Give the common name for the alkyl halide with the structure (CH3)2CH–CH2Br, that is, a four carbon chain where bromine is attached to the CH2 group of the isobutyl fragment.

Introduction / Context:
In organic chemistry, many compounds have both systematic IUPAC names and older common names based on traditional naming of alkyl groups. Understanding the common names of simple alkyl halides is useful because they frequently appear in textbooks, examination questions, and synthetic examples. This question presents the structural formula (CH3)2CH–CH2Br and asks you to identify its common name among several choices that refer to different butyl bromide isomers and other alkyl bromides.

Given Data / Assumptions:

• The structural formula is (CH3)2CH–CH2Br, which has four carbon atoms in total.
• The carbon attached directly to bromine is part of a CH2 group connected to a branched three carbon chain (isopropyl group).
• Common names like n-butyl bromide, sec-butyl bromide, isobutyl bromide, and tert-butyl bromide refer to different arrangements of the four carbon atoms.
• Methyl bromide and propyl bromide would have one and three carbon atoms respectively, not four.

Concept / Approach:
The general idea in common naming of butyl groups is to identify the arrangement of the carbon chain and the point of attachment. The normal butyl (n-butyl) group is a straight four carbon chain, CH3–CH2–CH2–CH2–. The sec-butyl group attaches at the second carbon of a straight chain. The isobutyl group can be thought of as an isopropyl group (CH3)2CH– attached to a CH2– group that carries the functional group. The given structure (CH3)2CH–CH2Br matches the isobutyl pattern because the CH2Br group is attached to a carbon bearing two CH3 groups. Therefore, its common name is isobutyl bromide.

Step-by-Step Solution:
Step 1: Count the total number of carbon atoms in (CH3)2CH–CH2Br. There are two CH3 groups (2 carbons), one CH group (1 carbon), and one CH2 group attached to bromine (1 carbon), giving a total of 4 carbons. Step 2: Recognise that with four carbons and a bromine atom, this compound is one of the butyl bromide isomers if named by common names. Step 3: Draw the structure in line form. The CH2Br group is at one end, attached to a CH that also carries two CH3 groups. This indicates a branching pattern rather than a straight chain. Step 4: Recall that the isobutyl group is defined as (CH3)2CH–CH2–, where the functional group is attached at the CH2– end, exactly as in the given structure. Step 5: Conclude that the common name of (CH3)2CH–CH2Br is isobutyl bromide.

Verification / Alternative check:
Compare with other possible butyl bromide structures. n-Butyl bromide would have the structure CH3–CH2–CH2–CH2Br, a straight chain with bromine at the terminal carbon, which does not match the branching seen in (CH3)2CH–CH2Br. sec-Butyl bromide would have bromine attached to the second carbon of a straight chain, for example CH3–CHBr–CH2–CH3. tert-Butyl bromide would have bromine attached to a carbon bearing three CH3 groups, (CH3)3C–Br. None of these match the CH2Br unit attached to an isopropyl group. The unique match is isobutyl bromide, confirming the answer.

Why Other Options Are Wrong:
Methyl bromide would have only one carbon, CH3Br, and propyl bromide would have three carbons, C3H7Br, so both have fewer carbons than the given structure. n-Butyl bromide has four carbons in a straight chain without branching, which does not resemble (CH3)2CH–CH2Br. sec-Butyl bromide places bromine on a secondary carbon in a straight chain, not on a CH2 attached to an isopropyl group. These differences in carbon skeleton and point of attachment mean that none of these names correctly describe the given structure.

Common Pitfalls:
Students often mix up the various butyl group names because they all have four carbons and similar formulas. A typical mistake is to pick n-butyl bromide for any four-carbon bromide or to confuse sec-butyl and isobutyl. To avoid confusion, it is helpful to memorise simple patterns: n-butyl is straight chain, sec-butyl has bromine on the second carbon of a straight chain, isobutyl has a CH2– group attached to an isopropyl fragment, and tert-butyl has the functional group attached to a carbon with three methyl groups. Matching these patterns to the given structure leads reliably to the correct name.

Final Answer:
The common name of the compound (CH3)2CH–CH2Br is isobutyl bromide.