14.4 Determination of relative activity
A group is classified as activating if the ring it is attached to is more reactive than benzene, and it is classified as deactivating if
the ring it is attached to is less reactive than benzene. The reactivities of benzene and a substituted benzene are compared in one of the following
ways.
The time required for reactions to occur under identical reaction conditions can be measured. Thus, as we just saw, toluene is found to react
with fuming sulfuric acid in about one-tenth to one-twentieth the time required by benzene. Toluene is more reactive than benzene, and -CH3
is therefore an activating group.
The severity of conditions required for comparable reaction to occur within the same period of time can be observed. For example, benzene is
nitrated in less than an hour at 60 degrees Celsius by a mixture of concentrated sulfuric acid and concentrated nitric acid; comparable nitration of
nitrobenzene requires treatment at 90 degrees Celsius with fuming nitric acid and concentrated sulfuric acid. Nitrobenzene is evidently less reactive
than benzene, and the nitro group, -NO2, is a deactivating group.
For an exact, quantitative comparison under identical reaction conditions, competitive reactions can be carried out, in which the compounds to
be compared are allowed to compete for a limited amount of a reagent. For example, if equimolar amounts of benzene and toluene are treated with a
small amount of nitric acid (in a solvent like nitromethane or acetic acid, which will dissolve both organic and inorganic reactants), about 25 times
as much nitrotoluene as nitrobenzene is obtained, showing that toluene is about 25 times as reactive as benzene. On the other hand, a mixture of
benzene and chlorobenzene yields a product in which nitrobenzene exceeds the nitrochlorobenzenes by 30:1, showing that chlorobenzene is only
one-thirtieth as reactive as benzene. The chloro group is therefore classed as deactivating, the methyl group as activating. The activation or
deactivation caused by some groups is extremely powerful: aniline, C6H5NH2, is roughly one million times as reactive
as benzene, and nitrobenzene, C6H5NO2, is roughly one-millionth as reactive as benzene.
14.5 Classification of substituent groups
The methods described in the last two sections have been used to determine the effects of a great number of groups on electrophilic substitution. As
shown in Table 14.3, nearly all groups fall into one of two classes: activating and ortho, para-directing, or deactivating and
meta-directing. The halogens are in a class by themselves, being deactivating but ortho, para-directing.
Activating: Ortho, para directors
Strongly activating
-NH2 (-NHR, -NR2)
-OH
Moderately activating
-OCH3 (-OC2H5, etc.)
-NHCOCH3
Weakly activating
-C6H5
-CH3 (-C2H5, etc.)
Deactivating: Meta directors
-NO2
-N(CH3)3+
-CN
-COOH (-COOR)
-SO3H
-CHO, -COR
Deactivating: Ortho, para directors
-F, -Cl, -Br, -I
Just by knowing the effects summarized in these short lists, we can now predict fairly accurately the course of hundreds of aromatic substitution
reactions. We now know, for example, that bromination of nitrobenzene will yield chiefly the meta isomer and that the reaction will go more
slowly than the bromination of benzene itself; indeed, it will probably require severe conditions to go at all. We now know that the nitration of
C6H5NHCOCH3 (acetanilide) will yield chiefly the ortho and para isomers and will take place more
rapidly than the nitration of benzene.
Although, as we shall see, it is possible to account for these effects in a reasonable way, it is necessary for you to memorize the calssifications in
table 14.3 so that you may deal rapidly with synthetic problems involving aromatic compounds. |