Give the common name for the following compound (CH$_{3}$)$_{2}$ CH CH$_{2}$ -Br

Analysis of the Question

The compound given is $(CH_{3})_{2}CHCH_{2}-Br$. We need to identify its common (trivial) name based on the arrangement of the carbon chain and the position of the functional group (Bromine).

Concept Explanation

In organic chemistry, common naming conventions use prefixes like n- (normal), iso-, and neo- to describe the structure of alkyl groups:

• n- (normal): A straight chain of carbons.
• iso-: A structure where there is a methyl group ($-CH_{3}$) attached to the second-to-last carbon atom of the chain.
• Butyl group: Contains 4 carbon atoms.

Looking at the structure $(CH_{3})_{2}CHCH_{2}-Br$:

1. Count the total number of carbons: There are 3 carbons in the $(CH_{3})_{2}CH$ part and 1 carbon in the $CH_{2}$ part, totaling 4 carbons. This makes it a butyl derivative.
2. Examine the branching: The structure has a $(CH_{3})_{2}CH-$ group at the end of the chain. This specific branching pattern (a "V" shape at the end of the chain) is the hallmark of an isobutyl group.

Step-by-Step Solution

1. Expand the structure: $CH_{3} - CH(CH_{3}) - CH_{2} - Br$
2. Identify the alkyl group: The carbon attached to the Bromine ($CH_{2}$) is connected to a $CH$ group, which is further branched with two methyl groups. This configuration is defined as isobutyl.
3. Attach the functional group: Since the functional group is Bromine ($-Br$), the compound is named isobutyl bromide.

Comparison with Options

• A. Isobutyl bromide: Matches our structural analysis.
• B. Methyl bromide: Incorrect; this would be $CH_{3}Br$.
• C. Propyl bromide: Incorrect; this would have only 3 carbons.
• D. Butyl bromide: This usually refers to n-butyl bromide ($CH_{3}CH_{2}CH_{2}CH_{2}Br$), which is a straight chain.

Exam Tip

When naming alkyl halides, always count the total number of carbons first to determine the root (methyl, ethyl, propyl, butyl). Then, look at the branching: if you see a $(CH_{3})_{2}CH-$ group at the end, it is almost certainly an "iso" compound!