Total number of structural isomers possible from molecular formula `C_(8)H_(18)` that contain `7` carbons in the parent chain are:

Options

A. 3
B. 4
C. 5
D. 6

Explanation

To determine the total number of structural isomers possible from the molecular formula \( C_8H_{18} \) that contain 7 carbons in the parent chain, we can follow these steps:

### Step 1: Understand the Molecular Formula
The molecular formula \( C_8H_{18} \) indicates that we are dealing with an alkane, which follows the general formula \( C_nH_{2n+2} \). For \( n = 8 \), the formula is satisfied, confirming that we have a saturated hydrocarbon.

**Hint:** Remember that alkanes are saturated hydrocarbons with single bonds only.

### Step 2: Draw the Parent Chain
We need to establish a parent chain with 7 carbon atoms. This will be our main chain, and we will denote it as follows:

```
C1 - C2 - C3 - C4 - C5 - C6 - C7
```

**Hint:** Visualize the carbon chain as a straight line of carbon atoms.

### Step 3: Identify the Remaining Carbon
Since we have 8 carbons in total and 7 are in the parent chain, we have 1 carbon atom left to place as a substituent.

**Hint:** Count the total number of carbons to ensure you are using all available carbons.

### Step 4: Determine Possible Substituent Positions
Now, we can place the remaining carbon atom (as a methyl group, \( -CH_3 \)) at different positions along the parent chain. The possible positions for the substituent are:
- Position 2
- Position 3
- Position 4

**Hint:** The substituent can be placed on any carbon in the chain except the terminal ones if you want to avoid redundancy.

### Step 5: Analyze the Substituent Positions
1. **Substituent at Position 2**:
- Structure:
```
CH3 - C - C - C - C - C - C - CH3
|
CH3
```
2. **Substituent at Position 3**:
- Structure:
```
CH3 - C - C - C - C - C - C - CH3
|
CH3
```
3. **Substituent at Position 4**:
- Structure:
```
CH3 - C - C - C - C - C - C - CH3
|
CH3
```

### Step 6: Count Unique Isomers
- The substituent at position 2 is unique.
- The substituent at position 3 is unique.
- The substituent at position 4 is equivalent to position 2 due to symmetry (the chain can be flipped).

Thus, the unique structural isomers are:
1. Substituent at position 2
2. Substituent at position 3
3. Substituent at position 4 (equivalent to position 2)

**Hint:** Remember to check for symmetry and equivalent positions to avoid counting duplicates.

### Conclusion
The total number of structural isomers possible from the molecular formula \( C_8H_{18} \) that contain 7 carbons in the parent chain is **3**.

**Final Answer:** **3** (Option A)