Alkene

Objectives:

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