Q.Out of 2-Bromobutane, 1-Bromobutane, 2-Bromopropane and 1-Bromopropane, the molecule which is chiral in nature is : (A) 2-Bromobutane (B) 1-Bromobutane (C) 2-Bromopropane (D) 1-Bromopropane

Concept: Chirality requires a carbon atom bonded to four different substituents (a stereocenter).

Reasoning:

1. 2-Bromobutane ($\mathrm{CH_3CHBrCH_2CH_3}$): The carbon bearing the bromine is attached to $\mathrm{H}$, $\mathrm{Br}$, $\mathrm{CH_3}$, and $\mathrm{CH_2CH_3}$ — all four groups are different. This carbon is a chiral center.
2. 1-Bromobutane ($\mathrm{CH_2BrCH_2CH_2CH_3}$): The carbon with bromine is attached to $\mathrm{H}$, $\mathrm{H}$, $\mathrm{Br}$, and $\mathrm{CH_2CH_2CH_3}$ — two hydrogens make it achiral.
3. 2-Bromopropane ($\mathrm{CH_3CHBrCH_3}$): The carbon with bromine is attached to $\mathrm{H}$, $\mathrm{Br}$, $\mathrm{CH_3}$, and $\mathrm{CH_3}$ — two identical methyl groups make it achiral. …

The Concept: What Makes a Molecule Chiral?

Chirality in organic chemistry is all about asymmetry. A molecule is chiral if it cannot be superimposed on its mirror image — like your left and right hands. The most common cause is a chiral centre (or stereocentre): a carbon atom bonded to four different groups.

The key test: look for a tetrahedral carbon where no two substituents are identical. If you find one, the molecule is almost certainly chiral. If every carbon has at least two identical groups attached, the molecule is achiral.

Let’s apply this to each compound.

Step-by-Step Analysis

1. 2-Bromobutane

Structure: $\mathrm{CH_3-CHBr-CH_2-CH_3}$

Focus on carbon-2 (the one with the bromine). Its four bonds go to:

• a hydrogen atom ($\mathrm{H}$)
• a bromine atom ($\mathrm{Br}$)
• a methyl group ($\mathrm{CH_3-}$)
• an ethyl group ($\mathrm{CH_3CH_2-}$)

Are all four different? Yes — $\mathrm{H}$, $\mathrm{Br}$, $\mathrm{CH_3}$, and $\mathrm{CH_3CH_2}$ are all distinct. This carbon is a chiral centre. Therefore, 2-bromobutane exists as a pair of enantiomers and is chiral.

2. 1-Bromobutane

Structure: $\mathrm{CH_2Br-CH_2-CH_2-CH_3}$

Check each carbon:

• Carbon-1: bonded to two hydrogens, one bromine, and one carbon chain — two hydrogens are identical, so not chiral.
• Carbon-2: bonded to two hydrogens, one $\mathrm{CH_2Br}$ group, and one $\mathrm{CH_2CH_3}$ group — again two identical hydrogens.
• Carbons-3 and -4: similarly have at least two identical substituents.

No carbon has four different groups. The molecule is achiral.

3. 2-Bromopropane

Structure: $\mathrm{CH_3-CHBr-CH_3}$

Carbon-2 is bonded to: one hydrogen, one bromine, and two methyl groups ($\mathrm{CH_3}$). The two methyl groups are identical. So this carbon has only three different substituents — not a chiral centre. The molecule is achiral.

4. 1-Bromopropane …