Multiple products in organic reactions are a common feature.

What governs the product proportions and how? is an interesting and fascinating feature.

Among others, concentration, temperature and even time can be factors, which influence the product ratio.

Sulphonation of Naphthalene

Consider the sulphonation of naphthalene. There are two isomeric products possible. 1-Naphthalene sulphonic acid (A) and its 2-isomer (B).

Experimental observations

At lower temperatures (80⁰C) it is found (A) is the major product and at higher temperatures (160⁰C) (B) is the major product.

Mechanistic explanation

Facts

• 1-Naphthalene sulphonic acid is less stable while the other isomer is more stable.

• The large sulphonic acid group encounters steric hindrance from the hydrogen in the adjacent aromatic ring.

• Such hindrance is almost absent in its isomer, 2-Naphthalene sulphonic acid.

Features

The activation energy barrier for the formation of (A) is less than for (B) Hence (A) is formed faster.

At low temperatures there isn’t sufficient energy to over come the activation barrier for the reverse process that is the desulphonation of (A). Hence (A) that is formed faster remains as the major product.

At higher temperatures there is sufficient energy to over come this energy barrier, the product can revert back to the reactant and go in the direction of the more stable product, making 2-naphthalene sulphonic acid as the major product.

Also the equilibrium is fairly and rapidly established between the product and reactant and gradually most of naphthalene is converted to (B)