Chemistry · General Principles Of Organic Chemistry · NEET
Chirality means a molecule has 'handedness'. Its mirror image is a different molecule that you cannot slide or turn to fit exactly on top of the original. A molecule with this property is called chiral. The most common cause is a carbon atom joined to four different groups. If the mirror image CAN be placed on top of the original, the molecule is achiral (not chiral).
A chiral carbon (also called an asymmetric carbon or stereocentre) is a carbon that is bonded to four different atoms or groups. Because all four are different, swapping any two of them gives a new arrangement that is a mirror image. For example, in CHFClBr the central carbon has H, F, Cl and Br — all different — so it is a chiral carbon.
Enantiomers are the two mirror-image forms of a chiral molecule. They are NON-superimposable — you cannot place one on top of the other. This is a favourite NEET trap: enantiomers are non-superimposable mirror images, never superimposable. They have identical physical properties except that they rotate plane-polarised light by equal amounts in opposite directions.
A racemic mixture is a 1:1 mixture of both enantiomers. One enantiomer rotates plane-polarised light to the right (+), the other rotates it exactly the same amount to the left (−). The two rotations cancel each other, so the net rotation is zero. That is why a racemic mixture shows zero optical rotation even though each pure enantiomer is optically active.
Both are types of stereoisomerism (same bonds, different arrangement in space). Geometrical isomerism (cis/trans, E/Z) comes from restricted rotation around a C=C double bond or a ring. Optical isomerism comes from chirality — non-superimposable mirror images, usually due to a carbon with four different groups. Geometrical isomers are NOT mirror images; optical isomers (enantiomers) ARE mirror images.
When plane-polarised light passes through a solution of a chiral compound, the light plane rotates. If it rotates clockwise (to the right), the compound is dextrorotatory, written (+) or d. If it rotates anticlockwise (to the left), it is laevorotatory, written (−) or l. The two enantiomers of a compound are one (+) and one (−), rotating light by equal angles in opposite directions.
Look for a chiral centre: a carbon with four different groups. If the molecule has at least one such centre and no internal plane of symmetry, it is usually chiral and optically active. If the molecule has a plane of symmetry or a centre of symmetry, its mirror image is superimposable, so it is achiral and optically inactive (this includes meso compounds).
The incorrect statement regarding chirality is:
Which of the following biphenyls is optically active?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. Only molecules that are chiral show optical isomerism. Most simple molecules have a plane of symmetry, so their mirror image is superimposable and they are optically inactive.
Yes, but it is rare at NEET level. Certain molecules like some substituted biphenyls (atropisomers) and allenes are chiral due to restricted rotation or shape, even without a classic chiral carbon.
A meso compound has chiral centres but also an internal plane of symmetry. This makes its mirror image superimposable, so it is optically inactive despite having stereocentres.
With a polarimeter. Plane-polarised light is passed through the solution, and the angle by which the light plane rotates (and its direction, + or −) is measured.
