All About Alkenes webblog

Alkene-Addition Reaction

Alkenes react in many addition reaction, which occur by opening up the double-bond.

  • Catalytic hydrogenation: Hydrogenation of alkenes produces the corresponding alkaness. The reaction is carried out under pressure in the presence of a metallic catalyst. Common industrial catalysts are based on platinumm, nickel or palladium. For laboratory syntheses, Raney nickel (an alloy of nickel and aluminium)is often employed. The simplest example of this reaction is the catalytic hydrogenation of ethylene to yield ethane:
CH2=CH2 + H2 → CH3-CH3
  • Electrophilic halogenation: Addition of elemental bromine or chlorine to alkenes yields vicinal dibromo- and dichloroalkanes, respectively. The decoloration of a solution of bromine in water is an analytical test for the presence of alkenes:
CH2=CH2 + Br2 → BrCH2-CH2Br
It is also used as a quantitive test of unsaturation, expressed as the bromine number of a single compound or mixture. The reaction works because the high electron density at the double bond causes a temporary shift of electrons in the Br-Br bond causing a temporary induced dipole. This makes the Br closest to the double bond slightly positive and therefore an electrophile.
  • Hydrohalogenation: Addition of hydrohalic acids such as HCl and HBr :CH3-CH=CH2 + HBr → CH3-CHBr-CH2H
If the two carbon atoms at the double bond are linked to a different number of hydrogen atoms, the halogen is found preferentially at the carbon with fewer hydrogen substituents (Markovnikov’s rule).
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January 31, 2010 Posted by | Addition Reaction Of Alkenes | Leave a comment