Question

What is the correct IUPAC name for the following?

A. $3$ -Isopropyl- $2$ -methylpentane
B. $3$ -Ethyl- $2$ , $4$ -dimethylpentane
C. $2$ , $4$ -dimethyl- $3$ -ethylpentane
D. $3$ -Isopropyl- $4$ -methylpentane

Answer 1

In order to answer this question, we should know about the rules of IUPAC nomenclature to name the organic compounds. First, the longest carbon chain is taken and the substituents are numbered in such a way that all the substituents get the lowest possible numbers.

Complete answer:
First, select the longest carbon chain as the parent chain.

The parent chain consists of five carbon atoms, so its name is pentane.
Now, start naming the substituents from the carbon $1$ . We see that at carbon $2$ and $4$ , two methyl groups are present so we write it as dimethyl. At carbon $3$ , an ethyl group is present. Now, while writing the name of the compound, we should keep the name of the substituents according to alphabetical order.
Hence, the IUPAC name of the given is $3$ -Ethyl- $2$ , $4$ -dimethylpentane.
Therefore, the correct answer is option (B).

Note:
It must be remembered that the ‘d’ alphabet of the term dimethyl does not consider. It is just a prefix to indicate the number of same substituents. This indicates that the option (C) is not correct. Also, the term ‘isopropyl’ is not used for denoting a substituent. It is just a common name and here the longest carbon chain is pentane so we cannot use the ‘isopropyl’ term. So, According to IUPAC nomenclature, option (A) and option (D) are not correct.