When phenol is treated with choloroform and aqueous sodium hydroxide at 340 K followed by hydrolysis, an aldehydic group, -CHO group is introduced in the ring at a position ortho to the phenol group (OH group).
Ortho hydroxy benzaldehyde or salicylaldehyde is formed as the product of the reaction.
In addition, small amount of p-salicylaldehyde is also formed
In place of chloroform, carbon tetrachloride can be used. I this case o-salicylic acid is formed as the major product.