Groups that donate electron density to the benzene ring, such as -OH and -NH2, are 2- and 4-directing.Why they are electron-donating
The oxygen in -OH and the nitrogen in -NH2 each have a lone pair in a p-orbital that overlaps with the ring's π-system. The lone pair is partially delocalised into the ring, increasing its electron density. The increase is greatest at positions 2, 4 and 6, so electrophiles substitute there.Activation
Because the ring is more electron-rich, these groups make it more reactive than benzene. Reactions go under milder conditions, and several substitutions often happen at once.Examples
• Phenol + bromine water → 2,4,6-tribromophenol (white precipitate) + 3HBr, at room temperature.
• Phenylamine, C6H5NH2, + bromine water → 2,4,6-tribromophenylamine (white precipitate) + 3HBr.
• Phenol + dilute nitric acid → 2-nitrophenol and 4-nitrophenol.Alkyl groups such as -CH3 are also weakly electron-donating and 2,4-directing, so methylbenzene reacts a little faster than benzene.Worked example: predict the products of chlorinating methylbenzene with Cl2 and AlCl3.
The -CH3 group is 2,4-directing, so chlorine goes to position 2 or 4. The products are 1-chloro-2-methylbenzene and 1-chloro-4-methylbenzene, together with HCl. No 3-substituted product forms in significant amounts.
The oxygen in -OH and the nitrogen in -NH2 each have a lone pair in a p-orbital that overlaps with the ring's π-system. The lone pair is partially delocalised into the ring, increasing its electron density. The increase is greatest at positions 2, 4 and 6, so electrophiles substitute there.Activation
Because the ring is more electron-rich, these groups make it more reactive than benzene. Reactions go under milder conditions, and several substitutions often happen at once.Examples
• Phenol + bromine water → 2,4,6-tribromophenol (white precipitate) + 3HBr, at room temperature.
• Phenylamine, C6H5NH2, + bromine water → 2,4,6-tribromophenylamine (white precipitate) + 3HBr.
• Phenol + dilute nitric acid → 2-nitrophenol and 4-nitrophenol.Alkyl groups such as -CH3 are also weakly electron-donating and 2,4-directing, so methylbenzene reacts a little faster than benzene.Worked example: predict the products of chlorinating methylbenzene with Cl2 and AlCl3.
The -CH3 group is 2,4-directing, so chlorine goes to position 2 or 4. The products are 1-chloro-2-methylbenzene and 1-chloro-4-methylbenzene, together with HCl. No 3-substituted product forms in significant amounts.