Question

The functional isomer of ethanenitrile is:
A. Ethyl carbylamine
B. Methanenitrile
C. Methyl isocyanide
D. Propanenitrile

Answer 1

Two or more compounds having the same molecular formula but different functional groups are called functional isomers. To find the functional isomer of ethanenitrile, we need to find the compound, which has the same molecular formula as ethanenitrile, but different functional groups.

Complete Solution :
The structural formula of ethanenitrile is $\text{C}{{\text{H}}_{\text{3}}}\text{C}\equiv \text{N}$, so it’s molecular formula will be ${{\text{C}}_{\text{2}}}{{\text{H}}_{\text{3}}}\text{N}$. The functional group of ethanenitrile is $-\text{C}\equiv \text{N}$.
The functional isomer of ethanenitrile will also have the same molecular formula ${{\text{C}}_{\text{2}}}{{\text{H}}_{\text{3}}}\text{N}$.
We need to find a compound with the same molecular formula and a different functional group.

A) The structural formula of ethyl carbylamine (also called ethyl isocyanide) is $\text{C}{{\text{H}}_{\text{3}}}\text{C}{{\text{H}}_{\text{2}}}-\text{N}\equiv \text{C}$, having molecular formula ${{C}_{3}}{{H}_{5}}N$.

B) There exists no compound named methanenitrile.

C) The structural formula of methyl isocyanide is $C{{H}_{3}}N\equiv C$, having molecular formula${{C}_{2}}{{H}_{3}}N$

D) The structural formula of propanenitrile is $\text{C}{{\text{H}}_{\text{3}}}\text{C}{{\text{H}}_{\text{2}}}\text{C}\equiv \text{N}$, having molecular formula ${{C}_{3}}{{H}_{5}}N$

We can see that methyl isocyanide has the same molecular formula as ethanenitrile, but its functional group is $-\text{N}\equiv \text{C}$. Hence, methyl isocyanide is the functional isomer of ethanenitrile.
So, the correct answer is “Option C”.

Note: In ethanenitrile and methyl isocyanide, the alkyl group is $\text{C}{{\text{H}}_{3}}$. However, in ethanenitrile, the alkyl bonding is through carbon atom, whereas in methyl isocyanide, the alkyl bonding is through nitrogen atom.
Ethanenitrile and propanenitrile are both cyanides having different alkyl groups attached to the functional group of cyanide, thus they are not functional isomers.