Properties of Alkenes:
The first three members (C2H4, C3H6 and C4H8) are colourless gases; the higher ones i.e. C5 to C18 are liquid and still higher members are solid at ordinary temperature. Alkenes are insoluble in water but soluble in organic solvent. Boiling and melting points of alkenes increases with increase of molecular mass.
Chemical Reactions:
Alkenes are more reactive then alkanes because of presence of double bond (pi-bond). Alkenes mostly undergo addition reactions with hydrogen, halogens, water, halogen halide and ozone.
1) Addition of hydrogen:
When alkene is heated with hydrogen in presence of Pd, Pt or Ni, give alkane.
CH2 = CH2 + H2 —[Ni]—-> CH3 — CH3
Ethylene Ethane
2) Addition of halogen:
Chlorine and bromine reacts with alkene at ordinary temperature to form dihalides.
CH2 = CH2 + Br2 ——–> Br—CH2 — CH2—Br
Ethylene Dibromoethane
3) Addition of Water:
Alkenes reacts with water in presence of dil. H2SO4 to produce alkyl hydrogen sulphate.
CH2 = CH2 + H—OH ——-> CH3 — CH2—OH
Ethylene Ethyl alcohol
4) Addition of hydrogen halide:
Alkenes reacts with halogen acids (HCl, HBr or HI) to form alkyl halides.
CH2 = CH2 + HBr ——-> CH3 — CH2—Br
Ethylene Ethyl Bromide
Markovnikov’s Rule:
When an unsymmetrical reagent adds to an unsymmetrical alkene, the positive part of the reagent becomes attached to that double bonded carbon which bear the higher number of hydrogen atoms.
Anti-Markovnikov’s Rule (Peroxide Effect):
When an unsymmetrical reagent adds to an unsymmetrical alkene in presence of organic peroxide, the positive part of the reagent becomes attached to the double bonded carbons which bear the least number of hydrogen atoms.
5) Ozonolysis:
When ozone is passed through an alkene in an inert solvent, it adds across the double bond to form an ozonide. Ozonides are explosive compounds, are not to isolate. On warming with zinc and water, ozonides cleave to give carbonyl compounds.
6) Oxidation:
Alkene oxidizes to glycol, acetic acid or epoxide depending on the oxidant. e.g.
