Q.Which of the following molecules is chiral in nature ? (A) 1-chloropropane (B) 2-chloropropane (C) 1-chlorobutane (D) 2-chlorobutane

Concept: A molecule is chiral if it has a non-superimposable mirror image, which almost always requires a carbon atom bonded to four different substituents (a stereocenter).

Reasoning:

1. 1-chloropropane ($\text{CH}_3\text{CH}_2\text{CH}_2\text{Cl}$): The second carbon is $\text{CH}_2$ (two H atoms), so no carbon has four different groups.
2. 2-chloropropane ($\text{CH}_3\text{CHClCH}_3$): The central carbon is $\text{CHCl}$ bonded to two identical methyl groups — not a stereocenter.
3. 1-chlorobutane ($\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{Cl}$): All internal carbons are $\text{CH}_2$ groups; no stereocenter. …

The key idea is the chirality centre (also called an asymmetric carbon or stereocentre). A carbon is chiral if it is sp³-hybridised and carries four different substituents. If any two substituents are the same, the molecule is achiral (it can be superimposed on its mirror image).

Let’s check each option systematically.

1. 1-chloropropane ($\mathrm{CH_3CH_2CH_2Cl}$)

   The carbon atoms are:

   • C1 (bonded to Cl, H, H, CH₂CH₃) — two H atoms → not chiral.
   • C2 (bonded to H, H, CH₃, CH₂Cl) — two H atoms → not chiral.
   • C3 (bonded to H, H, H, CH₂CH₂Cl) — three H atoms → not chiral. No carbon has four different groups. Achiral.

2. 2-chloropropane ($\mathrm{CH_3CHClCH_3}$)

   The central carbon (C2) is bonded to Cl, H, CH₃, and CH₃. Two of the groups are identical methyl groups. So it has a plane of symmetry and is achiral.

3. 1-chlorobutane ($\mathrm{CH_3CH_2CH_2CH_2Cl}$)

   All carbons are either primary or secondary with at least two identical substituents (usually two H atoms). No carbon has four different groups. Achiral.

4. 2-chlorobutane ($\mathrm{CH_3CHClCH_2CH_3}$)

   Look at C2: it is bonded to

   • Cl
   • H
   • CH₃ (methyl)
   • CH₂CH₃ (ethyl) …