Summary

• When treated with HX alkenes form alkyl halides.
• Hydrogen halide reactivity order : HI > HBr > HCl > HF (paralleling acidity order).
• Regioselectivity predicted by Markovnikov's rule : 
  "For addition of hydrogen halides to alkenes, the H atom adds to the C with the most H atoms already present"
• Reaction proceeds via protonation to give the more stable carbocation intermediate.
• Not stereoselective since reaction proceeds via planar carbocation.
• For HBr, care must be taken to avoid the formation of radicals when an alternate mechanism occurs.

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.
Step 2: Attack of the nucleophilic bromide ion on the electrophilic carbocation creates the alkyl bromide.

 

Markovnikov's Rule

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" Modern mechanistic knowledge indicates reaction occurs via protonation to give the more stable carbocation.

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

Reaction of Alkenes with HBr (radical)

Summary

• When treated with HBr, alkenes form alkyl bromides.
• However, under these conditions, the regioselectivity is anti Markovnikov

• Peroxides facilitate the formation of a bromine radical, RO^. + HBr ® ROH + Br^.
• Reaction proceeds via the  more stable radical intermediate.
• Contrast with non-radical alternative:

 

	Regular	Radical
Conditions	HBr (dark, N2 atmosphere)	HBr (peroxides, uv light)
Electrophile	H+	Br.
Intermediate	carbocation	radical
Regioselectivity	Markovnikov	Anti-Markovnikov

• The change in selectivity can be rationalized by the reversal of the order in which the -H and -Br add

MECHANISM FOR RADICAL REACTION OF ALKENES WITH HBr
Step 1: An electrophilic bromine radical adds to the alkene to generate the 2o radical.
Step 2: Radical abstracts a H atom from another molecule of HBr, creating the alkyl bromide and another bromine radical.

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