Chapter 6: Reactions of Alkenes: Addition Reactions

Summary

• When treated with aq. acid, most commonly H₂SO₄,  alkenes form alcohols.
• Electrophile : H+
• Regioselectivity predicted by Markovnikov's rule
• Reaction proceeds via protonation to give the more stable carbocation intermediate.
• Not stereoselective since reactions proceeds via a planar carbocation.

• Reaction of alkenes with hydrogen halides, HX
• Hydroboration / oxidation of alkenes
• Hydration of alkynes

MECHANISM FOR REACTION OF ALKENES WITH H3O+
Step 1: An acid / base reaction. Protonation of the alkene to generate the more stable carbocation.  The pi electrons act as a Lewis base.
Step 2: Attack of the nucleophilic water molecule on the electrophilic carbocation creates an oxonium ion.
Step 3: An acid / base reaction. Deprotonation by a base generates the alcohol and regenerates the acid catalyst.
In this scheme, examples of what the B: could be inlcude the conjugate base of the acid catalyst (e.g. HSO4-), the C=C in the starting material or a molecule of H2O.

© Dr. Ian Hunt, Department of Chemistry