When 2-methylpentane undergoes free radical monochlorination, the number of structurally isomeric products formed and the number of these that are chiral are, respectively:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 5, 3
In free radical monochlorination, Cl replaces one H. Each kind of equivalent hydrogen gives one product.
A product is chiral when it has a carbon joined to four different groups (an asymmetric carbon).
2-Methylpentane (main chain numbered C1 to C5 from left to right):
\( \displaystyle \ce{CH3-CH(CH3)-CH2-CH2-CH3} \)
C1 and the methyl group on C2 are the same, so there are 5 kinds of hydrogen: on C1, C2, C3, C4 and C5. Putting Cl on each in turn:
Cl on C1: 1-chloro-2-methylpentane, \( \ce{ClCH2-CH(CH3)-CH2CH2CH3} \) — C2 has H, \( \ce{CH3} \), \( \ce{CH2Cl} \), \( \ce{C3H7} \). Chiral.
Cl on C2: 2-chloro-2-methylpentane, \( \ce{(CH3)2C(Cl)-CH2CH2CH3} \) — C2 has two \( \ce{CH3} \) groups. Not chiral.
Cl on C3: 3-chloro-2-methylpentane, \( \ce{(CH3)2CH-CH(Cl)-CH2CH3} \) — C3 has H, Cl, isopropyl, ethyl. Chiral.
Cl on C4: 2-chloro-4-methylpentane, \( \ce{(CH3)2CH-CH2-CH(Cl)-CH3} \) — this carbon has H, Cl, \( \ce{CH3} \), isobutyl. Chiral.
Cl on C5: 1-chloro-4-methylpentane, \( \ce{(CH3)2CH-CH2-CH2-CH2Cl} \) — no carbon has four different groups. Not chiral.
Total: 5 products, of which 3 are chiral.
NCERT (2026-27 edition):
(I) From alkanes by free radical halogenation – “Free radical chlorination or bromination of alkanes gives a complex mixture of isomeric mono- and polyhaloalkanes, which is difficult to separate as pure compounds.”
(ii) 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.”