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Chapter 6: Reactions of Alkenes: Addition Reactions



Hydrogenation | Hydrohalogenation | Hydration | Hydroboration/Oxidation | Halogenation | Halohydration | Epoxidation | Ozonolysis |


Reaction of Alkenes with Hydrogen Halides

Chapter 6: Reactions of Alkenes : Addition Reactions

hydrohalogenation of alkenes
Reaction type: Electrophilic Addition

Summary

MECHANISM FOR REACTION OF ALKENES WITH HBr

Step 1:
An acid/base reaction. Protonation of the alkene to generate the more stable carbocation.  The p electrons act as a Lewis base.
addition of HBr to C=C
Step 2:
Attack of the nucleophilic bromide ion on the electrophilic carbocation creates the alkyl bromide.

 

Markovnikov's Rule
    hydrohalogenation of alkenes
  • Markovnikov's rule (1870s), based on experimental observation, states that :

  • "when an unsymmetrical alkene reacts with a hydrogen halide to give an alkyl halide, the hydrogen adds to the carbon that has the greater number of hydrogen substituents, and the halogen to the carbon having the fewer number of hydrogen substituents"

    example of Markovnikov's rule

  • Modern mechanistic knowledge indicates reaction occurs via protonation to give the more stable carbocation.
protonation of propene can give two carbocations
Propene can protonate to give two different carbocations, one 2o and the other 1o.  Formation of the more stable 2o carbocation is preferred.
  • The carbocation then reacts with the nucleophile to give the alkyl bromide
bromide reactions with the carbocation

Reaction of Alkenes with HBr (radical)

radical hydrobromination of alkenes
Reaction type: Radical Addition

Summary

addition of HBr to propene under radical conditions
MECHANISM FOR RADICAL REACTION OF ALKENES WITH HBr

Step 1:
An electrophilic bromine radical adds to the alkene to generate the 2o radical.
addition of HBr to C=C under radical conditions
Step 2:
Radical abstracts a H atom from another molecule of HBr, creating the alkyl bromide and another bromine radical.



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