Question

In the presence of a small amount of bromine, cyclohexene undergoes the following light-promoted reaction:

(a) Propose a mechanism for this reaction.

(b) Draw the structure of the rate-limiting transition state.

(c) Use Hammond’s postulate to predict which intermediate most closely resembles this transition state.

(d) Explain why cyclohexene reacts with bromine much faster than cyclohexane, which must be heated to react.

Short answer

(a)

Mechanism for free-radical bromination of cyclohexene

(b)

rate-limiting transition state

(c)

(d) Cyclohexene reacts faster with bromine than cyclohexane, because the allylic radical that is formed is stabilized by resonance.

Step-by-step solution

Free radicals

An atom or group of atoms containing odd or unpaired electrons is known as free radical. The unpaired electron is represented by a single unpaired dot in the formula. Free radicals are electrically neutral. They are highly reactive species formed by homolytic fission of a covalent bond.

Steps involved in free radical chain reaction

In a free-radical chain reaction, free radicals are generally created in the initiation step. A free radical and a reactant are combined to yield a product in the propagation step. Lastly, the number of free radicals generally decreases in the termination step

Transition state or activated complex

The highest energy state in a molecular collision that leads to reaction is the transition state. Transition state is the most unstable and[l1] gets converted into product molecules. During the formation of this complex, old bonds start breaking and the new bonds are formed.

Intermediate

Intermediate is a species that exists for a finite amount of time, and has some stability, although its lifespan is short.

Rate limiting step

It is the slowest step of a chemical reaction that determines the rate at which the overall reaction takes place.

Hammond’s postulate

According to Hammond’s postulate, related species which are closer in energy are also closer in structure, and the structure of the transition state resembles the structure of the closest stable species.

Explanation

(a) Free-radical bromination is highly selective. The mechanism consists of three parts—initiation step, propagation step I, and propagation step II.

Mechanism for free-radical bromination of cyclohexene

(b) Rate-limiting transition state is the abstraction of hydrogen by bromine radical

rate-limiting transition state

(c) From Hammond’s postulate, it is known that the transition state in the endothermic bromination must resemble the products more than the reactants, since they are closer in energy to the products.

(d) Cyclohexene reacts faster with bromine than cyclohexane, because the allylic radical that is formed is stabilized by resonance.

allylic radical stabilized by resonance