Hydrocarbons: JEE Main Chemistry Question with Solution
The number of possible isomeric products formed when 3-chloro-1-butene reacts with HCl through carbocation formation is ……
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Hint 1 of 3
What carbocations are formed upon the initial protonation of the double bond and subsequent rearrangement in 3-chloro-1-butene?
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Correct answer
The reaction forms a total of 4 isomeric products: 2,2-dichlorobutane (1 achiral isomer) and 2,3-dichlorobutane (3 stereoisomers: a pair of enantiomers and one meso compound).
Option analysis
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Reviewed route
Solution
StepWorking
01given
Reactant is 3-chloro-1-butene (CH2=CH−CH(Cl)−CH3). Reagent is HCl. Mechanism proceeds via carbocation intermediate.
02find
Total number of possible isomeric products formed (including stereoisomers).
03visualise
Protonation of the double bond at C1 gives a resonance-stabilised allylic carbocation: CH3−CH+−CH(Cl)−CH3↔CH3−CH=CH−C+H(Cl)↔CH3−CH(Cl)−CH+−CH3. Attack of Cl− can occur at different positions.
04strategise
1. Protonation forms an allylic carbocation. The positive charge delocalises along the chain: [CH3−CH(Cl)−CH+−CH3↔CH3−C+(Cl)−CH2−CH3].
2. Attack of Cl− at C2 gives 2,3-dichlorobutane, CH3−CH(Cl)−CH(Cl)−CH3.
3. Proton addition to CH2=CH−CH(Cl)−CH3 gives the secondary allylic carbocation CH3−C+H−CH(Cl)−CH3↔CH3−CH(Cl)−C+H−CH3.
4. Chloride attack at C2 yields 2,3-dichlorobutane. This molecule has 2 stereocenters with the same substituents. It gives a meso compound and a pair of enantiomers, which is 3 stereoisomers.
5. Hydride shift and attack yields 2,2-dichlorobutane. This molecule is achiral and gives 1 isomer.
6. Total isomeric products = 3+1=4.
Checking stereocenters: 2,3-dichlorobutane has n=2 identical chiral centers, giving 22−1+2(2/2)−1=2+1=3 stereoisomers. 2,2-dichlorobutane has 0 chiral centers, giving 1 product. Total = 4.
✓ Source and academic review↓
Question type
Numerical
Exam relevance
JEE Main · Chemistry
Concepts assessed
Chemistry
Academic status
Reviewed by official_key
Source
pyq
Editorial review
7 September 2026
Quick checks
Students also ask
Why does 2,3-dichlorobutane have only 3 stereoisomers instead of 4?
Because the molecule is symmetrical. The (2R,3S) and (2S,3R) configurations are superimposable mirror images (meso compound), representing the same molecule.