Pick up the error an write the correct IUPAC names for the following.
(i) 3-Pentyne (ii) 1, 6-Hexadiene (iii) 2-Ethyl-2-pentene (iv) 1, 2-Dihydroxyethane (v) Pent-4-en-1-yne.

Answer

To determine the correct IUPAC names for the given compounds, we will analyze each one step by step.

### Step 1: Analyze 3-Pentyne
- The compound has 5 carbon atoms and a triple bond.
- The correct numbering should prioritize the triple bond.
- If we number from the left, the triple bond is on the 2nd carbon.
- Therefore, the correct name is **2-Pentyne**.

### Step 2: Analyze 1,6-Hexadiene
- This compound has 6 carbon atoms and two double bonds.
- The correct numbering should start from the end closest to the first double bond.
- The double bonds are at positions 1 and 5, not 1 and 6.
- Therefore, the correct name is **1,5-Hexadiene**.

### Step 3: Analyze 2-Ethyl-2-pentene
- This compound has 5 carbon atoms in the main chain and an ethyl group as a substituent.
- The main chain can be extended to include the ethyl group, making it a 6-carbon chain.
- The double bond is on the 3rd carbon when considering the longest chain.
- Therefore, the correct name is **3-Ethyl-1-Hexene**.

### Step 4: Analyze 1,2-Dihydroxyethane
- This compound has 2 carbon atoms and two hydroxyl (-OH) groups.
- The correct name for a 2-carbon alcohol with two hydroxyl groups is **Ethylene glycol** or **1,2-Ethanediol**.

### Step 5: Analyze Pent-4-en-1-yne
- This compound has both a double bond and a triple bond.
- The correct numbering should prioritize the double bond over the triple bond.
- The double bond is on the 4th carbon, and the triple bond is on the 1st carbon.
- Therefore, the correct name is **1-Pentyn-4-ene**.

### Summary of Correct IUPAC Names:
1. 3-Pentyne → **2-Pentyne**
2. 1,6-Hexadiene → **1,5-Hexadiene**
3. 2-Ethyl-2-pentene → **3-Ethyl-1-Hexene**
4. 1,2-Dihydroxyethane → **1,2-Ethanediol** (or Ethylene glycol)
5. Pent-4-en-1-yne → **1-Pentyn-4-ene**