Chemistry · General Principles Of Organic Chemistry · NEET
A carbon is a chiral centre when all four groups attached to it are different from one another. If even two of the four groups are the same, the carbon is not chiral. Quick test: number the four bonds and check that no two lead to the same group. Example: in CHFClBr the carbon holds H, F, Cl and Br — all four different — so it is chiral.
Usually yes for NEET-level questions with one chiral carbon, but not always. If a molecule has two chiral carbons that are mirror images inside the same molecule, an internal plane of symmetry can cancel the chirality. That special case is called a meso compound (for example meso-tartaric acid). So: one chiral carbon means chiral; more than one means check for a symmetry plane.
The central carbon carries H, Cl and two CH3 groups. Two of the four groups are identical (both CH3), so the carbon is not a chiral centre. The molecule can be placed exactly on its mirror image, so it is achiral and optically inactive. Compare with CH3-CHCl-CH2CH3 (2-chlorobutane), where the groups are H, Cl, CH3 and CH2CH3 — all different — so that carbon IS chiral.
A single pure chiral molecule (one enantiomer) rotates the plane of plane-polarised light; this is optical activity. Achiral molecules do not rotate it. Caution: a 50:50 mixture of the two mirror-image forms (a racemic mixture) shows zero net rotation because the two rotations cancel — but each molecule is still chiral.
Look only at sp3 carbons (four single bonds). Ignore CH3, CH2 and =CH carbons because they always have at least two identical bonds. For each remaining CH carbon, trace its four attached groups; if all four paths are different, that carbon is the chiral centre. Carbons in double bonds, triple bonds, or carbons with an H2 are never chiral centres.
The incorrect statement regarding chirality is:
How many products (including stereoisomers) are expected from the monochlorination of 2-methylbutane, (CH3)2CH-CH2-CH3?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A carbon atom joined to four different groups. Because the four groups are all different, the atom and its mirror image cannot be overlapped, which makes the molecule chiral.
It is also called a stereocentre, a stereogenic centre, or an asymmetric carbon. In NEET questions these terms mean the same thing: a carbon with four different attached groups, often marked with a star.
A single pure chiral compound rotates plane-polarised light and is optically active. But a racemic mixture (equal amounts of both mirror images) gives zero net rotation, and meso compounds are achiral despite having chiral carbons.
Yes. Some molecules are chiral due to restricted rotation, such as certain biphenyls (atropisomers) tested in NEET 2016. But for most NEET problems, look for a carbon with four different groups first.
Chirality is about mirror images that will not overlap (needs four different groups on a carbon). Cis-trans isomerism is about groups on the same or opposite sides of a rigid double bond or ring. Both are stereoisomerism, but the cause is different.
