Read the passage carefully and answer the Questions.
Haloalkanes undergo nucelophilic substitution reactions owing to the polarity of C-X bond. The nucleophile reacts with the haloalkane on the carbon possessing a partial positive charge holding the halogen atom. The halogen atom X is replaced by a nucleophile. Depending on the kinetics and mode of bond breaking, the mechanism can be either \( S_N1 \) or \( S_N2 \) reaction. The rate of \( S_N1 \) reaction is governed by the stability of carbocation and in \( S_N2 \) reaction, the rate of reaction is governed by steric factor. Chirality is the main factor in both \( S_N1 \) and \( S_N2 \). In \( S_N1 \) reaction, the chirality of alkyl halide is accompanied by racemization of the product, while in \( S_N2 \) reaction, the product is characterized by inversion of configuration. The structure of alkyl halide and the nature of the solvent also governs the mechanism of the substitution.
What is chiral carbon?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Attached to four different atoms or groups
A carbon atom bonded to four different atoms or groups is called a chiral (asymmetric) carbon, or a stereocentre. The four groups point to the corners of a tetrahedron, and when all four are different, the molecule and its mirror image cannot be superimposed. A molecule with one such carbon is therefore chiral and optically active.
In 2-chlorobutane, C-2 carries H, Cl, \( \ce{CH3} \) and \( \ce{C2H5} \): four different groups, so C-2 is a chiral carbon. In propan-2-ol, C-2 carries two identical \( \ce{CH3} \) groups, so it is not a chiral carbon and propan-2-ol is achiral.
Why the other options are incorrect
Option (1) Forms Superimposable mirror images — Incorrect. A molecule that is superimposable on its mirror image is achiral; a chiral carbon makes the mirror images non-superimposable.
Option (3) Optically inactive — Incorrect. A compound with a single chiral carbon is optically active; it rotates the plane of plane polarised light.
Option (4) Symmetrical carbon — Incorrect. A carbon carrying two identical groups, such as C-2 of propan-2-ol, has a symmetrical environment and is not chiral.
NCERT (2026-27 edition):
Molecular Asymmetry, Chirality and Enantiomers – “… Dutch scientist, J. Van’t Hoff and French scientist, C. Le Bel in the same year (1874), independently argued that the spatial arrangement of four groups (valencies) around a central carbon is tetrahedral and if all the substituents attached to that carbon are different, the mirror image of the molecule is not superimposed (overlapped) on the molecule; such a carbon is called asymmetric carbon or stereocentre. The resulting molecule would lack symmetry and is referred to as asymmetric molecule. The asymmetry of the molecule along with non superimposability of mirror images is responsible for the optical activity in such organic compounds.”