Chapter 25, Problem 25.20YT

Interpretation Introduction

Interpretation:

For the propagation steps from Equation 25-33, the redundant species and sum of the two steps to arrive at the net reaction are to be crossed out.

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.