Then number of structurally isomeric compound(s) possible with molecular formula `C_(8)H_(18)` containing `5` carbons in main chain having methyl group(s) as side chain are

Explanation

To determine the number of structurally isomeric compounds possible with the molecular formula \( C_8H_{18} \) that contain 5 carbons in the main chain and methyl groups as side chains, we can follow these steps:

### Step 1: Identify the Base Structure
The molecular formula \( C_8H_{18} \) indicates that it is an alkane because it follows the formula \( C_nH_{2n+2} \). Here, \( n = 8 \).

### Step 2: Establish the Main Chain
We need to have 5 carbons in the main chain. We can represent this as:
- Main chain: \( C_1 - C_2 - C_3 - C_4 - C_5 \)

### Step 3: Determine the Remaining Carbons
Since we have 8 carbons total and 5 in the main chain, we have \( 8 - 5 = 3 \) carbons left to be added as methyl groups (–CH₃) as side chains.

### Step 4: Positioning the Methyl Groups
We can place the 3 methyl groups on different positions of the main chain. The positions available for substitution are the 2nd, 3rd, and 4th carbons of the main chain.

### Step 5: Generate Isomers
Now we will generate the possible isomers by varying the positions of the methyl groups:

1. **Isomer 1**: Methyl groups on the 2nd, 3rd, and 4th carbons.
- Structure:
\[ CH_3 - CH(CH_3) - CH(CH_3) - CH(CH_3) - CH_2 \]

2. **Isomer 2**: Methyl groups on the 2nd and 3rd carbons, and one on the 4th carbon.
- Structure:
\[ CH_3 - CH(CH_3) - CH_2 - CH(CH_3) - CH_2 \]

3. **Isomer 3**: Methyl groups on the 2nd carbon and two on the 3rd carbon.
- Structure:
\[ CH_3 - CH(CH_3) - CH(CH_3) - CH_2 - CH_2 \]

4. **Isomer 4**: Methyl groups on the 3rd and 4th carbons, and one on the 2nd carbon.
- Structure:
\[ CH_3 - CH_2 - CH(CH_3) - CH(CH_3) - CH_2 \]

### Step 6: Count the Isomers
After considering all possible arrangements of the methyl groups while keeping the main chain intact, we find that there are **4 distinct structural isomers** for the given molecular formula \( C_8H_{18} \).

### Final Answer
The total number of structurally isomeric compounds possible is **4**.

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