Ch12: Substituent Effects

Chapter 12 : Reactions of Arenes. Electrophilic Aromatic Substitution

Substituent Effects

So far we have looked at the electrophilic aromatic substitution of benzene itself. Of course substituted benzenes can undergo further substitution to give polysubstituted systems:

preparation of disubstituted aromatics

Two questions arise based on this general scheme...

How easy is the second substitution reaction compared to the first ? Is it faster or slower or the same rate ?
Where does E₂ substitute relative to E₁ ? There are 3 possibilities:

ortho-, meta- and para- disubstituted benzene regioisomers

QUESTION Before we look at some experimental evidence is there a prediction we could make ? ANSWER

Let's now look at some experimental evidence from nitration reactions :

Starting Material	Relative rate*	ortho	meta	para
methylbenzene	25	63%	3%	34%
trifluoromethylbenzene	2.5 x 10^-5	6%	91%	3%

* the relative rate is expressed with respect to rate of recation of benzene under the same conditions

What can we conclude from this data ?

Since methylbenzene reacts more rapidly than benzene, it is more reactive than benzene, i.e. it is said to be activated
The methyl group, -CH₃, preferentially directs the new substituent to the ortho- and para- positions

In contrast,

Since trifluoromethylbenzene reacts more slowly than benzene, it is less reactive than benzene, i.e. it is said to be deactivated
The trifluromethyl group, -CF₃, directs the new substituent to the meta position.

The origin of these effects and application to substituents in general is discussed on the following page.