Which of the following IUPAC name is incorrect

Options

A. 3-Bromo-2-chloropentane
B. Pent-2-en-4-yne
C. 3-Bromo-butan-2-ol
D. 1-Aminopentane-2-thiol

Answer

To determine which IUPAC name is incorrect, we will analyze each of the provided names step by step.

### Step 1: Analyze the first name - 3-bromo-2-chloropentane
1. **Identify the carbon chain**: The name indicates a pentane, which means there are 5 carbon atoms in the longest chain.
2. **Number the carbon chain**: Start numbering from the end closest to the first substituent (bromo or chloro).
- 1: CH3
- 2: CH
- 3: CH
- 4: CH2
- 5: CH3
3. **Assign substituents**:
- Bromo at position 3
- Chloro at position 2
4. **Check the order**: According to IUPAC rules, substituents should be listed in alphabetical order. Since "bromo" comes before "chloro", the name is correctly written as 3-bromo-2-chloropentane.

**Conclusion**: This name is correct.

### Step 2: Analyze the second name - pentoene-4-yne
1. **Identify the carbon chain**: The name suggests a pentane structure with both double and triple bonds.
2. **Number the carbon chain**:
- If we start from the end with the double bond, we have:
- 1: CH
- 2: CH
- 3: CH
- 4: CH2
- 5: CH3
3. **Assign double and triple bonds**:
- The double bond is at position 2 and the triple bond at position 4.
4. **Check the naming**:
- The correct way to name this compound is to give the double bond the lowest number possible, which means it should be named as pent-3-ene-1-yne (not pentoene-4-yne).

**Conclusion**: This name is incorrect.

### Step 3: Analyze the third name - 3-bromobutane-2-ole
1. **Identify the carbon chain**: The name indicates a butane structure with a hydroxyl (OH) group.
2. **Number the carbon chain**:
- Start numbering from the end closest to the OH group:
- 1: CH2 (with OH)
- 2: CH
- 3: CH
- 4: CH3
3. **Assign substituents**:
- Bromo at position 3 and OH at position 2.
4. **Check the order**: The name should be 3-bromobutan-2-ol (the suffix should be "ol" for alcohol).

**Conclusion**: This name is correct.

### Step 4: Analyze the fourth name - 1-aminopentane-2-thiol
1. **Identify the carbon chain**: The name indicates a pentane structure with an amino (NH2) group and a thiol (SH) group.
2. **Number the carbon chain**:
- Start numbering from the end closest to the thiol group:
- 1: CH2 (with SH)
- 2: CH (with NH2)
- 3: CH2
- 4: CH2
- 5: CH3
3. **Assign substituents**:
- Amino at position 1 and thiol at position 2.
4. **Check the order**: The correct name should be 2-thiol-1-aminopentane.

**Conclusion**: This name is correct.

### Final Answer
The incorrect IUPAC name from the options provided is **pent-4-yne-2-ene**.