Q.Haloalkanes contain halogen atom(s) attached to the $sp^3$ hybridised carbon atom of an alkyl group. Identify haloalkane from the following compounds. (Two or more than two options may be correct.)

The key idea is that the definition given in the question has two conditions: the halogen must sit on an $sp^3$ hybridised carbon, AND that carbon must belong to an alkyl group — a group derived from an alkane, containing only single-bonded carbon and hydrogen.

Reasoning:

1. 2-Bromopentane — Br on C2 of a plain pentane chain: an $sp^3$ carbon of a genuine alkyl group. A haloalkane. ✓
2. Vinyl chloride ($\mathrm{CH_2{=}CHCl}$) — Cl sits directly on an $sp^2$ carbon of the C=C double bond: a vinylic halide, not a haloalkane. ✗ …

The question's own opening line is the definition to apply: "Haloalkanes contain halogen atom(s) attached to the $sp^3$ hybridised carbon atom of an alkyl group." Notice that this is a two-part test — hybridisation AND the kind of group the carbon belongs to. An alkyl group is obtained from an alkane by removing one hydrogen; it contains only $sp^3$ carbons and hydrogens, with no double bonds, rings of the aromatic type, or other functional groups built into it.

Let us examine each option against both parts.

1. 2-Bromopentane

   The structure is $\mathrm{CH_3{-}CHBr{-}CH_2{-}CH_2{-}CH_3}$. The bromine is on C2 of a plain saturated pentane chain: the carbon is $sp^3$ (four single bonds) and the group it belongs to is a genuine alkyl group. Both conditions pass — a textbook haloalkane.

2. Vinyl chloride (chloroethene)

   The structure is $\mathrm{CH_2{=}CHCl}$. The chlorine is attached to a carbon of the C=C double bond — an $sp^2$ carbon. The very first condition fails; compounds of this kind are vinylic halides, a class of their own with quite different (much lower) reactivity toward substitution. Not a haloalkane.

3. 2-Chloroacetophenone

   Acetophenone is $\mathrm{C_6H_5{-}CO{-}CH_3}$, numbered with C1 = the carbonyl carbon and C2 = the methyl carbon, so "2-chloro" places the chlorine on the side-chain carbon: $\mathrm{C_6H_5{-}CO{-}CH_2Cl}$ (phenacyl chloride). That $\mathrm{CH_2}$ carbon is $sp^3$ — but look at what it is attached to: a ketone carbonyl. It is part of the phenacyl (α-keto) framework of an aromatic ketone, not of a group derived from an alkane. The compound's parent is a ketone, and its class is α-halo ketone — its chemistry (activated toward substitution by the adjacent C=O) is taught with carbonyl compounds, not with haloalkanes. It passes the hybridisation half of the test but fails the "alkyl group" half, so it is not counted a haloalkane. …

Concept: The two-part definition of a haloalkane

The question states the definition itself: halogen attached to an $sp^3$ hybridised carbon of an alkyl group. Both parts must hold:

• Part 1 — hybridisation: the carbon bearing X must be $sp^3$ (four single bonds). This excludes vinylic halides (X on a C=C carbon) and aryl halides (X on an aromatic-ring carbon).
• Part 2 — the group: that carbon must belong to an alkyl group, i.e. a group derived from an alkane (only C and H, all single bonds). This excludes carbons built into another functional framework — such as the α-carbon of a ketone.

Method: Two-Condition Check

Steps

1. Locate the carbon bonded to the halogen (draw the structure if the name needs decoding).
2. Check Part 1: is that carbon $sp^3$? If not → not a haloalkane (vinylic/aryl halide).
3. Check Part 2: is the carbon part of a plain alkane-derived group, or is it attached into a carbonyl/aromatic/alkene functional framework? If the latter → the compound belongs to that other class (e.g. α-halo ketone), not to the haloalkanes.

Applying to each option:

(i) 2-Bromopentane — $\mathrm{CH_3{-}CHBr{-}CH_2{-}CH_2{-}CH_3}$

C2 bears Br: $sp^3$ ✓, plain pentane chain ✓.

✓ Haloalkane …

Here are the common mistakes students make when identifying haloalkanes from a list of compounds, along with how to avoid each.

Mistake 1: Stopping at "the carbon is $sp^3$" — and selecting 2-chloroacetophenone

• The Mistake: Students check the hybridisation of the halogen-bearing carbon, find it $sp^3$, and immediately tick the option. For 2-chloroacetophenone ($\mathrm{C_6H_5{-}CO{-}CH_2Cl}$, phenacyl chloride), the $\mathrm{CH_2Cl}$ carbon is indeed $sp^3$ — so this trap is very easy to fall into.
• Why it's wrong: The definition in the question has a second clause: the carbon must be part of an alkyl group — a group derived from an alkane. The $\mathrm{CH_2Cl}$ carbon here is bonded directly to a ketone carbonyl; it belongs to the phenacyl (α-keto) framework of an aromatic ketone, so the compound is classified as an α-halo ketone, not a haloalkane. It passes the hybridisation check but fails the alkyl-group check.
• How to Avoid: Apply BOTH halves of the definition every time: (1) $sp^3$? (2) part of a plain alkane-derived group? A halogen on a carbon that is welded onto another functional group's framework (a C=O here) puts the compound in that other family.

Mistake 2: Selecting vinyl chloride "because it contains chlorine"

• The Mistake: Any molecule with a halogen gets labelled a haloalkane.
• Why it's wrong: In vinyl chloride ($\mathrm{CH_2{=}CHCl}$) the chlorine sits directly on a carbon of the C=C double bond — an $sp^2$ carbon. That makes it a vinylic halide, a separate class with very different reactivity (the C–Cl bond has partial double-bond character and resists substitution).
• How to Avoid: Always find the exact carbon holding the halogen and check its hybridisation first. X on an $sp^2$ carbon (alkene or aromatic ring) is never a haloalkane.

Mistake 3: Wrongly excluding trichloromethane

• The Mistake: Students exclude $\mathrm{CHCl_3}$ because it carries three halogens, or because the familiar name "chloroform" doesn't sound like an alkyl halide.
• Why it's wrong: Trichloromethane is methane with three hydrogens replaced by chlorine — a halogen derivative of the simplest alkane. Its carbon is $sp^3$ and alkane-derived, so it satisfies the definition completely. The question itself allows "halogen atom(s)", plural. …