During conversion of alcohol to alkyl halide, which of the below is correct? 

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DSSSB PGT Chemistry (Female) Official Paper (Held On: 06 Jul, 2018 Shift 1)
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  1. 1° > 2° > 3° 
  2. 3° > 2° > 1°
  3. 2° > 3° > 1°
  4. 2° > 1° > 3°

Answer (Detailed Solution Below)

Option 2 : 3° > 2° > 1°
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Detailed Solution

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CONCEPT:

Reactivity of Alcohols in Conversion to Alkyl Halides

  • When alcohols are converted to alkyl halides, the reactivity order depends on the stability of the carbocation intermediate formed during the reaction.
  • In these reactions, tertiary (3°) alcohols are the most reactive, followed by secondary (2°) alcohols, and then primary (1°) alcohols.
  • This trend is due to the stability of the carbocation intermediate formed during the reaction, which increases in the order: 3° > 2° > 1°.

EXPLANATION:

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  • During the conversion of alcohols to alkyl halides, the reaction typically proceeds via a carbocation intermediate when using reagents like HCl, HBr, or HI.
  • The formation of the carbocation is the rate-determining step.
    • Tertiary alcohols (3°) form the most stable carbocations due to the +I (inductive) and hyperconjugation effects of surrounding alkyl groups.
    • Secondary alcohols (2°) form moderately stable carbocations.
    • Primary alcohols (1°) form the least stable carbocations, making them the least reactive in this process.

Thus, the reactivity order for alcohols in this reaction is: 3° > 2° > 1°.

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