Chapter 11, Problem 11.22P

Interpretation Introduction

Interpretation:

The rate of reaction of cyclohexene with hydrogen halides $\text{HF}$, $\text{HCl}$, $\text{HBr}$, and $\text{HI}$ in the order of slowest reaction rate to fastest, is to be ranked.

Concept introduction:

In electrophilic addition of a Bronsted acid (hydrogen halide) to an alkene, the reaction takes place in two steps. In the first step, the electrophilic addition of the proton takes place in which a pair of electrons from the electron-rich pi bond in the alkene forms a bond with the hydrogen atom in the Bronsted acid. This generates a carbocation intermediate. Step two is the coordination step in which the anion produced in step one attacks the carbocation intermediate to yield the final product via an ${\text{S}}_{\text{N}}\text{1}$ mechanism.

Step one of the mechanism is slow because it has a high-energy transition state, so it is the rate determining step. The rate of the reaction is determined by how fast the $\text{H-X}$ bond breaks in the first (rate-determining) step, generating ${\text{X}}^{\text{-}}$ (the anion). More stable the anion ${\text{(X}}^{\text{-}}\text{)}$ generated, lower is the energy barrier, and faster will be the reaction.