Final answer:
The second step of an alkyne reaction with excess HX or molecular halogen is slower because the initial addition of the halogen or HX to the alkyne forms a relatively unstable intermediate called a vinyl halide or a vinylic halide. This intermediate contains a partial positive charge on the carbon atom that is bonded to the halogen. The partial positive charge makes the carbon atom more susceptible to attack by another nucleophile, such as water or the halide ion, in a subsequent step.
Step-by-step explanation:
The second step of an alkyne reaction with excess HX or molecular halogen is slower because the initial addition of the halogen or HX to the alkyne forms a relatively unstable intermediate called a vinyl halide or a vinylic halide. This intermediate contains a partial positive charge on the carbon atom that is bonded to the halogen. The partial positive charge makes the carbon atom more susceptible to attack by another nucleophile, such as water or the halide ion, in a subsequent step.
For example, in the reaction with excess HX, the first step involves the addition of HX to the alkyne to form a vinylic halide. In the second step, water acts as a nucleophile and attacks the carbon atom in the vinylic halide, displacing the halogen and forming the corresponding alcohol.
The second step is slower because the vinylic halide is less reactive than the alkyne, and the attack of the nucleophile requires breaking the carbon-halogen bond, which is more difficult than breaking the carbon-carbon triple bond in the alkyne.