Question

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

Answer 1

The “yne” suffix of hexyne represents that it contains a triple bond. Hex stands for the carbon chain containing six carbon atoms. Thus the parent carbon chain contains six carbon atoms. The prefix ethyl represents that the parent carbon chain has a substituent of ethyl group. Thus with the help of numbering the carbon atoms in the parent carbon chain we can draw its structure.

Complete answer:
According to IUPAC nomenclature the suffix “yne ” refers to the triple bond between the carbon atoms.
The word prefix “hex” refers to six carbon atoms in the parent carbon chain. Thus 3-hexyne represents the triple bond present at the third carbon atom. Thus we have to give number to carbon atom according to IUPAC rule and then the 3-hexyne can be represented as:
${}^1C{H_3} - {}^2C{H_2} - {}^3C \equiv {}^4C - {}^5C{H_2} - {}^6C{H_3}$
Now the ethyl group is present at second carbon position which can be shown as:

{{\text{ }}C{H_2} - C{H_3}} \\\ | \\\ {{\text{ }}{}^1C{H_3} - {\text{ }}{}^2CH - {}^3C \equiv {}^4C - {}^5C{H_2} - {}^6C{H_3}} \end{array}$$ Ethyl group is an alkyl group which consists of two carbon atoms and five hydrogen atoms. Ethyl is a common name for ethane molecules. Thus the structure of 2-ethyl-3-hexyne can be shown as: $$\begin{array}{*{20}{c}} {{\text{ }}C{H_2} - C{H_3}} \\\ | \\\ {{\text{ }}{}^1C{H_3} - {\text{ }}{}^2CH - {}^3C \equiv {}^4C - {}^5C{H_2} - {}^6C{H_3}} \end{array}$$ But here we can see that longest carbon chain is of seven carbon atoms as: $$\begin{array}{*{20}{c}} {{\text{ }}{}^2C{H_2} - {}^1C{H_3}} \\\ | \\\ {{\text{ }}C{H_3} - {\text{ }}{}^3CH - {}^4C \equiv {}^5C - {}^6C{H_2} - {}^7C{H_3}} \end{array}$$ According to IUPAC rules we have to search for the longest carbon chain which is a seven carbon atom chain, not a six carbon atom chain. Thus it is a heptyne derivative not a hexyne derivative. **Note:** We can write its condensed formula as $$C{H_3}CH\left( {C{H_2}C{H_3}} \right) - C \equiv C - C{H_2}C{H_3}$$ and also we can write its bond line formula in which each bond can be shown including carbon-hydrogen bonds. The parent carbon chain must be noted carefully and then we can attach the substitute which is at the beginning of IUPAC name of compound.