Chemistry · Haloalkanes And Haloarenes · NEET
Chirality means a molecule and its mirror image cannot be placed exactly on top of each other. Think of your two hands. They look the same but you cannot lay your right hand perfectly over your left hand. A molecule like this is called chiral. If the mirror image CAN overlap the original, the molecule is achiral. For NEET, most chiral molecules have one carbon attached to four different groups.
Look for a carbon that is bonded to four DIFFERENT atoms or groups. This carbon is called a chiral centre (or asymmetric carbon). Example: in CHFClBr, the central carbon has H, F, Cl and Br, all four different, so it is chiral. If even two of the four groups are the same, that carbon is NOT chiral. Quick trick for NEET: circle each carbon, list its 4 groups, and check if all four are different.
Enantiomers are NON-superimposable mirror images. This is the single most tested point in NEET. They are mirror images of each other, but you can never make one overlap the other exactly. NEET loves to write the WRONG statement 'enantiomers are superimposable mirror images' as a trap. That statement is false. Remember: enantiomers = mirror images that do NOT overlap.
Optical activity is the ability of a chiral compound to rotate the plane of plane-polarised light when the light passes through it. This is measured in an instrument called a polarimeter. It happens only because the molecule is chiral (non-superimposable on its mirror image). One enantiomer rotates the light to the right (dextrorotatory, d or +) and its mirror partner rotates it to the left by the same amount (laevorotatory, l or -).
A racemic mixture is a 50:50 mixture of the two enantiomers. One enantiomer rotates light to the right by some angle, and the other rotates it to the left by the exact same angle. So the two effects cancel out and the net rotation is zero. The mixture is optically inactive, and we say it is 'inactive by external compensation'. NEET point: SN1 reactions at a chiral centre give a racemic mixture, so the product shows zero optical rotation.
Stereoisomers is the big family: molecules with the same formula and same bonds but different arrangement of atoms in space. Enantiomers are one type of stereoisomer, specifically the pair that are non-superimposable mirror images. The other type is diastereomers, which are stereoisomers that are NOT mirror images. So all enantiomers are stereoisomers, but not all stereoisomers are enantiomers.
The incorrect statement regarding chirality is:
Which of the following biphenyls is optically active? (a molecule whose rotation about the central bond is blocked by bulky ortho groups on both rings)
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. A carbon is chiral only when all four groups attached to it are different. If two or more groups are the same, the carbon is not a chiral centre, even though it has four bonds.
Yes, for most properties like melting point, boiling point and density they are identical. They differ in only two things: the direction they rotate plane-polarised light (equal size, opposite sign) and how they react with other chiral molecules.
Dextrorotatory (d or +) rotates plane-polarised light to the right (clockwise). Laevorotatory (l or -) rotates it to the left (anticlockwise). The two enantiomers of a chiral compound are one d and one l.
Yes, in special cases like the locked biphenyls asked in NEET 2016. Optical activity really depends on the whole molecule having no plane or centre of symmetry, not only on having a chiral carbon.
When a haloalkane has a chiral carbon at the reaction site, SN1 gives a racemic mixture (racemisation) and SN2 gives inversion of configuration. NEET tests this link directly, so you must first understand chirality and enantiomers.
