Chapter 11, Problem 11.14P

Interpretation Introduction

(a)

Interpretation:

How to synthesize the given compound from an alkyne is to be described.

Concept introduction:

The carbon-carbon triple bond in terminal alkynes is electron rich and thus undergoes electrophilic addition reaction with a strong Bronsted acid such as hydrogen halide. Addition of two equivalents of hydrogen halide to a terminal alkyne produces a geminal dihalide, in which both the halide atoms are on the same carbon atom. The regioselectivity is in accordance with the Markovnikov’s rule. The hydrogen halide adds twice to generate the geminal dihalide. If hydrogen chloride is used as a reagent, the reaction goes through vinylic carbocation intermediate as the chlorine atom will stabilize the positive charge on the carbon atom through resonance. The alkyne must be symmetric to avoid producing a mixture of isomers.

Interpretation Introduction

(b)

Interpretation:

How to synthesize the given compound from an alkyne is to be described.

Concept introduction:

The carbon-carbon triple bond in terminal alkynes is electron rich and thus undergoes electrophilic addition reaction with a strong Bronsted acid such as hydrogen halide. Addition of two equivalents of hydrogen halide to a terminal alkyne produces a geminal dihalide, in which both the halide atoms are on the same carbon atom. The regioselectivity is in accordance with the Markovnikov’s rule. The hydrogen halide adds twice to generate the geminal dihalide. If hydrogen bromide is used as a reagent, the reaction goes through formation of a pi complex with an alkyne. This pi complex is stabilized by the attraction between electron rich triple bond and electron poor proton of the acid. The alkyne must be symmetric to avoid producing a mixture of isomers. An unsymmetric alkyne produces a mixture of isomers.