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Hydrohalogenation of Hydrocarbons

Key Concepts

  • Unsaturated hydrocarbons (alkenes and alkynes) react with hydrogen halides (HX) in addition reactions producing halogenated compounds.

      H
    |
     H
    |
      
    R-C=C-R'
           
    + H-X
      H
    |
     H
    |
      
    R-C-C-R'
      |
    H
     |
    X
      

  • If the alkene or alkyne molecule is symmetrical, R is the same as R', then only one product is formed.

  • If the alkene or alkyne molecule molecule is unsymmetrical, R is NOT the same as R', then more than one product will be produced.

  • Markovnikov's Rule is applied in order to determine which product is produced with the the greater yield, the major product.

  • Markovnikov's Rule:
    In additions of HX to unsymmetrical alkenes, the H+ of HX goes to the double-bonded carbon that already has the greatest number of hydrogens

Animated Tutorial

Examples

Symmetrical Alkenes

Ethene (ethylene) is a symmetrical molecule.
  H
|
  H
|
 
  C = C  
  |
H
  |
H
 

Each carbon atom is covalently bonded to two hydrogen atoms.
Ethene reacts with hydrogen bromide to produce bromoethane.
ethene + hydrogen bromide bromoethane
C2H4 + HBr C2H5Br
  H
|
  H
|
 
  C = C  
  |
H
  |
H
 
+ H-Br
 H
|
 Br
|
 
H-C-C-H
 |
H
 |
H
 

Bromoethane is the only product possible for this reaction.

Unsymmetrical Alkenes

Propene (propylene) is an unsymmetrical molecule.
  H    
|    
      H
|
 
  C(1) = C(2) - C -H
  |    
H    
  |    
H    
  |
H
 

C(1) is covalently bonded to two hydrogen atoms.
C(2) is covalently bonded to one hydrogen atom.
When propene reacts with a halogen halide such as hydrogen bromide, two structural isomers are possible products:

  • If the bromine atom bonds with C(1), the product is 1-bromopropane.

  • If the bromine atom bonds with C(2), the product is 2-bromopropane.

propene + hydrogen bromide 1-bromopropane + 2-bromopropane
  H
|
      H
|
 
  C = C - C -H
  |
H
  |
H
  |
H
 
+ H-Br
 Br
|
 H
|
 H
|
 
H-C-C-C-H
 |
H
 |
H
 |
H
 
+
 H
|
 Br
|
 H
|
 
H-C-C-C-H
 |
H
 |
H
 |
H
 

Markovnikov's Rule states that the hydrogen adds to the carbon atom of the double bond which is already bonded to more hydrogen atoms, that is, the hydrogen atom adds to C(1) and the bromine atom adds to C(2) so the major product of the reaction will be 2-bromopropane.


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