Q.21 Consider the following reaction:
The CORRECT statement(s) related to mono-chlorination at carbon-2 position
is/are
(A) The reaction proceeds through alkyl radical intermediate
(B) Complete inversion of configuration at carbon-2 takes place
(C) Complete retention of configuration at carbon-2 takes place
(D) A mixture of enantiomers is formed
Free radical monochlorination at the carbon-2 position of the given chiral alkane (CH₃-CH(Cl)-CH₃) proceeds through a planar radical intermediate, leading to a mixture of enantiomers. Options A and D are correct.
Reaction Mechanism
The reaction is initiated by UV light cleaving Cl₂ into chlorine radicals, which abstract the hydrogen from carbon-2 to form a secondary alkyl radical. This radical is sp²-hybridized and planar, allowing Cl- attack from either face. The product is thus racemic, with no stereospecificity.
Option Analysis
A: The reaction proceeds through alkyl radical intermediate
Correct. Free radical halogenation always involves alkyl radical formation in the propagation step (R-H + Cl- → R- + HCl).
B: Complete inversion of configuration at carbon-2 takes place
Incorrect. Inversion requires backside attack (SN2-like), but the planar radical permits front and back attack equally.
C: Complete retention of configuration at carbon-2 takes place
Incorrect. Retention needs a concerted mechanism preserving stereochemistry, absent here due to radical planarity.
D: Mixture of enantiomers is formed
Correct. The planar radical loses chirality memory, yielding 50:50 R and S products at carbon-2.
Free radical monochlorination at carbon-2 position exemplifies radical reaction stereochemistry for CSIR NET Life Sciences aspirants. This UV-initiated Cl₂ reaction on chiral secondary alkanes like CH₃-CH(Cl)-CH₃ forms alkyl radical intermediates, crucial for grasping racemization in organic mechanisms.
Key Mechanism Steps
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Initiation: hv breaks Cl-Cl (Cl- formation).
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Propagation: Cl- abstracts H from chiral C2 (planar R- ), then R- + Cl₂ → product + Cl- .
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Termination: Radical recombination ends chain.
The secondary alkyl radical’s planarity erases original configuration, unlike SN1/SN2 ionic paths.
Why Racemization Occurs
Planar sp² radicals allow equal Cl- approach from both sides, yielding enantiomer mixtures. No skeletal rearrangement; selectivity favors secondary > primary H (3.9:1).
| Stereochemical Outcome | Mechanism Type | Example Reaction |
|---|---|---|
| Complete Inversion | SN2 | Primary alkyl halides |
| Partial Racemization | SN1 | Tertiary carbocations |
| Full Racemization | Free Radical | Monochlorination C2 |
| Retention | Double SN2 | Neighboring group participation |
This table contrasts free radical monochlorination stereochemistry with ionic substitutions, vital for exam comparisons.
