Q.Which of the following molecules is chiral in nature ? (A) Propan-2-ol (B) Butan-2-ol (C) 1-Bromobutane (D) 2-Bromopropane

Concept: Chirality requires a carbon atom bonded to four different substituents (a chiral centre). A molecule with no such centre is achiral.

Step 1 – Check each molecule for a chiral carbon.

• (A) Propan-2-ol: $\text{CH}_3\text{–CH(OH)–CH}_3$ — the central carbon has two identical methyl groups → no chiral centre.
• (B) Butan-2-ol: $\text{CH}_3\text{–CH(OH)–CH}_2\text{–CH}_3$ — the carbon bearing the –OH is attached to –H, –OH, –CH$_3$, and –CH$_2$CH$_3$ (all different) → chiral centre present. …

The Concept: What Makes a Molecule Chiral?

Chirality in organic chemistry is a property of "handedness" — a molecule is chiral if it is not superimposable on its mirror image. For most simple organic compounds (like the ones in this question), chirality arises from the presence of a stereocenter (often called a chiral centre or asymmetric carbon). This is a carbon atom that is bonded to four different substituents.

If a carbon has even two identical groups attached, its mirror image can be rotated to match the original — it is achiral. So the task is to check each molecule for a carbon with four distinct groups.

Step-by-Step Analysis

Let's examine each molecule one by one.

1. Propan-2-ol — Structure: $\text{CH}_3-\text{CH}(\text{OH})-\text{CH}_3$

The central carbon (the one with the -OH group) is bonded to:

• A hydrogen atom (H)
• A hydroxyl group (-OH)
• A methyl group ($\text{CH}_3$)
• Another methyl group ($\text{CH}_3$)

Since two of the groups are identical (both are $\text{CH}_3$), this carbon is not a chiral centre. The molecule is achiral.

2. Butan-2-ol — Structure: $\text{CH}_3-\text{CH}(\text{OH})-\text{CH}_2-\text{CH}_3$

Look at the carbon that carries the -OH group (the second carbon in the chain). Its four bonds go to:

• A hydrogen atom (H)
• A hydroxyl group (-OH)
• A methyl group ($\text{CH}_3$)
• An ethyl group ($\text{CH}_2\text{CH}_3$)

Are all four groups different? Yes — H, -OH, $\text{CH}_3$, and $\text{CH}_2\text{CH}_3$ are all distinct. This carbon is a stereocenter, so Butan-2-ol is chiral.

3. 1-Bromobutane — Structure: $\text{CH}_3-\text{CH}_2-\text{CH}_2-\text{CH}_2\text{Br}$

Scan each carbon: …