Which nomenlature is not correct according to IUPAC system ?

Explanation

To determine which nomenclature is not correct according to the IUPAC system, we will analyze the given options step by step.

### Step-by-Step Solution:

1. **Identify the Structures and Nomenclature**:
- We need to look at the nomenclature provided in the question and the corresponding structures.

2. **Check the First Option**:
- The first nomenclature given is "1-bromo-prop-2-ene".
- For prop-2-ene, the double bond is between the second and third carbon atoms.
- According to IUPAC rules, the numbering should start from the end of the chain closest to the double bond. Therefore, the correct numbering should start from the double bond side.

3. **Correct the First Option**:
- If we start numbering from the double bond, the correct structure should be:
- 1 (double bond) - 2 (bromo group) - 3 (propene).
- Thus, the correct nomenclature should be "3-bromo-prop-2-ene".

4. **Check the Second Option**:
- The second nomenclature is "2,4-dimethylhexane".
- Counting the carbon atoms from either end, we find that there are six carbons (hexane) and the methyl groups are correctly placed at the 2nd and 4th positions.
- This nomenclature is correct.

5. **Check the Third Option**:
- The third nomenclature is "5-oxohexanoic acid".
- The carboxylic acid group is at the first position, and the ketone group (oxo) is correctly placed at the 5th position.
- This nomenclature is also correct.

6. **Conclusion**:
- The first option "1-bromo-prop-2-ene" is incorrect according to IUPAC rules. The correct nomenclature should be "3-bromo-prop-2-ene".

### Final Answer:
The incorrect nomenclature according to the IUPAC system is **"1-bromo-prop-2-ene"**.

---