A Guidebook to Mechanism in Organic Chemistry

The Strengths of Acids and Bases

C«H 5 C0 2 H 4-20

o-0 2 NC 6 H 4 C0 2 H 2-17

m-0 2 NC 6 H 4 C0 2 H 3-45

P-0 2 NC,H 4 C0 2 H 3-43

:5-(0 2 N) 2 C,H 3 C0 2 H 2-83

The particularly marked effect with^-N0 2 is probably due to the

very short distance over which the powerful inductive effect is opera­

ting.

The presence of groups such as OH, OMe or halogen having an

electron-withdrawing inductive effect but an electron-donating meso-

meric effect when in the o- and />-positions irjay, however, cause the

/7-substituted acids to be weaker than the m- and, on occasion,

weaker even than the unsubstituted acid itself, e.g. />-hydroxybenzoi»

acid:

pKaot XC 6 H 4 C© 2 H

n© H ci

Br OMe OH

o- 4-20 2-94 2-85 4 09 2-98

m- 4-20 3-83 3-81 4-09 4 08

P- 4-20 3-99 400 4-47 4-58

It will be noticed that this .compensating effect becomes more
pronounced in going CI «Br-*OH, i.e. in increasing order of
readiness with which the atom attached to the nucleus will part with
its electron pairs. The behaviour of o-substituted acids is,-as seen
above, often anomalous. Their strength is sometimes considerably
greater than expected due to direct interaction of the adjacent groups,
e.g. intramolecular hydrogen bonding stabilises the anion (IV) from