Explaining directing effects in Friedel-Crafts reactions

The reactivity of aromatic pi bonds in S_EAr reactions is very sensitive to the presence of electron-donating groups (EDG) and electron-withdrawing groups (EWG) on the aromatic ring.  This is due to the carbocation nature of the intermediate, which is stabilized by electron-donating groups and destabilized by electron-withdrawing groups.

Alkyl groups are weakly ring-activating groups, as their electron-donating ability stems only from weak inductive effects. Substituents with heteroatoms connected to the aromatic ring are significantly more ring-activating than alkyl groups, because resonance electron-donating effects are possible.  Amines, for example, are very powerful ring-activating substituents, due to the ability of the lone pair on the nitrogen to stabilize the carbocation intermediate through resonance:

Other ring-activating groups are shown below (in these figures, the R group can be a hydrogen).  All of these groups are able, in varying degrees, to stabilize the carbocation intermediate in an electrophilic aromatic substitution reaction. Notice that plain old alkyl groups are also (weakly) ring-activating.

Substituent groups that are ring-activating due to resonance effects also tend to exert a strong regiochemical influence on further substitution reactions.   Specifically, substitution tends to occur in the ortho and para positions relative to the existing group. This is known as the ortho-para directing effect.  The effect can be explained by drawing resonance contributors for the carbocation intermediate of the S_EAr reaction: the positive charge is in position to be delocalized by resonance only in reactions leading to ortho or para substitution.

The carbocation which leads to the meta-substituted product, however, cannot be stabilized by resonance with the ring-activating group:

As an example, the Friedel-Crafts alkylation of methoxy benzene would be expected to produce a mixture of the ortho and para substituted products, but no meta-substituted product.

In addition, the para product would be expected to be preferred over the ortho product, due to steric considerations.

Electron-withdrawing substituents on an aromatic ring are ring-deactivating, making it harder for further substitution reactions to occur. These are mostly carbonyl-containing groups, as well as alkyl halides.

When substitution does occur on an aromatic ring with deactivating group already attached, it tends to occur specifically at the meta position – deactivating groups are generally meta-directing.  The exception to this rule is the halogens, which are ring-deactivating but ortho-para directing (see next section).