Question

Explain how does the −OH group attached to a carbon of benzene ring activate it towards electrophilic substitution?

Answer 1

Phenol is considered to be a resonance hybrid of the following structures:

The +R effect of the –OH group increases the electron density on the benzene ring, which makes the attack of the electrophile easier. Hence, the presence of the –OH group activates the benzene ring towards electrophilic substitution reactions. Since the electron density at the ortho and para positions is relatively high, electrophilic substitution is mainly more at the ortho and para positions.

Answer 2

In an electrophilic substitution reaction, an electron-deficient species attacks the benzene ring, which is electron-rich. When an –OH group is attached to the benzene ring, it increases the ring's electron density, making it more reactive and a good site for electrophilic attack.

The increase in electron density can be visualised as:

Structures (I)-(V) indicate that the addition of a hydroxyl group to benzene has increased the electron density (negative charge) on the carbon atoms of the ring, particularly at C-2, C-4, and C-6. Therefore, it is said that the ring has been activated towards electrophiles, which are drawn to the increased electron density.