Rules for Naming Hydrocarbons:-
Rule-I Selection of the longest carbon chain.
Rule-II Numbering the longest chain: Longest chain is numbered starting from a terminal carbon. Numbering is done is such a way that lowest possible number is given to the side chain or a functional group.
Rule-III Writing names of branched, saturated hydrocarbons, locants are written first in alphabetical order followed by name of parent chain.
Rule-IV Some common names are retained in IUPAC system for unsubstituted hydrocarbon only.
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As −Cl is attached to right side carbon; so it is given 1 number.
As −CH3 group is attached to 2−C so, methyl substituent will be given in name.
So, 1−chloro−2−methyl−propane