Question

Which one of the following is not an allylic halide?

• A. 4-bromopent-2-ene
• B. 3-bromo-2-methylbut-1-ene
• C. 1-bromobut-2-ene
• D. 4-bromobut-1-ene
• E. 3-bromo-2-methylpropene

Hint:

To identify an allylic halide, look for the pattern \(C=C-C-X\). If there is an extra carbon between the double bond and the carbon holding the halogen (\(C=C-C-C-X\)), it is no longer allylic.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
The problem requires identifying which compound does not fit the classification of an "allylic halide".
An allylic halide is a specific type of organic halide where the halogen atom (like bromine, chlorine) is bonded to an $sp^3$-hybridized carbon atom that is immediately adjacent to a carbon-carbon double bond ($C=C$).
The general structure is $C=C-C-X$, where the $-C-X$ carbon is the allylic carbon.
Step 2: Key Formula or Approach:
To classify these compounds, follow these steps for each option:
1. Draw the structural formula based on the IUPAC name.
2. Locate the halogen atom (bromine in this case).
3. Check the hybridization of the carbon bonded to the halogen (it must be $sp^3$, meaning it has all single bonds).
4. Check if that specific $sp^3$ carbon is directly attached to a carbon atom that is part of a double bond.
Step 3: Detailed Explanation:
Let's analyze each option systematically:
(a) 4-bromopent-2-ene: The structure is $CH_3-CH=CH-CH(Br)-CH_3$.
The Br is attached to C-4. Carbon-4 is $sp^3$ hybridized and is directly adjacent to C-3, which is part of the $C=C$ double bond. Therefore, this is an allylic halide.
(b) 3-bromo-2-methylbut-1-ene: The structure is $CH_2=C(CH_3)-CH(Br)-CH_3$.
The Br is attached to C-3. Carbon-3 is $sp^3$ hybridized and is directly adjacent to C-2, which is part of the $C=C$ double bond. Therefore, this is an allylic halide.
(c) 1-bromobut-2-ene: The structure is $Br-CH_2-CH=CH-CH_3$.
The Br is attached to C-1. Carbon-1 is $sp^3$ hybridized and is directly adjacent to C-2, which is part of the $C=C$ double bond. Therefore, this is an allylic halide.
(d) 4-bromobut-1-ene: The structure is $CH_2=CH-CH_2-CH_2-Br$.
The Br is attached to C-4. Carbon-4 is $sp^3$ hybridized. However, it is adjacent to C-3, which is also an $sp^3$ carbon, not a double-bonded carbon. The double bond is between C-1 and C-2. Since the halogen-bearing carbon is separated from the double bond by another saturated carbon, this is an ordinary alkyl halide (specifically, a homoallylic halide), not an allylic halide.
(e) 3-bromo-2-methylpropene: The structure is $CH_2=C(CH_3)-CH_2-Br$.
The Br is attached to C-3. Carbon-3 is $sp^3$ hybridized and is directly adjacent to C-2, which is part of the $C=C$ double bond. Therefore, this is an allylic halide.
Step 4: Final Answer:
The compound 4-bromobut-1-ene is not an allylic halide.