Q.20 Each of the following alkenes undergoes addition reaction with bromine. Under the
same reaction conditions, the CORRECT trend in the reaction rates
is
(A) I > II > III
(B) II > III > I
(C) I > III > II
(D) III > II > I

The correct order of rates of bromine addition to the three alkenes is: II > III > I.

Interpreting the three alkenes

From the figure:

• Alkene I: an alkene conjugated with a carbonyl that also bears an –OH (α,β‑unsaturated carboxylic acid type). The carbonyl–OH group is strongly electron withdrawing, making the double bond electron poor.

• Alkene II: an alkene with a methyl group attached to the double bond (monosubstituted alkene with an electron‑releasing alkyl group).

• Alkene III: a simple unsubstituted alkene (ethene‑like), with only hydrogens on the double bond.

Bromine addition to an alkene proceeds through formation of a bromonium ion, which is favored when the double bond is electron rich and well stabilized by adjacent groups.

Why option (B) II > III > I is correct

1. Alkene II – fastest rate

   • The methyl group is electron releasing by inductive and hyperconjugation effects.

   • This increases electron density on the double bond, allowing it to attack Br–Br more readily and stabilize the developing positive charge in the bromonium ion.

   • Therefore, the rate of bromine addition is highest for II.

2. Alkene III – intermediate rate

   • This double bond has no substituents other than hydrogen, so it is less electron rich than II.

   • It can still form the bromonium ion, but with less stabilization, so it reacts slower than II but faster than I.

3. Alkene I – slowest rate

   • The double bond is conjugated with a strongly electron‑withdrawing carbonyl–OH group.

   • The carbonyl pulls electron density away from the alkene, making the π bond less nucleophilic and therefore less reactive toward Br–Br.

   • As a result, I shows the lowest rate of bromine addition among the three.

Thus, the overall trend in the rate of bromine addition to these alkenes is:

$$\text{II > III > I}$$

which corresponds to option (B).

Analysis of each option

Option (A) I > II > III

• This would imply the conjugated, electron‑poor alkene I reacts fastest, which contradicts the principle that electron‑rich alkenes add bromine more rapidly.

• Therefore, option (A) is incorrect.

Option (B) II > III > I

• Correctly places the electron‑rich, methyl‑substituted alkene II first, the unsubstituted alkene III next, and the electron‑poor conjugated alkene I last.

• This agrees with expected reactivity patterns, so option (B) is correct.

Option (C) I > III > II

• Again assumes I is the most reactive and also places unsubstituted III above methyl‑substituted II.

• This contradicts the stabilizing, activating effect of the methyl group; thus option (C) is incorrect.

Option (D) III > II > I

• Correctly places I as slowest but incorrectly claims unsubstituted III is faster than methyl‑substituted II.

• Since alkyl substitution activates the double bond toward electrophilic addition, option (D) is also incorrect.

Short SEO‑friendly introduction

Understanding the rate of bromine addition to alkenes is essential for predicting outcomes of electrophilic addition reactions in organic chemistry. By comparing the structures of alkenes I, II and III, and considering the effects of alkyl substitution and electron‑withdrawing groups, the correct trend in bromine addition rates can be deduced as II > III > I.