Chemistry Hydrocarbons MDCAT 2012
PMDC Verified Question 57 of 65
In the reaction of ethene with bromine the intermediate formed is
A
Cyclic bromonium ion
B
Primary carbocation
C
Bromine free radical
D
Carbanion
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option A: Cyclic bromonium ion
Concept:

Electrophilic addition of halogens to alkenes does not proceed through a standard open carbocation, but rather a stable three-membered cyclic intermediate.

Solution:

  • When ethene (\( \text{CH}_2=\text{CH}_2 \)) reacts with \( \text{Br}_2 \), the pi electrons attack the electrophilic bromine.


  • Instead of a full positive charge resting on one carbon, the bromine atom shares its lone pairs to bridge both carbons, forming a three-membered ring with a positive charge on bromine.


  • This intermediate is called the cyclic bromonium ion.


Why other options are incorrect:

  • A primary carbocation is highly unstable and would lead to rearrangement or unstereospecific addition; the cyclic intermediate explains the strict anti-addition of halogens.


  • Free radicals are involved in UV-catalyzed substitution, not cold electrophilic addition.


  • Carbanions are negatively charged species, impossible to form in electrophilic attack.

Quality & Fidelity Assurance: Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.

Previous Question All Hydrocarbons MCQs Next Question

Want to solve full-length papers under timed exam conditions?

Practice with zero-scroll lockdown sprints, dynamic latency zone timers, live peer selection telemetry, and the automated Amber mistake recovery loop.

Enter Arena Sprint iOS App Android App