Which of the following substances is not an isomer of 3-ethyl 2-methyl pentane ?

Options

A.
B.
C.
D. all are isomers

Explanation

To determine which substance is not an isomer of 3-ethyl-2-methylpentane, we first need to establish the molecular formula of 3-ethyl-2-methylpentane and then compare it with the molecular formulas of the given options.

### Step 1: Determine the Structure of 3-Ethyl-2-Methylpentane
- The name indicates that we have a pentane (5 carbon chain) with:
- An ethyl group (C2H5) at the 3rd carbon
- A methyl group (CH3) at the 2nd carbon

### Step 2: Draw the Structure
- The structure can be represented as follows:
```
CH3
|
CH3-CH-CH-CH2-CH3
|
CH2-CH3
```
- This structure shows that the main chain has 5 carbons, with a methyl group on the second carbon and an ethyl group on the third carbon.

### Step 3: Count the Atoms for the Molecular Formula
- Count the carbon (C) and hydrogen (H) atoms:
- Carbons: 8 (5 from the pentane chain + 1 from the methyl + 2 from the ethyl)
- Hydrogens: 18 (calculated from the structure)

Thus, the molecular formula for 3-ethyl-2-methylpentane is **C8H18**.

### Step 4: Analyze the Given Options
- For each option, we will count the number of carbon and hydrogen atoms to check if they match C8H18.

1. **Option 1**: Count the atoms.
- Carbons: 8
- Hydrogens: 18
- Molecular formula: **C8H18** (isomer)

2. **Option 2**: Count the atoms.
- Carbons: 7
- Hydrogens: 16
- Molecular formula: **C7H16** (not an isomer)

3. **Option 3**: Count the atoms.
- Carbons: 8
- Hydrogens: 18
- Molecular formula: **C8H18** (isomer)

### Step 5: Conclusion
- The only option that does not have the same molecular formula (C8H18) as 3-ethyl-2-methylpentane is **Option 2 (C7H16)**. Therefore, **Option 2 is not an isomer of 3-ethyl-2-methylpentane**.

### Final Answer
**Option 2** is not an isomer of 3-ethyl-2-methylpentane.