Question

Draw the organic products formed when each alkyne is treated with two equivalents of HBr.

a.

b.

c.

Short answer

a.

b.

c.

Step-by-step solution

Addition of hydrogen bromide to an alkyne

Alkynes can undergo the addition of hydrogen bromide, and this is an electrophilic addition.Two moles of HBr are used. The first mole will react with alkyne to form vinyl halide, which will then react with another mole of HBr and form a geminal halide.

Mechanism

The mechanism of the reaction taking place is shown using an example below:

Mechanism of the addition of HBr to but-1-yne

Formation of the final product

a. The reaction taking place between hex-1-yne and HBr is shown below:

Addition of HBr to hex-1-yne

b. The reaction taking place between ethynylcyclohexane and HBr is shown below:

Addition of HBr to ethynylcyclohexane

c. The reaction taking place between 3-ethynyl-1,1-dimethylcyclopentane and HBr is shown below:

Addition of HBr to 3-ethynyl-1,1-dimethylcyclopentane