Q.(a) Define Rearrangement reaction and Isomerisation reaction with examples. [3 marks]

Reaction Intermediates: The Transient Guests of a Chemical Reaction

Imagine you are watching a relay race. The first runner sprints, hands the baton to the second runner, and the second runner finishes the race. The baton itself is passed in a split second — you barely see it in mid-air. That baton, for that fleeting moment, is a reaction intermediate.

In a chemical reaction, molecules rarely transform directly from reactants to products in one clean step. Instead, they often climb a hill, form a short-lived species, and then tumble down to the final product. That short-lived species — the baton in mid-air — is the reaction intermediate.

The Intuition: Why Do Intermediates Exist?

Most chemical bonds do not break and form simultaneously in a single, graceful motion. The energy required to rip a molecule apart is often too high to happen all at once. So, the reaction takes a detour: it first rearranges into a high-energy, unstable species that exists for a very brief time (often microseconds or less), and then this species quickly reacts further to give the final product.

Think of it like rolling a ball over a hill. The ball does not teleport from the bottom of one valley to the bottom of the next. It must first climb to the peak (the transition state), then roll down into a small dip (the intermediate), and then climb a smaller hill before finally reaching the final valley. The intermediate sits in that small dip — it is more stable than the peak, but far less stable than the starting material or the product.

The Precise Statement

A reaction intermediate is a molecular entity that is formed during the conversion of reactants to products and has a lifetime long enough to be detected (or at least inferred) but is too unstable to be isolated under the reaction conditions. It occupies a local minimum on the potential energy surface of the reaction.

The Three Classic Examples

You will meet three archetypes repeatedly in organic chemistry. Each arises from a different way a bond breaks.