Question

Number of possible alkynes with formula ${C_5}{H_8}$ is:
(A) $2$
(B) $3$
(C) $4$
(D) $5$

Answer 1

Hint: Alkynes are the hydrocarbons having carbon-carbon triple bonds. Isomers in chemistry are compounds having the same molecular formula but different structural formula or arrangements of atoms. All the hydrocarbons show isomerism.

Complete step by step answer:
The chemical name of ${C_5}{H_8}$ is Pentyne. We can find out its isomers by writing its structures. Let us write down the extended structural formulae of pentyne showing the atoms attached to each carbon atom.
$
  1.\,C{H_3} - C{H_2} - C{H_2} - C \equiv CH,\,Pent - 1 - yne \\
  2.\,CH - C \equiv C - C{H_2} - C{H_3},Pent - 2 - yne \\
  3.\,C{H_3} - CH(C{H_3}) - C \equiv CH,\,3 - methylbut - 1 - yne \\
 $
No more arrangements are possible with the same formula. Thus, there can be only three above mentioned isomers of ${C_5}{H_8}$ (pentyne).
The final answer is option ‘B’.
Additional information: Alkynes show two types of isomerism position isomerism and chain isomerism. Chain isomers are those in which the structure of the chain is different i.e. number of C atoms in the main chain vary. Position isomers are those in which the position of the double bond is different. In the above example the isomers $Pent - 1 - yne$ and $Pent - 2 - yne$ are position isomers because they differ in the position of triple bonds. Whereas isomers $1$ and $3$ are chain isomers as the position of double bond is the same. The isomers $2$ and $3$ are both chain and position isomers as they have different positions of triple bonds as well as the number of C atoms in the main chain.
Note:
The structure of all the possible isomers should be drawn for finding out the total number of possible isomers. The number of C and H atoms should remain the same while writing different structure and valency of C should be satisfied in each structure i.e. each C should have four bonds with neighbouring atoms.