Propyne is treated with $\mathrm{NaNH}{}_2$ in liquid $\mathrm{NH}{}_3$, and then 2-bromo-2-methylpropane is added. The main outcome is

$\mathrm{NaNH}{}_2$ converts propyne into sodium propynide, a strong base and good nucleophile. Acetylide alkylation works by ${\mathrm{S}}_{\mathrm{N}}2$ only with methyl or primary halides. With a tertiary halide ${\mathrm{S}}_{\mathrm{N}}2$ is impossible and the strong base removes a $\beta$-hydrogen (E2), giving 2-methylpropene and regenerating propyne.