The alkane that gives only one monochloro product on chlorination with $Cl_2$ in presence of diffused sunlight

Question

The alkane that gives only one monochloro product on chlorination with $Cl_2$ in presence of diffused sunlight is:

Accepted Answer

1. **Principle of Monochlorination:** Free radical chlorination replaces a hydrogen atom with a chlorine atom. The number of isomeric monochloro products formed depends on the number of **chemically non-equivalent sets of hydrogen atoms** present in the alkane molecule [NCERT 11th, Ch 13, Sec 13.5.2; NCERT 12th, Ch 10, Ex 10.3]. If an alkane has only one type of hydrogen, it yields a single monochloro product.
2. **Analyze Option B (Neopentane / 2,2-Dimethylpropane):**
   - Structure: A central quaternary carbon bonded to four methyl groups ($C(CH_3)_4$).
   - Symmetry: All 12 hydrogen atoms belong to identical methyl groups attached to the same central atom. They are chemically equivalent.
   - Product: Replacing any one of the 12 hydrogens results in the same compound: **1-chloro-2,2-dimethylpropane**.
3. **Analyze Other Options:**
   - **n-Pentane ($CH_3CH_2CH_2CH_2CH_3$):** Has 3 sets of equivalent hydrogens ($C_1/C_5$, $C_2/C_4$, and $C_3$), yielding **3 isomers**.
   - **Isopentane (2-Methylbutane):** Has 4 different sets of hydrogens, yielding **4 isomers**.
   - **2,2-Dimethylbutane:** Has 3 different sets of hydrogens, yielding **3 isomers**.
4. **Conclusion:** Neopentane is the only alkane listed that yields a single monochloro product.

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