Chapter 25, Problem 25.23YT

Interpretation Introduction

Interpretation:

The curved arrow notation and intermediate are to be shown to the radical and closed-shell mechanisms for the addition of $\text{HBr}$ to propene. Each mechanism is to be completed by drawing the more stable intermediate in the box provided and adding the curved arrow notation. Realizing that both reactions proceed through the more stable intermediate, the opposite regiochemistry of the reactions is to be explained.

Concept introduction:

A hydrogen halide adds across the double bond of an alkene in a Markovnikov fashion under normal conditions. ${\text{H}}^{\text{+}}$ adds to the less alkyl-substituted $\text{C}$ atom whereas the halide anion adds to the more alkyl-substituted $\text{C}$ atom. The reason is that the first step of the mechanism is protonation of the $\text{C=C}$ double bond, which proceeds to give the more stable carbocation intermediate. If a small amount of peroxide ($\text{RO-OR}$) is present, however, then the major product is produced according to anti-Markovnikov addition. The $\text{H}$ atom adds to the more substituted $\text{C}$, and the halide atom adds to the less substituted one. Peroxides are radical initiators because they contain a weak $\text{O-O}$ bond. Hence, the reaction in Equation 25-32 proceeds by radical mechanism. Radical addition to an alkene generally takes place so as to produce the more stable alkyl radical intermediate.