Question 12

Buta-1,3-diyne is $H-C \equiv C-C \equiv C-H$. Treatment with 2 equivalents of $NaNH_2$ forms the dianion $Na^{+ -}C \equiv C-C \equiv C^- Na^+$. Reaction with excess $trans-CH_3-CH=CH-CH_2-Br$ (an alkyl halide) results in alkylation at both ends: $X = CH_3-CH=CH-CH_2-C \equiv C-C \equiv C-CH_2-CH=CH-CH_3$. In the diyne system, the carbons involved in the triple bonds ($sp$ hybridized) and the carbons directly attached to them ($sp^3$ methylenes) are collinear because the bond angle at $sp$ carbon is $180^\circ$. The sequence of collinear carbon atoms is: $C_{methylene} - C \equiv C - C \equiv C - C_{methylene}$. Counting these atoms: 1 ($sp^3$) + 4 ($sp$) + 1 ($sp^3$) = 6 carbon atoms. The next carbons in the chain (part of the double bonds) are not collinear as the $C(sp)-C(sp^3)-C(sp^2)$ bond angle is approximately $109.5^\circ$.