Question

In how many wayts can the structure for the alkyne with molecular formula `C_5H_8` be constucted?
Strategy: First arrange five carbon atoms with a continuous C chain and then four C atoms with a side chain. Introduce the triple bond at different locations.

Answer 1

(i). `overset(5)(C)H_(3)overset(4)(C)H_(2)overset(3)(C)H_(2)-overset(2)(C)-=overset(1)(C)H`, pent-1-yne
(ii). `overset(5)(C)H_(3)overset(4)(C)H_(2)-overset(3)(C)-=overset(2)(C)-overset(1)(C)H_(3)`, pent-2-yne
(iii). `H_(3)overset(4)(C)-underset(CH_(3))underset(|)(overset(3)(C))-overset(2)(C)-=overset(1)(C)H`, 3-methylbut-1-yne Strutures (i) and (ii) are position isomers while strutures (i) and (iii) or (ii) and (iii) are chain isomers. Strutures (ii) and (iii) exhibit chain as well as positionn isomerism.
Note that skeleton with two side chains is not possible here because the central C atom would then have 6 bonds which is not possible:
`C-underset(C)underset(|)overset(C)overset(|)(C)-=C` (not possible)