Chapter 6: Reactions of Alkenes: Addition Reactions

Reaction of Alkenes with Hydrogen Halides

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 (an empirical rule based on experimental observations):

"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 of the C=C to give the more stable carbocation intermediate followed by attack of the nucleophile with the carbocation.
• Reaction is not stereoselective since reaction proceeds via planar carbocation (similar to SN1 reactions).
• HBr is usually used in the dark and under an inert atmosphere (e.g. N₂) since care must be taken to avoid the formation of radicals when an alternate mechanism occurs.

• Hydration of alkenes
• Addition of HBr to alkenes under radical conditions

MECHANISM FOR REACTION OF ALKENES WITH HBr
Step 1: An acid/base reaction. Protonation of the alkene to generate the more stable carbocation.  The π electrons act pairs as a Lewis base.
Step 2: Attack of the nucleophilic bromide ion on the electrophilic carbocation creates the alkyl bromide.

© Dr. Ian Hunt, Department of Chemistry