Question

The reactivity order of the following alkyl halides towards dehydrohalogenation:
(A) R-F
(B) R-Br
(C) R-Cl
(D) R-I
Choose the correct answer from the options given below:

• A. A $>$ B $>$ C $>$ D
• B. A $>$ C $>$ B $>$ D
• C. D $>$ B $>$ C $>$ A
• D. D $>$ C $>$ B $>$ A

Answer 1

The Correct Option is D

The rate of dehydrohalogenation (elimination) depends on the ability of the halide ion to act as a leaving group. Iodide (I$^-$) is the best leaving group, followed by bromide (Br$^-$), chloride (Cl$^-$), and fluoride (F$^-$).
Therefore, the correct order of reactivity is: \(R-I > R-Br > R-Cl > R-F\).

Answer 2

The rate of dehydrohalogenation (elimination) reactions depends on the ability of the halide ion to act as a leaving group. A leaving group is an atom or group that departs with its bonding electrons during a chemical reaction, and its effectiveness greatly influences the reaction rate.

Iodide (I^-) is the best leaving group among halides due to its large size and the ability to stabilize the negative charge after departure. This makes the iodide ion highly willing to leave, facilitating faster dehydrohalogenation.

Bromide (Br^-) follows iodide as a good leaving group. Although it is smaller than iodide, bromide is still effective due to its relatively good ability to stabilize the negative charge after departure.

Chloride (Cl^-) is less effective than both iodide and bromide because it is smaller and holds onto the electrons more tightly. This makes it less likely to leave during the elimination reaction.

Fluoride (F^-) is the poorest leaving group among halides due to its small size and high electronegativity, which results in a strong bond with the carbon atom and poor ability to stabilize the negative charge upon departure.

Therefore, the correct order of reactivity for dehydrohalogenation based on the leaving group ability is:

\(R-I > R-Br > R-Cl > R-F\)