Wednesday, May 4, 2011


1.        define the electrophiles and nucleophiles.
2.        Explain the mechanism of electrophilic addition in alkenes.
3.        Explain the chemistry of alkenes as exemplified by the reaction with :
        Hydrogen, hydrogen bromide, bromine in CCl4, bromine water, water and concentrated sulphuric acid.

Activity 1 :
Electrophile : An electron deficient species that can accept a pair of electrons. Examples : AlCl3, BF3, NO2+, H+
Nucleophile : An electron rich species that can donate a pair of electrons. Example : H2O, NH3, OH-, Cl-
  1.   The p bond in alkenes is electron rich and hence, alkenes are more reactive than alkanes. 
  2.  The electrophile attacks the electron rich centre at the C=C forming a carbonium ion (carbocation) as an intermediate.
  3.  This carbonium ion then combines with a nucleophile to form a product. Hence, alkenes undergo electrophilic addition reactions.
Activity 2:
1.        Hydrogenation
        CH2=CH2 + H2  à CH3CH3                                  catalysts : Ni, Pd or Pt
        Uses : catalytic hydrogenation is used in industry to convert liquid vegetable oil to semi-solid fats in the 
             manufacture of margarine.
2.        Addition of hydrogen halides à Electrophilic addition reaction.

4.        Markovnikov’s rule : When hydrogen halides are added to unsymmetric alkenes, the hydrogen atom is added to the carbon atom with a higher number of hydrogen atoms.
      CH3 – CH=CH2 + HBr à CH3 – CH – CH2



Bromination of Alkene

Bromination of Alkenes in water
The reaction between bromine water and ethene follows a similar mechanism as in CCl4. A brominium ion is also formed but there are now two nucleophiles involved ; a bromide ion and a water molecule.

Q: Why is 2-bromoethanol the major product ?  Water is in excess in the reaction.

Addition of water to alkenes
Addition of sulphuric acid

No comments:

Post a Comment