Final answer:
The acetylide ion is generated by treating acetylene with a strong base, which removes a proton, allowing it to then act as a nucleophile in SN2 reactions.
Step-by-step explanation:
An acetylide ion is formed by the treatment of acetylene with a strong base. This base abstracts a proton from the terminal alkyne, like acetylene, which is relatively acidic due to the sp-hybridization of the carbon atom bonded to the hydrogen. The acetylide ion, being a strong nucleophile, can then participate in SN2 reactions. It is important to note that while the reagent is a nucleophile in SN2 reactions, it can also act as a base in E2 reactions. Thus, the choice of base and reaction conditions significantly affect whether an SN2 or E2 reaction mechanism will predominate.