An unknown compound is analyzed to have a molecular mass of 84 and elements has carbon and hydrogen only.When subjected to chlorination in the presence of light, three monochlorinated products are isolated . This compound must be

Options

A. methylcyclopentane
B. cyclohexane
C. hexane
D. 1,3-dimethylcyclobutane

Answer

To determine the unknown compound based on the provided information, we can follow these steps:

### Step 1: Analyze the Molecular Mass
The molecular mass of the unknown compound is given as 84 g/mol. Since the compound consists only of carbon (C) and hydrogen (H), we can use the general formula for alkanes, which is CₙH₂ₙ₊₂.

### Step 2: Determine Possible Hydrocarbon Formula
From the molecular mass (84 g/mol), we can set up the equation:
- Let the number of carbon atoms be \( n \).
- The molecular mass can be expressed as \( 12n + 1(2n + 2) = 84 \).

This simplifies to:
\[ 12n + 2n + 2 = 84 \]
\[ 14n + 2 = 84 \]
\[ 14n = 82 \]
\[ n = \frac{82}{14} \approx 5.857 \]

Since \( n \) must be a whole number, we can try different values of \( n \) to find a suitable hydrocarbon.

### Step 3: Check Possible Values for n
1. **If \( n = 6 \)**:
- Formula: C₆H₁₄
- Molecular mass: \( 6 \times 12 + 14 = 72 + 14 = 86 \) (not suitable)

2. **If \( n = 5 \)**:
- Formula: C₅H₁₂
- Molecular mass: \( 5 \times 12 + 12 = 60 + 12 = 72 \) (not suitable)

3. **If \( n = 4 \)**:
- Formula: C₄H₁₀
- Molecular mass: \( 4 \times 12 + 10 = 48 + 10 = 58 \) (not suitable)

4. **If \( n = 7 \)**:
- Formula: C₇H₁₄
- Molecular mass: \( 7 \times 12 + 16 = 84 \) (suitable)

### Step 4: Identify the Structure
The compound with the formula C₇H₁₄ is heptane. However, we need to check for structural isomers that can give three monochlorinated products.

### Step 5: Analyze Chlorination Products
When subjected to chlorination in the presence of light, we need to consider the possible positions for chlorine substitution:
- For heptane, we can have different isomers that allow for three unique monochlorinated products.

### Step 6: Identify the Correct Compound
After analyzing the possible structures, **1,3-dimethylcyclobutane** fits the criteria:
- It has a molecular mass of 84 g/mol.
- It contains only carbon and hydrogen.
- It can yield three distinct monochlorinated products when chlorinated.

### Conclusion
The unknown compound must be **1,3-dimethylcyclobutane**.

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