Chapter 25, Problem 25.21P

Interpretation Introduction

(a)

Interpretation:

The major product of the given reaction is to be drawn by paying attention to stereochemistry.

Concept introduction:

HBr and peroxide are sources of $\text{Br}·$ radical. $\text{Br}·$ radical adds to the alkene via radical addition, and the resulting alkyl radical abstracts an H atom from HBr, yielding the overall product. Thus the addition is anti-Markovnikov addition. When the radical intermediate is formed from radical addition of $\text{Br}·$ to the alkene, the carbon that forms the C-Br bond becomes the chiral center, and thus both configurations are formed. The more substituted alkyl radical forms C-H bond, and that carbon becomes a chiral center, and both of its configurations are formed.

Interpretation Introduction

(b)

Interpretation:

The major product of the given reaction is to be drawn by paying attention to stereochemistry.

Concept introduction:

In radical halogenation reactions, N-bromosuccinimide (NBS) is the source of a low, steady concentration of ${\text{Br}}_{\text{2}}$. The homolysis of the N - Br bond to produce $\text{Br}·$ takes place in the presence of light. The alkyl radical is formed when $\text{Br}·$ abstracts a H atom from $\text{H-Br}$, to form a secondary allylic $\text{C}·$. This intermediate has two resonance structures. When $\text{C}·$ gains Br, that carbon atom becomes a chiral center, and both configurations can be produced.