What mechanistic intermediate is used to explain the preference for addition of the Br atom, of HBr, to the internal carbon of a terminal alkyne?
A) Formation of an intermediate with partial positive charge on a primary carbon
B) Formation of an intermediate with partial positive charge on a secondary carbon
C) Addition of the H atom to the carbon already with an H.
D) Addition of H, from HBr, to the least hindered carbon.
E) Steric hindrance for the approach of the Br atom to the primary carbon.

Question

Quizplus · Accepted Answer

The preferred mechanism for addition of HBr to a terminal alkyne involves the formation of a bridged intermediate with a secondary carbon bearing the partial positive charge. This intermediate is more stable than one with a primary carbon due to the greater stabilization provided by the adjacent sp hybridized carbon atom. This leads to regioselectivity towards the internal carbon atom.

What Mechanistic Intermediate Is Used to Explain the Preference for Addition