Animated Solution for Chemistry - Organic Chemistry: The major products A and B in the following reactions are :

Initiating the Radical Journey

Welcome to a thrilling exploration of free radical addition! In this problem, we are presented with a complex nitrile compound, 2,4-dimethylpentanenitrile, reacting in the presence of peroxide and heat.

Peroxide is a classic radical initiator. When heated, the weak oxygen-oxygen bond in peroxide undergoes homolytic cleavage, generating highly reactive alkoxy radicals. These radicals are hungry for a hydrogen atom and will abstract the most acidic or reactive hydrogen from our starting material to form a stable carbon-centered radical.

Identifying Radical A

Our reactant has several types of hydrogen atoms: primary, secondary, and tertiary. However, not all hydrogens are created equal. The hydrogen attached to the carbon bearing the cyano ($-C\equiv N$) group is the prime target.

Why? Because when this hydrogen is abstracted, the resulting unpaired electron is located on a tertiary carbon that is directly adjacent to the cyano group. This allows the unpaired electron to delocalize into the $\pi$ system of the cyano group through resonance:

This resonance stabilization is incredibly powerful, making this specific tertiary radical the major intermediate in the reaction. This is our compound A. By simply identifying this stable radical, we can immediately eliminate half of the given options!

Anti-Markovnikov Addition to the Diene

Now that we have our stable radical A, it acts as the attacking species in the next phase of the reaction. It encounters 1,3-butadiene, a conjugated diene.

Free radical addition to alkenes and dienes typically follows the anti-Markovnikov rule. The bulky radical A will preferentially attack the less sterically hindered terminal carbon of the diene. This regioselectivity is driven not only by steric factors but also by the stability of the resulting intermediate.

When radical A attacks the terminal carbon, it breaks one of the $\pi$ bonds, creating a new radical on the adjacent secondary carbon. This newly formed radical is an allylic radical, meaning it is adjacent to the remaining double bond. Allylic radicals are highly stabilized by resonance, which makes this addition pathway highly favorable.

The Birth of Product B

We are now at the final stage of our reaction mechanism. The allylic radical intermediate needs to terminate to form a stable, neutral molecule. It does this by abstracting a hydrogen atom from another molecule in the reaction mixture (often another molecule of the starting material or a solvent molecule).

The hydrogen atom adds to the secondary carbon of the allylic system. This specific regiochemistry occurs because the secondary carbon has a higher spin density and is more accessible, leading to the formation of a terminal alkene.

The resulting molecule, featuring the newly attached chain with a terminal double bond, is our final major product B. Comparing our meticulously derived structures for A and B with the given choices, we find a perfect match with option (c). A beautiful demonstration of how radical stability dictates the course of complex organic reactions!