Question

Consider the following compounds

Hyperconjugation occurs in
A) III only
B) I and III
C) I only
D) II only

Answer 1

To solve this we must know hyperconjugation with respect to free radical intermediates. We are given three free radicals. First find out the type of electronic interaction that occurs in each of the given intermediates. The electronic interactions increase the stability of the free radical intermediates. Then find out the intermediate that is stabilised by hyperconjugation

Complete answer:
We know that the interactions of the ${{\sigma }}$-electrons of the carbon-hydrogen bond $\left( {{\text{C}} - {\text{H}}} \right)$ bond with the adjacent empty non-bonding p-orbital or ${{\pi }}$-orbitals are the stabilizing interaction. These interactions lead to the formation of extended molecular orbital that increases the stability of the system. This is known as hyperconjugation. The hyperconjugation occurs in carbocations and free radical intermediates.

The important condition for any system to be stabilized by hyperconjugation is that the system should have an adjacent alkyl group having at least one free or ${{\beta }}$-hydrogen atom bonded to it. A hydrogen atom directly bonded to a ${{\beta }}$-carbon atom is known as a ${{\beta }}$-hydrogen atom. The ${{\beta }}$-carbon atom is the second carbon atom that is attached to the functional group.

In the (I) compound, the carbon radical is bonded to a phenyl ring and a tertiary butyl group. Thus, there is no ${{\beta }}$-hydrogen atom. Thus, compound (I) is not stabilized by hyperconjugation. Thus, hyperconjugation does not occur in compound (I).

In compound (II), the carbon radical is attached to the three phenyl rings. Thus, there is no ${{\beta }}$-hydrogen atom. Thus, compound (II) is not stabilized by hyperconjugation. Thus, hyperconjugation does not occur in compound (II).
In compound (III), there are two ${{\beta }}$-hydrogen atoms. Thus, compound (III) is stabilized by hyperconjugation. Thus, hyperconjugation occurs in compound (III). Thus, hyperconjugation occurs in compound (III).

Thus, the correct answer is option (A) III only.

Note: Remember that the important condition for any system to be stabilized by hyperconjugation is that the system should have an adjacent alkyl group having at least one free or ${{\beta }}$-hydrogen atom bonded to it. The hyperconjugation effect stabilizes the carbocation and free radical intermediates.