How many asymmetric carbon atoms are present in -
(i) 1,2-dimethyl cyclohexane
(ii) 3-methyl cyclopentene
(iii) 3-methyl cyclohexene

Options

A. Two, one, one
B. One, one one
C. Two, none, two
D. Two, none, one

Answer

To determine the number of asymmetric (chiral) carbon atoms in the given compounds, we will analyze each compound step by step.

### Step 1: Analyze 1,2-Dimethylcyclohexane

1. **Structure Drawing**: Draw the cyclohexane ring and add two methyl (CH₃) groups at the 1 and 2 positions.

```
CH3
|
CH2 - C - CH2
|
CH3
```

2. **Identify Carbon Atoms**: In cyclohexane, all carbon atoms are connected to two hydrogens (CH₂) except for the carbons where the methyl groups are attached.

3. **Check Chirality**:
- For the carbon at position 1 (attached to CH₃ and CH₂), it has:
- CH₃ (methyl group)
- CH₂ (the rest of the ring)
- H (hydrogen)
- For the carbon at position 2 (attached to CH₃ and CH₂), it has:
- CH₃ (methyl group)
- CH₂ (the rest of the ring)
- H (hydrogen)
- Both of these carbons have four different groups attached, making them chiral.

4. **Conclusion**: There are **2 chiral centers** in 1,2-dimethylcyclohexane.

### Step 2: Analyze 3-Methylcyclopentene

1. **Structure Drawing**: Draw the cyclopentene ring and add a methyl group at the 3 position.

```
CH2
|
CH2 - C = CH
|
CH3
```

2. **Identify Carbon Atoms**: In cyclopentene, there are five carbon atoms, with one double bond.

3. **Check Chirality**:
- For the carbon at position 3 (attached to CH₃ and CH₂), it has:
- CH₃ (methyl group)
- C (the double bond)
- H (hydrogen)
- The carbon at position 2 (double bond) is not chiral because it is bonded to two hydrogens (CH₂).
- The carbon at position 3 is chiral because it has four different groups.

4. **Conclusion**: There is **1 chiral center** in 3-methylcyclopentene.

### Step 3: Analyze 3-Methylcyclohexene

1. **Structure Drawing**: Draw the cyclohexene ring and add a methyl group at the 3 position.

```
CH2
|
CH2 - C = CH
|
CH3
```

2. **Identify Carbon Atoms**: In cyclohexene, there are six carbon atoms, with one double bond.

3. **Check Chirality**:
- For the carbon at position 3 (attached to CH₃ and CH₂), it has:
- CH₃ (methyl group)
- CH₂ (the rest of the ring)
- H (hydrogen)
- The carbon at position 2 (double bond) is not chiral because it is bonded to two hydrogens.
- The carbon at position 3 is chiral because it has four different groups.

4. **Conclusion**: There is **1 chiral center** in 3-methylcyclohexene.

### Final Summary of Chiral Centers:
- **1,2-Dimethylcyclohexane**: 2 chiral centers
- **3-Methylcyclopentene**: 1 chiral center
- **3-Methylcyclohexene**: 1 chiral center