Question

Explain why the methoxy group is a good activator and ortho- para director in electrophilic aromatic substitution reactions. Use resonance forms and the Wheland intermediate in your answer.

Answer 1

The availability of the π-electrons makes benzene rings nucleophilic , that is,they are susceptible to attack by electrophiles, despite the stability of the ring.

A reaction in which an electrophile adds to an aromatic ring, replacing an aromatic ring hydrogen atom, is called an electrophilic substitution reaction.

When a monosubstituted benzene undergoes an electrophilic aromatic substitution reaction, three possible disubstitution products might be obtained. For example, nitration of bromobenzene could in principle give ortho-, meta-, or para-bromonitrobenzene.A substituent group is either an ortho, para-directing group or a meta-directing group in all electrophilic aromatic substitution reactions.

When compared with benzene, the rate of the reaction of a monosubstituted benzene may be slower or faster. This is the Reactivity of the reaction. If the reaction is slower the substituent is said to deactivate the ring; if faster it activates it.

Substituents that donate electrons make ring more nucleophilic and they are called electron donating groups and activate the ring toward electrophilic aromatic substitution. Halogen( -Cl, -F,-Br), -OH, alkoxyl (OR), and amino substituents donate electrons.

Substitutients that withdraw electrons make ring less nucleophilic and they are called electron withdrawing groups (ewg) and deactivate the ring toward electrophilic aromatic substitution. C=O, CN, NO2 substituents withdraw electrons from the aromatic ring by resonance.

Methoxy group is a good activator and ortho , parab director in electrophilic aromatic substitution reactions because  the atom directly attached to the benzene ring (the oxygen of the methoxy group) has unshared electron pairsand it increases the electron density on the ring through a resonance donating effect. The resonance only allows electron density to be positioned at the ortho- and para- positions. Hence these sites are morenucleophilic, and the system tends to react with electrophiles at these ortho- and para- sites.

Reaction of E⁺ at the para position of anisole gives a carbocation intermediate with the following four important resonance structures:

The colored structure shows that the unshared electron pair of the methoxy group can delocalize the positive charge on the carbocation. This is an especially important structure because it contains more bonds than the others, and every atom has an octet.

If the electrophile reacts with anisole at the meta position, the carbocation intermediate that is formed has fewer resonance structures. The charge cannot be delocalized onto the -OCH3 group when reaction occurs at the meta position.

Activating groups (here -OCH3) donate electrons to the ring, stabilizing the Wheland intermediate (carbocation).