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 C8H18, where there are 5 carbons in the main chain and methyl groups as side chains, we will follow these steps:

### Step 1: Understand the molecular formula
The molecular formula C8H18 indicates that we are dealing with an alkane (since it follows the general formula CnH2n+2). Here, n = 8, which means we have 8 carbon atoms and 18 hydrogen atoms.

### Step 2: Establish the main chain
We need to have a main chain consisting of 5 carbon atoms. This can be represented as a pentane chain:
```
C-C-C-C-C
```
This is our main chain (pentane).

### Step 3: Identify remaining carbons
Since we have 8 carbon atoms in total and 5 are in the main chain, we have 3 carbon atoms left to be used as methyl groups (−CH3) as side chains.

### Step 4: Determine possible positions for methyl groups
The methyl groups can be attached to the main chain at the available positions. The positions for substituents on a 5-carbon chain (pentane) are:
- Carbon 1
- Carbon 2
- Carbon 3
- Carbon 4
- Carbon 5

However, substituents on Carbon 1 and Carbon 5 are equivalent due to symmetry, so we will only consider positions 2, 3, and 4 for placing the methyl groups.

### Step 5: Generate structural isomers
We can now generate the structural isomers by placing the 3 methyl groups at different positions. The possible combinations are:
1. **2,2-Dimethylpentane**: Two methyl groups on Carbon 2 and one on Carbon 3.
2. **2,3-Dimethylpentane**: One methyl group on Carbon 2 and one on Carbon 3, and one on Carbon 4.
3. **3,3-Dimethylpentane**: Two methyl groups on Carbon 3 and one on Carbon 2.
4. **2,2,3-Trimethylpentane**: Two methyl groups on Carbon 2 and one on Carbon 3.

### Step 6: Count the isomers
After analyzing the positions and combinations, we find that there are 4 distinct structural isomers:
1. 2,2-Dimethylpentane
2. 2,3-Dimethylpentane
3. 3,3-Dimethylpentane
4. 2,2,3-Trimethylpentane

### Conclusion
Thus, the total number of structurally isomeric compounds possible with the molecular formula C8H18, containing 5 carbons in the main chain and methyl groups as side chains, is **4**.

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