# What is the structure of 2-ethyl-3-hexyne?

Jan 2, 2017

Are you sure this is an hexyne, and not an heptyne?

#### Explanation:

$\text{3-hexyne}$ $\equiv$ ${H}_{3} C - C {H}_{2} C \equiv C - C {H}_{2} C {H}_{3}$,

Because this is an even-numbered acetylene, we could number from either end. The third and fourth carbon along the chain are triply bound.

Given this structure, we have defined the numbering system. $C 2$ is an ethyl group.........

$\text{2-ethyl-3-hexyne}$ $\equiv$ ${H}_{3} C - C H \left(C {H}_{2} C {H}_{3}\right) C \equiv C - C {H}_{2} C {H}_{3}$,

but this is not a valid name, because the longest chain is now 7 carbons long. Organic nomenclature insists that first you find the LONGEST hydrocarbyl chain. This is therefore an heptyne derivative.