Question

Explain the fact that in aryl alkyl ethers 

1. the alkoxy group activates the benzene ring towards electrophilic substitution and
2. it directs the incoming substituents to ortho and para positions in the benzene ring.

Answer 1

i. In aryl alkyl ethers, the alkoxy group increases the electron density on the benzene ring due to the +R effect and activates the benzene ring towards electrophilic substitution reactions.

ii. Since the electron density is higher at ortho and para positions than at m-positions, electrophilic substitution reactions mainly take place at ortho and para positions.

Aromatic ethers also give Friedel-Crafts alkylation and Friedel-Crafts acylation reactions.

Answer 2

Consider the following resonance structures of aryl alkyl ethers.

1. The benzene ring is considered to have been activated toward incoming electrophiles since the presence of the −OR group has raised the electron density on the ring, as can be seen from the structures above. 

2. We can see that the electron density has increased on C-2, C-4 and C-6, or in the ortho and para positions, from structures (II), (III), and (IV). Consequently, the −OR group is considered to have directed the E⁺ to ortho and para locations as the electrophile E⁺ binds itself to these e⁻ rich sites.