Q.Match the structures of compounds given in Column I with the classes of compounds given in Column II.
Column I:

The key idea is Structural Isomerism — specifically, classifying organic halides based on the hybridisation of the carbon directly bonded to the halogen.

Reasoning:

1. In (i), the halogen is attached to an $sp^3$-hybridised carbon of an alkane chain. This is an alkyl halide.
2. In (ii), the halogen is on a carbon adjacent to a $\mathrm{C=C}$ double bond (the allylic position). This is an allyl halide. …

This question tests your grasp of structural isomerism in organic halides — specifically, how the position of the halogen relative to a double bond or an aromatic ring changes the compound’s classification. The names (alkyl, vinyl, allyl, aryl) aren’t arbitrary; they tell you exactly what kind of carbon the halogen is bonded to.

Let’s break each one down.

1. Compound (i): $\mathrm{CH_3-\underset{\underset{\displaystyle X}{|}}{CH}-CH_3}$

   The halogen X is attached to a carbon that is sp³ hybridized and part of a saturated chain (no double bonds, no aromatic ring). This carbon is simply an alkyl carbon.

   → This is an alkyl halide.

   Match: (i) → (b).

2. Compound (ii): $\mathrm{CH_2{=}CH-CH_2-X}$

   Here, X is attached to a carbon that is one carbon away from a C=C double bond. That carbon is sp³ hybridized, but it’s “allylic” — meaning it’s adjacent to a vinylic carbon. The name allyl halide specifically refers to a halogen on a carbon next to a double bond.

   → This is an allyl halide.

   Match: (ii) → (d).

3. Compound (iii): $\mathrm{C_6H_5-X}$ (halobenzene)

   The halogen is directly attached to a carbon of a benzene ring. That carbon is part of an aromatic system (sp² hybridized, but in a ring with delocalized π electrons). The term aryl halide is used when X is bonded directly to an aromatic ring.

   → This is an aryl halide.

   Match: (iii) → (a).

4. Compound (iv): $\mathrm{CH_2{=}CH-X}$ …

Method: Functional Group Classification Based on Hybridisation and Attachment

Concept: The class of a halide depends on the hybridisation of the carbon to which the halogen is attached, and whether that carbon is part of an aromatic ring or an unsaturated chain.

Steps

1. Identify the carbon directly bonded to X (the halogen).

2. Check the hybridisation of that carbon:

   • sp³ → alkyl or allyl (check further)
   • sp² → vinyl or aryl (check further)

3. Check the immediate environment:

   • If the carbon is sp³ and part of a saturated chain → Alkyl halide
   • If the carbon is sp³ but adjacent to a C=C double bond → Allyl halide
   • If the carbon is sp² and part of a benzene ring → Aryl halide
   • If the carbon is sp² and part of a C=C double bond (not aromatic) → Vinyl halide

Applying to the given compounds

| Compound | Carbon bonded to X | Hybridisation | Adjacent to C=C? | Class |

|---|---|---|---|---| …

This is a classic matching problem from organic chemistry that tests your understanding of halide classification based on the hybridisation of the carbon attached to the halogen.

Let’s first get the correct match, then list the common mistakes.

✓ Correct Matching

✗ Common Mistakes & How to Avoid Them

1. Confusing Allyl and Vinyl halides

• Mistake: Matching (ii) $\mathrm{CH_2{=}CH-CH_2-X}$ with “Vinyl halide” and (iv) $\mathrm{CH_2{=}CH-X}$ with “Allyl halide”.
• Why it happens: Students see a double bond and assume “vinyl” without checking where the halogen is.
• How to avoid:
  • Vinyl halide = halogen directly on the double-bond carbon ($sp^2$).
  • Allyl halide = halogen on the carbon next to the double bond ($sp^3$).
  • Draw the structure and circle the carbon attached to X — is it part of the C=C or one bond away?

2. Calling halobenzene an “Alkyl halide”

• Mistake: Matching (iii) $\mathrm{C_6H_5-X}$ with “Alkyl halide”.
• Why it happens: Students think “benzene ring = carbon chain” or forget that “alkyl” means saturated, open-chain.
• How to avoid:
  • Alkyl halide = X on $sp^3$ carbon of an aliphatic (non-aromatic) chain.
  • Aryl halide = X directly on an aromatic ring ($sp^2$).
  • Remember: benzene is not an alkyl group — it’s an aryl group.

3. Misidentifying (i) as a “Vinyl” or “Allyl” halide

• Mistake: Matching (i) $\mathrm{CH_3-\underset{\underset{\displaystyle X}{|}}{CH}-CH_3}$ with “Vinyl” or “Allyl”.
• Why it happens: Students see a branched structure and think “something special” — or confuse it with isopropyl halide being “different”.
• How to avoid:
  • Check for a double bond — there is none. …