A Guidebook to Mechanism in Organic Chemistry

Ortho/Para Ratios

nitrating mixture is due to the small, residual concentration of free

aniline, a very weak base (cf. p. 52), that is still in equilibrium with

the anilinium cation in the acid medium. The free base, having an

activated nucleus, undergoes o/p-substitution very, very much faster

than the deactivated cation suffers attack at the m-position. The

difference in rate is so marked that the presence of less than one part

per million of free base will still lead to more than 50 per cent ojp-

substitution, but the proportion of m-nitroaniline obtained does

increase as the acid concentration of the medium increases, as would

be expected*

As soon as more than one saturated atom is interposed between a

positive charge and the nucleus, however, its inductive effect falls off

very sharply (cf. strengths of acids, p. 43) and so does the percentage

of the m-isomer produced, as seen in the nitration of:

Compound Percentage m-

PhNMe, 100

PhCHjNMe, -88

Ph • CH, • CH, • NMe, 19

Ph • CH, • CH, • CH, • NMe, 5

(vi) o/p-ratios
It might, at first sight, be expected that the relative proportions of
o- and p-isomers obtained during substitution of a nucleus contain­
ing an o/p-directive substituent would be 67 per cent o- and 33 per
cent /»-,as there are two o-positions to be substituted for every one
Apart from the fact that a little w-product is often obtained (the ex­
tent to which a position is substituted is merely a matter of relative
rates of attack, after all), the above ratio is virtually never realised and
more often than not more p- than o-product is obtained. This may
be due to the substituent already present hindering attack at the re­
positions adjacent to it by its very bulk, an interference to which the