Question

Which among the followings is an allylic halide?

• A. 1-Chloropropene
• B. 2-Chloropropene
• C. 3-Chloropropene
• D. 4-Chlorobut-1-ene

Hint:

To quickly distinguish between vinylic and allylic positions: remember that "Vinyl" means the halogen is glued directly onto the double bond (C=C-X), whereas "Allyl" means it is one single step away from the double bond (C=C-C-X).

Answer 1

The Correct Option is C

Step 1: Define an allylic halide.
In an allylic halide the halogen sits on an $sp^3$ carbon that is directly next to a C=C double bond, i.e. the pattern C=C-CH$_2$-X. Compare each option against this template.
Step 2: Examine 1-Chloropropene.
Here Cl is attached to a double-bond carbon itself ($\text{CH}_3\text{-CH=CH-Cl}$). That is a vinylic halide, not allylic.
Step 3: Examine 2-Chloropropene.
Cl is again on a double-bond carbon ($\text{CH}_3\text{-CCl=CH}_2$), so this too is vinylic.
Step 4: Examine 3-Chloropropene.
Structure $\text{Cl-CH}_2\text{-CH=CH}_2$: the Cl-bearing carbon is $sp^3$ and lies immediately beside the double bond. This is exactly the allylic pattern.
Step 5: Examine 4-Chlorobut-1-ene.
Structure $\text{Cl-CH}_2\text{-CH}_2\text{-CH=CH}_2$: the Cl carbon is one CH$_2$ removed from the double bond (homoallylic), so it does not qualify.
Step 6: Select the answer.
Only 3-Chloropropene has the halogen on the carbon adjacent to C=C, so it is the allylic halide - option (3).
\[ \boxed{\text{Allylic halide: 3-Chloropropene (option 3)}} \]