H 2 • OH