Question

The IUPAC name of the following compound is-
  

(This question has multiple correct options.)
A.$4-$methylchlorobenzene
B.$4-$chlorotoluene
C.$1-$ chloro $-4-$ methylbenzene
D.$1-$ methyl $-4-$ chlorobenzene

Answer 1

Hint: Nomenclature means assignment of names to organic compounds. The main system for nomenclature of compounds is IUPAC. IUPAC stands for International Union of Pure and Applied Chemistry. There are rules laid down by this system for naming any compound.

Complete answer:
The old system of naming compounds is called a trivial system. Compounds were named either after their discoverers or by their source. But such names used to differ in every region. So we needed one universal system for naming compounds.
IUPAC system of nomenclature is accepted internationally for assigning names to the compounds. In order to name any compound we must follow the rules laid down.
Now let us look at the given example,
-It is a substituted benzene compound. The aromatic ring at the centre is benzene. In case of substituted benzene, the substituent name is added as prefix to the word ‘benzene’ . for example,

The above molecule is named as methylbenzene.
-Now there are some common names of the compounds that are accepted universally. For example the above compound is known as toluene commonly and this is accepted by IUPAC.

-If the benzene ring is dis-substituted then the position of substituents is defined by numbering the carbon atoms of the ring such that the substituents are located at the lowest possible numbers. Now for higher substituted benzene, compounds are identified by numbering the substituents in such a way that each of them gets a lower possible number and are arranged in alphabetical manner. In some cases, the common name of benzene derivatives is taken as the base name.
-Substituents of the base compound are numbered as $1$ and then the further substituents are numbered in such a way that they get the lowest possible number.
-Now in the given compound if we considered benzene then the IUPAC name can be given as-

-Benzene is considered as a base compound. There are two substituents-chlorine and methyl group. Chlorine will get first priority alphabetically and will be numbered as $1$ and methyl will be numbered as $4$ . So one possible name is $1-$ chloro $-4-$ methylbenzene. So option C is correct.
-Now methyl group cannot get first priority over chlorine because it will be alphabetically incorrect. So the name $1-$ methyl $-4-$ chlorobenzene is not possible. So the option D is incorrect.
-Also in option A, the substituent chlorine has not been assigned any number. So it is also the incorrect answer.
-Now in the given compound, toluene can be considered as the base compound and chlorine as its substituent. We know that toluene though common name is accepted universally. So the compound can also be named as-

Since the base compound is toluene, methyl will be numbered as $1$ and then count the position of chlorine. So option B is also correct.
Hence the correct options are B and C.

Note:The nomenclature is less formal for substituted benzene ring compounds, when compared to alkanes, alkenes and alkynes. While naming organic compounds, the substituents on the ring are given priority alphabetically and are assigned the lowest possible number. Some of the common names of the compounds are also accepted by IUPAC.