Q.Molecules whose mirror image is non superimposable over them are known as chiral. Which of the following molecules is chiral in nature?

Concept: Chirality requires a carbon bonded to four different substituents (a chiral centre). A molecule is chiral if it is non-superimposable on its mirror image.

Reasoning:

1. Check each molecule for a chiral carbon.
   • (i) 2-Bromobutane: $\text{CH}_3\text{CHBrCH}_2\text{CH}_3$ — the second carbon is attached to H, Br, CH$_3$, and CH$_2$CH$_3$ (all different).
   • (ii) 1-Bromobutane: $\text{CH}_2\text{BrCH}_2\text{CH}_2\text{CH}_3$ — no carbon with four different groups.
   • (iii) 2-Bromopropane: $\text{CH}_3\text{CHBrCH}_3$ — the second carbon has two identical methyl groups, so it is achiral. …

Why chirality matters — and how to spot it

Chirality is a property of molecular handedness: a chiral molecule and its mirror image cannot be superimposed, like your left and right hands. For most organic molecules at the JEE/NEET level, chirality arises from a stereogenic centre — typically a carbon atom with four different substituents. If any two groups on that carbon are identical, the molecule is achiral (it has a plane of symmetry).

So the task is simple: check each molecule for a carbon with four distinct attachments.

Step-by-step analysis

1. 2-Bromobutane

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

Number the carbons:

• C1: $\mathrm{CH_3-}$ (three H's, one C — not a chiral centre)
• C2: $\mathrm{-CHBr-}$ — this carbon is bonded to:
  • a hydrogen (H)
  • a bromine (Br)
  • a methyl group ($\mathrm{CH_3-}$)
  • an ethyl group ($\mathrm{-CH_2CH_3}$)

All four groups are different. Therefore, C2 is a chiral centre. The molecule exists as a pair of enantiomers.

2. 1-Bromobutane

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

• C1: $\mathrm{-CH_2Br}$ — two hydrogens, one bromine, one carbon. Two H's are identical → not chiral.
• C2, C3, C4: each has at least two identical substituents (e.g., two H's on a $\mathrm{CH_2}$ group). No chiral centre.

The molecule is achiral. …

Method: Chirality Detection via Asymmetric Carbon (Stereocenter) Analysis

Concept First — Why This Works

A molecule is chiral if it has a non-superimposable mirror image. The most common cause is the presence of an asymmetric carbon (a carbon bonded to four different groups). If no such carbon exists, the molecule is usually achiral (superimposable on its mirror image).

Steps

1. Draw the structure of each molecule (condensed or line formula).
2. Identify each carbon that is bonded to four different atoms/groups.
3. Check for symmetry — even if a carbon has four different groups, the molecule may still be achiral if it has a plane of symmetry.
4. Conclude: If at least one asymmetric carbon exists and the molecule lacks a plane of symmetry, it is chiral.

Applying to the Options

(i) 2-Bromobutane

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

• Carbon-2: bonded to $\mathrm{H}$, $\mathrm{Br}$, $\mathrm{CH_3}$, $\mathrm{CH_2CH_3}$ — four different groups ✓
• No plane of symmetry → Chiral ✓

(ii) 1-Bromobutane

Structure: $\mathrm{Br-CH_2-CH_2-CH_2-CH_3}$ …

This is a classic trap in stereochemistry. Let's break down the common mistakes students make when tackling this exact problem, and how to avoid each.

✗ Mistake 1: Confusing "chiral" with "having a chiral centre"

Many students think: If a molecule has a chiral carbon, it must be chiral.

But that's not always true — a molecule can have chiral centres and still be achiral if it has a plane of symmetry (meso compound).

✓ How to avoid:

• Always check for internal symmetry (plane of symmetry) before concluding chirality.
• A chiral centre is a necessary but not sufficient condition for chirality.

✗ Mistake 2: Forgetting to check for symmetry in the whole molecule

Students often look only at one carbon and ignore the rest of the molecule.

For example, in 2-Bromopropan-2-ol (iv), the central carbon has four different groups? Let's check:

• Carbon: attached to –Br, –OH, –CH₃, and –CH₃ → Two identical methyl groups → not a chiral centre.

✓ How to avoid:

• Draw the full structure.
• Check every substituent on the carbon in question — if any two are identical, it's not a chiral centre.

✗ Mistake 3: Assuming all halogenated alkanes are chiral

Just because a molecule has a bromine atom doesn't make it chiral.

Example: 1-Bromobutane (ii) has Br at the end — the carbon with Br is attached to two H atoms → achiral.

✓ How to avoid:

• A carbon must have four different groups to be a chiral centre.
• Count groups carefully: –H counts as a group!

✗ Mistake 4: Misidentifying the chiral centre in 2-Bromobutane

2-Bromobutane (i) has the structure:

CH₃–CHBr–CH₂–CH₃

The carbon with Br is attached to:

• –H
• –Br
• –CH₃
• –CH₂CH₃

All four are different → chiral centre exists.

No plane of symmetry → molecule is chiral.

✓ How to avoid:

• Write the full condensed formula.
• List the four groups explicitly.
• Check for symmetry in the whole molecule.

✗ Mistake 5: Overlooking that 2-Bromopropane is symmetric

2-Bromopropane (iii):

CH₃–CHBr–CH₃

The central carbon is attached to:

• –H
• –Br
• –CH₃
• –CH₃

Two identical methyl groups → not a chiral centre. …