Question

Which of the following is true for the leaving group ability of halides (R-X) by elimination reaction?

• A. $E1:R-I>R-Br>R-Cl>R-F$
  $E2:R-I>R-Br>R-Cl>R-F$
• B. $E1:R-I>R-Br>R-Cl>R-F$
  $E2:R-I<R-Br<R-Cl<R-F$
• C. $E1:R-I<R-Br<R-Cl<R-F$
  $E2:R-I<R-Br<R-Cl<R-F$
• D. $E1:R-I<R-Br<R-Cl<R-F$
  $E2:R-I>R-Br>R-Cl>R-F$

Answer 1

The correct option is A $E1:R-I>R-Br>R-Cl>R-F$

$E2:R-I>R-Br>R-Cl>R-F$
A good leaving group favours both E1 and E2 elimination reactions.
A good leaving group must have a weak C-X bond.
Strength of $C-X$ bond decrease in the order,
$C-F>C-Cl>C-Br>C-I$
Thus, the leaving group ability of alkyl halide for elimination reactions are

$E1:R-I>R-Br>R-Cl>R-F$
$E2:R-I>R-Br>R-Cl>R-F$