IUPAC name of following compound (P) is:

Solution

Chemistry ma’am · 3 steps

Get the lowest locant set first. Alphabetical order only decides the order you write the prefixes in.

1. Read the structure. The ring has six carbons and no functional group, so the parent is cyclohexane. One ring carbon carries two methyl groups, and the carbon two positions away carries an ethyl group.
2. Try each numbering. Start at a substituted carbon and go round the ring in the direction that reaches the next substituent soonest. Then compare the full locant sets at the first point of difference.

   C1 is	Direction	Locant set	Verdict
   the C(CH₃)₂ carbon	towards ethyl	{1, 1, 3}	lowest set
   the ethyl carbon	towards dimethyl	{1, 3, 3}	loses at the 2nd number (3 > 1)
   the C(CH₃)₂ carbon	away from ethyl	{1, 1, 5}	loses at the 3rd number (5 > 3)
   the ethyl carbon	away from dimethyl	{1, 5, 5}	loses at the 2nd number (5 > 1)

3. So. The locants are 1,1 for the methyls and 3 for the ethyl. Write the prefixes alphabetically and ignore "di" when you sort: ethyl comes before methyl. The name is 3-Ethyl-1,1-dimethylcyclohexane, option (b).

The idea to keep. First choose the numbering that gives the lowest locant set. Alphabetical order breaks a tie only when two numberings give the same set. Multiplying prefixes like di and tri never count when you alphabetise.

Answer from the NTA final answer key. Solution by PrepWiser.