Step 1: Understanding the Concept:
A benzylic halide is a specific class of organic compound where the halogen atom is directly attached to an $sp^3$ hybridized carbon atom, which in turn is directly attached to an aromatic benzene ring. This specific bridging carbon is termed the benzylic carbon.
Step 2: Key Formula or Approach:
Approach: Examine the IUPAC name or structural formula of each compound to precisely locate the position of the halogen atom relative to the phenyl group.
Step 3: Detailed Explanation:
Let's draw and analyze the structures to check the attachment point of the bromine atom:
- (A) Bromophenylmethane: Commonly known as benzyl bromide. Structure: $\text{Ph-CH}_2\text{-Br}$. The bromine is attached to the $-\text{CH}_2-$ carbon, which is directly attached to the phenyl ($\text{Ph}$) ring. This is a primary benzylic halide.
- (B) 1-Bromo-1-phenylethane: Structure: $\text{Ph-CH(Br)-CH}_3$. The bromine is attached to Carbon-1. Because Carbon-1 is also directly attached to the phenyl ring, this is a secondary benzylic halide.
- (C) 2-Bromo-2-phenylpropane: Structure: $\text{Ph-C(CH}_3\text{)(Br)-CH}_3$. The bromine is on Carbon-2. The phenyl ring is also attached to Carbon-2. Thus, this is a tertiary benzylic halide.
- (D) 1-Bromo-2-phenylbutane: Structure: $\text{Ph-CH(CH}_2\text{CH}_3\text{)-CH}_2\text{-Br}$. Numbering starts from the carbon with the primary functional group (bromine). The bromine is on Carbon-1. The phenyl ring is attached to Carbon-2. Because the bromine is attached to an alkyl carbon that is one carbon atom further away from the benzylic position, this is a standard primary alkyl halide, not a benzylic halide.
Step 4: Final Answer:
1-Bromo-2-phenylbutane is not a benzylic halide.