Chapter 12, Problem 12.63P

Interpretation Introduction

(a)

Interpretation:

A method for conversion of $\text{hex-1-yne}$ to $\text{2, 2-dibromohexane}$ is to be proposed.

Concept introduction:

The carbon-carbon triple bond of an alkyne undergoes electrophilic addition when treated with a strong Bronsted acid. In the first step, the proton is added to a carbon-carbon triple bond to form a vinylic cation. The conjugate base of the acid then coordinates with the cation. An alkene is formed which adds another molecule of the acid as it has one more $\text{π}$ bond. The result is a geminal di-substituted product. When alkynes are treated with an excess of a hydrogen halide, a geminal dihalide is formed.

Interpretation Introduction

(b)

Interpretation:

A method for conversion of $\text{hex-1-yne}$ to $\text{1, 2-dibromohexane}$ is to be proposed.

Concept introduction:

The oxidation of terminal alkyne is carried out by using bulky dialkylborane like disiamylborane $\left[{\left({\text{C}}_{\text{5}}{\text{H}}_{\text{11}}\right)}_{\text{2}}\text{BH}\right]$ instead of simple borane (${\text{BH}}_{\text{3}}$) to avoid the formation of a mixture of products. After the first addition, no further addition takes place as the reagent does not have another $\text{B-H}$ bond. Oxidation of the adduct by the action of $\text{NaOH}$ and ${\text{H}}_{\text{2}}{\text{O}}_{\text{2}}$ initially forms an enol form which tautomerizes to the more stable keto form.

Sodium borohydride, ${\text{NaBH}}_{\text{4}}$, reduces aldehydes to corresponding primary alcohols. Alcohol is dehydrated on heating with a strong acid like ${\text{H}}_{\text{2}}{\text{SO}}_{\text{4}}$ forming the corresponding alkene.

The electrophilic addition of molecular bromine across the double bond of an alkene forms a vicinal dibromide. The addition of molecular bromine involves a cyclic bromonium intermediate, either above or below the plane of the double bond, leading to the formation of a racemic mixture of the products.