Answer to Question #181715 in Organic Chemistry for Mons

Question

Stereochemistry of ethynylcyclohexane + excess HBr

Answer 1

The reaction is an electrophilic addition reaction in which positively charged hydrogen ions (electrophiles) from HBr attacks the pi electrons in the triple bonds(of ethynyl group) first. This saturates the bond but creates a carbocation on the other carbon which then accommodates the nucleophile (bromide ions). The product formed is 2,2-dibromoethylcyclohexane.