Question

How many monochloro structural isomers are expected in free radical monochlorination of 2-methylbutane?

• A. 2
• B. 3
• C. 4
• D. 5
• E. 6

Hint:

To find isomers quickly, look for symmetry. 2-methylbutane has no plane of symmetry except for the two methyls on the same carbon. Therefore, all other carbons are unique "targets" for chlorine.

Answer 1

The Correct Option is C

Concept: Monochlorination via a free radical mechanism replaces one hydrogen atom with one chlorine atom. The number of structural isomers depends on the number of non-equivalent sets of hydrogen atoms in the molecule.
• Structural Isomers: Only consider different connectivity; ignore stereoisomers (optical/geometrical) unless specified.
• Equivalent Hydrogens: Hydrogens attached to carbons that are in identical chemical environments.

Step 1: Analyze the structure of 2-methylbutane. Structure: $(CH_3)_2CH-CH_2-CH_3$ Let's label the carbons: 1. Two terminal methyl groups on the left are equivalent (C1 and the branch). Replacing any H here gives 1-chloro-2-methylbutane. (Set 1) 2. The CH group (C2). Replacing this H gives 2-chloro-2-methylbutane. (Set 2) 3. The $CH_2$ group (C3). Replacing an H here gives 2-chloro-3-methylbutane (or 3-chloro-2-methylbutane). (Set 3) 4. The terminal methyl group on the right (C4). Replacing an H here gives 1-chloro-3-methylbutane. (Set 4)

Step 2: Count the distinct products. There are 4 distinct chemical environments for the hydrogens, leading to 4 distinct structural isomers.