Question

The number of stereoisomers possible for the major product formed in the reaction

is ________.

Answer 1

Correct Answer: 2

The reaction involves an alkyne (PhC≡CH) reacting with HBr. In the presence of one equivalent of HBr, an addition reaction occurs. This can form different structural isomers based on the position of the bromine atom.

Step 1: Addition of HBr to Alkyne

The reaction proceeds via Markovnikov’s rule, where HBr adds across the triple bond. The hydrogen atom preferentially bonds to the carbon with more hydrogens, leading to the formation of a vinyl bromide (PhCHBr=CH₂).

Step 2: Formation of Stereoisomers

The product PhCHBr=CH₂ has a double bond, meaning it can exhibit cis-trans isomerism. The addition of H and Br across the triple bond can lead to the formation of:

• Cis isomer: Both hydrogen atoms on the same side of the double bond.
• Trans isomer: Hydrogen atoms on opposite sides of the double bond.

Therefore, there are two possible stereoisomers.

Conclusion: The total number of stereoisomers for the major product is 2.