During this reaction,
- 2 bonds are broken, pi bond and H-X bond
- 2 bonds are formed, C-X and C-H
- In H-X and H has a small positive charge X is negatively charged.
- H is attracted to electron rich double bond.
- Therefore Electrophilic addition occurs.
- Addition reactions are exothermic.
- 2 sigma bonds formed are stronger than the the sigma and pi bond broken.
consists of 2 successive Lewis acid-base reactions. ( 2 step mechanism)
 Alkene is the Lewis base. H-X is the Lewis acid.
The Activation energy is higher than that of .
Therefore  is slower than .
2 Bonds break only one is formed.
Therefore ΔH01 is (+)
Results with the formation of a carbocation.
Rate determining step.
 X- is the Lewis base. Carbocation is the Lewis acid.
The Activation energy is lower than that of .
Therefore  is faster than .
Only bond making occurs,one bond is formed.
Therefore ΔH01 is (-)
- Markovnikov's Rule - With an unsymmetric alkene H atom bonds to the less substituted carbon.
- Hammond postulate - Structure of the transition state of an endothermic reaction resembles the products while that of an exothermic reaction resembles reactants.
A new stereogenic center is formed. New enantiomers are formed in equal amounts making a racemic mixture.This is due to the fact that the nucleophilic attack of the X- occurs from either side of the trigonal planar carbocation and have equal probability.
Therefore syn and anti addition can occur.