Explain the fact that in aryl alkyl ethers (i) the alkoxy group activates the benzene ring towards electrophilic substitution and (ii) it directs the incoming substituents to ortho and para positions in benzene ring.
The alkoxy group (-OR) activates the benzene ring towards electrophilic substitution in the same way as in phenol. The resonance effect caused by the alkoxy group makes the ring electron-rich, facilitating attack by electrophiles. This activation is so effective that reactions like bromination of anisole occur even in the absence of a catalyst like iron (III) bromide.
The alkoxy group directs the incoming substituents to ortho and para positions because these positions become electron-rich due to the resonance effect caused by the -OR group. The resonance structures show that the electron density is concentrated at ortho and para positions, making them more reactive towards electrophilic attack. As a result, para isomer is obtained as the major product (e.g., 90% yield in bromination of anisole) while ortho isomer is minor.
Explanation
The textbook explains that the alkoxy group behaves similarly to the -OH group in phenols. The resonance effect delocalizes electrons from the oxygen atom into the benzene ring, activating it and directing substitution to ortho and para positions. This is evidenced by the bromination of anisole, which proceeds without a catalyst and yields predominantly para-bromoanisole.