Markovnikov double addition to terminal alkyne

Steps:

• Protonate the triple bond of the terminal alkyne to form the more stable vinyl carbocation (Markovnikov orientation).
• Bromide adds to the carbocation, yielding a vinyl bromide intermediate.
• With excess HBr, the alkene in the intermediate undergoes a second protonation to form a secondary carbocation.
• Second bromide addition gives the geminal dibromide as the major product.

Why B is correct:

• B is the geminal dibromide (e.g., CH3-CHBr2 for propyne), following Markovnikov's rule where both Br atoms add to the same carbon.

Why the others are wrong:

• A is the vinyl monobromide, incorrect as excess HBr drives double addition.
• C is the anti-Markovnikov vinyl bromide, violating regioselectivity without peroxides.
• D is the vicinal dibromide, incorrect as additions to alkynes favor geminal over vicinal products.

Final answer: B