Reactions of Phenolic Benzene Rings

The hydroxy group in a phenol molecule exhibits a strong activating effect on the benzene ring because it provides a ready source of electron density for the ring. This directing influence is so strong that you can often accomplish substitutions on phenols without the use of a catalyst.

Halogenation

Phenols react with halogens to yield mono‐, di‐, or tri‐substituted products, depending on reaction conditions. For example, an aqueous bromine solution brominates all ortho and para positions on the ring.





Likewise, you can accomplish monobromination by running the reaction at extremely low temperatures in carbon disulfide solvent.




Nitration

Phenol, when treated with dilute nitric acid at room temperature, forms ortho‐ and para‐nitrophenol.




Sulfonation

The reaction of phenol with concentrated sulfuric acid is thermodynamically controlled. At 25°C, the ortho product predominates while at 100°C, the para product is the major product.





Notice that at both 25° and 100°, initially an equilibrium is established. However, at the higher temperature, the equilibrium is destroyed and the more thermodynamically stable product is produced exclusively.

Kolbe reaction

The reaction of a phenoxide ion with carbon dioxide to produce a carboxylate salt is called the Kolbe reaction.





The Kolbe Reaction progresses via a carbanion intermediate.





In this reaction, the electron deficient carbon atom in carbon dioxide is attracted to the electron rich π system of the phenol. The resulting compound undergoes keto‐enol tautomerization to create the product.