The Question is from neet-chemistry hydrocarbons alkenes, and it was created by the author Mr. S M HUSSAIN

Question

Which of the following alkenes exhibits the highest rate of reaction with HCl in an electrophilic addition reaction under standard conditions?

Mr. S M HUSSAIN · Accepted Answer

## CORE INFORMATION
✓ **Answer**: D
## CONCEPT FRAMEWORK
 **Primary Concept**: Electrophilic addition reactions of alkenes
 **Related Topics**: Carbocation stability, Markovnikov's rule
 **Prerequisites**: Understanding of carbocation intermediates and their stability, knowledge of Markovnikov's rule.
## SOLUTION PATHWAY
**Given Information**:
  The reaction is an electrophilic addition of HCl to different alkenes.
  We need to determine which alkene reacts fastest.
**Method & Steps**:
 1. Identify the carbocation intermediate formed in each reaction.
  Reasoning: The rate-determining step in electrophilic addition to alkenes is the formation of the carbocation intermediate. The more stable the carbocation, the faster the reaction.
  Formula/Rule:  None explicitly, but understanding of carbocation stability is crucial.
 2. Compare the stability of the carbocations formed from each alkene.
  Working: The stability of carbocations increases with increasing substitution.  Tertiary carbocations ($3^\circ$) are more stable than secondary ($2^\circ$), which are more stable than primary ($1^\circ$), which are more stable than methyl carbocations.
  Key Point: Hyperconjugation stabilizes carbocations; more alkyl groups donating electron density increases stability.
 3. Determine which alkene forms the most stable carbocation.
  Result: 2,3-Dimethyl-2-butene forms a highly stable tertiary carbocation, making it the fastest reacting alkene.
  Verification:  The other options form less stable carbocations (secondary or primary).
 ## OPTION ANALYSIS
A :x: Ethene forms a primary carbocation.
B :x: Propene forms a secondary carbocation.
C :x: 2-Methylpropene forms a tertiary carbocation, but less stable than that formed from 2,3-Dimethyl-2-butene.
D ✓: 2,3-Dimethyl-2-butene forms the most stable tertiary carbocation due to two methyl groups providing significant hyperconjugation.
 ## LEARNING AIDS
 **Common Mistakes**:
  1.Ignoring carbocation stability: Focusing solely on the number of carbons without considering the substitution pattern of the carbocation.
   How to avoid: Draw out the carbocation intermediate for each reaction and assess its stability based on substitution and hyperconjugation.
  2.Misapplying Markovnikov's rule: Markovnikov's rule predicts the regioselectivity of the addition, not the relative rates of reaction.
   How to avoid: Remember Markovnikov's rule applies to the product formed, not the reaction rate. Rate is determined by carbocation stability.
 **Quick Tips**:
  1.Prioritize tertiary carbocations: Tertiary carbocations are the most stable and lead to the fastest reactions.
  2.Consider hyperconjugation: The more alkyl groups attached to the carbocation, the more stable it is due to hyperconjugation.

Mr. S M HUSSAIN · Answer

Ethene

Mr. S M HUSSAIN · Answer

Propene

Mr. S M HUSSAIN · Answer

2-Methylpropene

Mr. S M HUSSAIN · Answer

The correct Option is "2,3-Dimethyl-2-butene"

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