Wednesday, July 20, 2011

Hydrohalogenation - Electrophilic Addition of HX


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.

Mechanism 
consists of 2 successive Lewis acid-base reactions. ( 2 step mechanism)
[1] Alkene is the Lewis base. H-X is the Lewis acid.
     The Activation energy is higher than that of [2].
     Therefore [1] is slower than [2].
     2 Bonds break only one is formed.
     Therefore ΔH01 is (+)
     Results with the formation of a carbocation. 
     Rate determining step.
[2] X- is the Lewis base. Carbocation is the Lewis acid.
     The Activation energy is lower than that of [1].
     Therefore [2] is faster than [1].
     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.


Stereo Chemistry
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.  

Extra reading




2 comments:

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