Which nomenclature is not according to IUPAC system?

Options

A. `underset("1-bromo prop-2-ene")(Br - CH_2 - CH =CH_2)`
B. `underset("4-bromo-2,4-dimethylhexane")(CH_3 - CH_2 - underset(Br )underset(|)overset(CH_3 ) overset(|) C-CH_2 -underset(CH_3) underset(|) CHCH_3)`
C.
D. `underset("5-oxohexanoic acid")(CH_3 - underset( O) underset(|| C- CH_2 -CH_2 - CH_2 - CH_2 COOH) `

Explanation

To determine which nomenclature is not according to the IUPAC system, we will analyze the names given for the four structures provided in the question.

### Step-by-Step Solution:

1. **Identify the Structures and Their Names**:
- We have four structures with their respective names:
1. 1-bromo-prop-2-ene
2. 4-bromo-2,4-dimethylhexane
3. 2-methyl-3-phenylpentane
4. 5-oxohexanoic acid

2. **Analyze Each Name According to IUPAC Rules**:
- **First Structure**:
- The name is "1-bromo-prop-2-ene".
- According to IUPAC rules, the correct name should be "3-bromo-prop-1-ene" because the double bond takes precedence in numbering, and the bromine substituent should be at the third carbon.
- **Second Structure**:
- The name is "4-bromo-2,4-dimethylhexane".
- This name follows IUPAC rules correctly. The bromine is on the fourth carbon, and the dimethyl groups are correctly positioned.
- **Third Structure**:
- The name is "2-methyl-3-phenylpentane".
- This name is also correct according to IUPAC nomenclature. The substituents are correctly numbered.
- **Fourth Structure**:
- The name is "5-oxohexanoic acid".
- This name is valid as it correctly indicates the presence of a ketone (oxo) and a carboxylic acid functional group.

3. **Determine Which Name is Incorrect**:
- From the analysis, the first structure's name "1-bromo-prop-2-ene" is incorrect as per IUPAC nomenclature rules. The correct name should be "3-bromo-prop-1-ene".

4. **Conclusion**:
- The nomenclature that is not according to the IUPAC system is "1-bromo-prop-2-ene".

### Final Answer:
The nomenclature that is not according to the IUPAC system is **1-bromo-prop-2-ene**.