Hydrogen on a terminal alkyne is somewhat acidic, with a pK_a of approximately 25.  This means that, given a strong enough base, terminal alkyne can be deprotonated, yielding a powerful carbanion nucleophile, which we used in S_N2 reactions.  Sodium hydride, or sodium amide in liquid ammonia is often used for this purpose.

The alkynyl carbanion can then be combined with a suitable electrophile, such as a primary alkyl bromide, in a carbon-carbon bond-forming  S_N2 displacement reaction.  However it can also add to polar bonds in the same way as a Grignard reagent, for example giving an alcohol when added to a ketone.

Grignard reagents will not react efficiently in S_N2 reactions with alkyl halides (the Gilman reagent, described below, can be used for this purpose).