Names of some organic compounds are given. Which one is not in IUPAC system?

Step-by-step solution

We evaluate the IUPAC names provided for each structure: 1. 1,3,3-trimethylcyclohex-1-ene: The numbering starts from the double bond. If we number the double bond as 1 and 2, the methyl groups are at positions 1 and 3,3. This is correct. 2. but-3-enoic acid: The carboxylic acid carbon is C1. The chain is C₁OOH-C₂H₂-C₃H=C₄H₂ . The double bond starts at C3. This is correct. 3. 2-methyl-3-phenylpentane: The longest chain is 5 carbons (pentane). Numbering from the left gives 2-methyl and 3-phenyl. This is correct. 4. 4