For an E2 reaction, which of the following is correct?  

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DSSSB PGT Chemistry (Female) Official Paper (Held On: 06 Jul, 2018 Shift 1)
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  1. R-Br < R - I < R-Cl 
  2. R-CL < R-I < R-Br
  3. R-Cl < R-Br < R-I
  4. R-Cl > R-Br > R-I

Answer (Detailed Solution Below)

Option 3 : R-Cl < R-Br < R-I
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Detailed Solution

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CONCEPT:

E2 Reaction and Leaving Group Ability

  • An E2 reaction is a bimolecular elimination reaction where the rate depends on both the substrate and the base.
  • The leaving group plays a critical role in E2 reactions. A better leaving group facilitates the elimination process.
  • The ability of a halide to act as a leaving group increases with its size and polarizability. In general, the order of leaving group ability for halogens is:

    Iodine (I) > Bromine (Br) > Chlorine (Cl)

EXPLANATION:
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  • In the given options, the reactivity order for an E2 reaction depends on the leaving group ability of the halides (R-Cl, R-Br, R-I).
  • Among halogens:
    • Iodine is the largest and most polarizable, making it the best leaving group.
    • Bromine is less polarizable than iodine but better than chlorine.
    • Chlorine is the smallest and least polarizable, making it the poorest leaving group.

Therefore, the correct answer is Option 3: R-Cl < R-Br < R-I.

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