Explain why ethyne should be alkylated before, rather than after, nucleophilic addition. g

This is due to the fact that akyl caborcation, that is formed, is more stable hence the reaction is also faster. Alkenes are more reactive that alkynes (that give vinyl caborcation) towards nucleophilic addition. This is attributed to the linear structure of alkynes, that has a higher surface for reactions, that the planar structure of alkenes.