Sodium ethynide, $\mathrm{HC}\equiv \mathrm{C}{}^{-}\mathrm{Na}{}^{+}$, is treated with 2-bromo-2-methylpropane. The main outcome is

The acetylide ion is a strong base as well as a nucleophile. A tertiary halide cannot undergo $\mathrm{SN}{}_2$ and in this strongly basic medium E2 dominates: the acetylide removes a beta hydrogen, giving 2-methylpropene and ethyne. Acetylide alkylation works well only with methyl and primary halides.