A Guidebook to Mechanism in Organic Chemistry

Structure, Reactivity and Mechanism

CHoO-H CH,Oe

^ H®+!

(XXII)

In other words, the phenoxide ion (XXI) is stabilised with respect to

the phenol molecule whereas the benzyl oxide ion (XXII) is not so

stabilised with respect to the benzyl alcohol molecule. A somewhat

similar situation occurs in the dissociation of the carboxylic acids:

O

R—C
\

^ H® +

O—H

R—C y

o®

(XXIII)

R—C

\

O

o"

i.e. R—C

V

The more the carboxylate ion (XXIII) is stabilised with respect to*he
undissociated acid molecule, the more readily the latter will lose a
proton and the stronger the acid wiUthen be. Here again delocalisa­
tion can take place in the undissociated molecule

JO O^0

R—C «-» R—<j

\>H OH

and contributes to its stability but it requires separation of charge and
so will be less effective than that in the carboxylate anion which does
not. It will be observed that the stabilisation effected in the carboxy­
late ion will be particularly marked as the two canonical structures
that can be written are of equal energy.
The most common examples of mesomeric effects are encountered
in substituted aromatic systems: the IT electrons of suitable substi-
tuents interact with the delocalised n orbitals of the nucleus and thus
profoundly influence its reactivity, i.e. its aromaticity. The delocalised
IT orbitals of the benzene nucleus are particularly effective in trans­
mitting the electrical influence of a substituent from one part of the
molecule to another: