How do you know which mixture of products are favoured when phenol is brominated?

Question

Jacob Anderson · Accepted Answer

How else but by measurement...? Do the reaction, and assess the results.

I presume you refer to electrophilic aromatic substitution of phenols. Now the hydroxyl substitution activates all positions on the ring; it especially activates the ortho and para positions. Specific conditions might be found to give ortho or para substitution...treatment of phenol with bromine would likely give #"1,3,5-tribromophenol"#.

Santiago Alfonso · Answer

undefined When phenol is brominated, the major product formed depends on the reaction conditions. Under mild conditions, such as room temperature and with a nonpolar solvent, the para-substitution product is favored due to steric hindrance. This is because the bromine atom prefers to substitute at the para position (opposite to the hydroxyl group) because it is less hindered by the bulky hydroxyl group.

However, under more vigorous conditions, such as high temperature or using a polar solvent like water, the ortho and para positions are equally accessible, leading to a mixture of ortho and para substitution products. This is because the electrophilic bromine species can attack either the ortho or para positions with similar ease.

Therefore, the favored mixture of products in bromination of phenol depends on the reaction conditions, with para-substitution being favored under mild conditions and a mixture of ortho and para substitution products under more vigorous conditions.