Question

The isomer of propyne is:
A. Allene
B. Propene
C. Cyclopropane
D. Propane

Answer 1

The compounds that have the same molecular formula but different structures (differ in physical and chemical properties) are called isomers. In propyne the two carbon atoms have sp hybridization.

Complete step by step answer:
To understand this question we should know about functional isomerism- when two more substances with the same molecular formula but different functional groups are functional isomers.
Now we already know that propyne
$C{{H}_{3}}-C=CH$
In this $\alpha$ carbon atoms have $sp$ hybridization and one carbon has $s{{p}^{3}}$
look at the hybridization of each option given-
allene - $C{{H}_{2}}=C=C{{H}_{2}}$
In this the central carbon is sp-hybridized and the two terminal carbon atoms are $s{{p}^{2}}$ hybridized.
Propene - $C{{H}_{2}}=CH-C{{H}_{3}}$
In this the two carbon atoms are $s{{p}^{2}}$ and one carbon atom is $s{{p}^{3}}$ hybridized.
Cyclopropane-
It is also $s{{p}^{3}}$ hybridized
Propane – $C{{H}_{3}}-C{{H}_{2}}-C{{H}_{3}}$
In this all bonds are single, hence it undergoes $s{{p}^{3}}$ hybridization.
So we can conclude that allene $\left( C{{H}_{2}}=C=C{{H}_{2}} \right)$ is a structural isomer of propyne because it has same molecular formula but different functional groups.

So, the correct answer is Option A.

Note: Isomers have different arrangement of atoms due to molecular rotating. Molecules having the same molecular formula have different isomers due to arrangement of atoms. Functional isomerism occurs when substances have the same molecular formula but different functional groups. Proyne can also show ring chain isomers called cyclopropene.