Question

The third member of the family of alkenynes has the molecular formula:
A.${C_6}{H_6}$
B.${C_5}{H_6}$
C.${C_6}{H_8}$
D.${C_4}{H_4}$

Answer 1

An alkenyne is an aliphatic compound which contains both double bond as well as a triple bond in it. For such a compound to form, its first member must have at least four carbon atoms to form the correct structure keeping in mind the tetravalency of carbon.

Complete step by step answer:
An alkenyne is a hydrocarbon having a carbon chain with hydrogen atoms bonded in a special manner, one double bond and one triple bond is compulsory other than single bonds present. As the name suggests, it has ene bond as well as a yne bond.
The first member of this family must have four carbons at least so that the structure formed is stable and must fulfil the tetravalency of all the carbons. Therefore, the first member comes out to be ${C_4}{H_4}$. It has a double bond as well as a triple bond at alternate positions with respect to the single bond between them.
If we move on to the next member, the second member will have five carbon atoms having one double and one triple bond. Such a compound is ${C_5}{H_6}$. While coming to the third member as asked in the question, it must have six carbons in its hydrocarbon chain with ene and yne bonds present at respective corners. Such a compound will be ${C_6}{H_8}$.
$HC \equiv C - C{H_2} - C{H_2} - CH = C{H_2}$

Hence, the correct option is (C).

Note:
In order to name alkenynes, remember that the chain should always be numbered to give the multiple bonds the lowest possible numbers, and when there is a choice, double bonds are given lower numbers than triple bonds.