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ALKENES

Methods of Preparation of Alkenes:

1. From Alkynes: By hydrogenation Pd deactivated with charcoal or with Na-liquid NH 3
R-C C-R + H2 Pd / C R-CH=CH-R (cis Alkene)
R-C C-R + H2 Na/ liquid NH3 R-CH=CH-R (trans Alkene)
2. From Alkyl halides: By Dehydrohalogenation or β-Elimination reaction
R-CH2-CH2-X + alc. KOH heat R-CH=CH2 + KX + H2O
3. From Vicinal dihalides:
CH2Br-CH2Br + Zn CH2=CH2 + ZnBr2
4. From Alcohols: By acidic dehydration
R-CH2-CH2-OH Conc. H2SO4/443K R-CH=CH2 + H2O
Physical properties:
(i)Cis isomer has higher boiling point than trans isomer due to more polarity of cis isomer.
(ii)Trans isomer has higher melting point than cis isomer due symmetry in trans isomer.
Chemical properties:
1. Addition of Hydrogen:
R-CH=CH2 + H2 Ni/Pt/Pd R-CH2-CH3
2. Addition of Halogen: Test for unsauration, Reddish brown colour of Br2 is decolourised.
CH2=CH2 + Br2 CCl4 CH2(Br)CH2(Br)
3. Addition of Hydrogen halides:
(a) Addition of HBr to symmetrical alkene:
CH2=CH2 + HBr CH3-CH2-Br
(b) Addition of HBr to unsymmetrical alkene: Markownikov’s Rule
It states that when a polar compound is added to unsymmetrical alkene, the
negative part of the addendum goes to that carbon atom which contains less no. of
H-atoms around C=C double bond.
CH3-CH=CH2 + HBr CH3-CH(Br)CH3
(c) Anti-Markownikov’s Rule or Peroxide effect or Kharash effect:
It states that when HBr is added to unsymmetrical alkene in presence of peroxide,
negative part of the addendum goes to that carbon atom which contains more no.
of H-atoms around C=C double bond.
CH3-CH=CH2 + HBr peroxide CH3-CH2-CH2-Br
4. Addition of H2SO4:
CH2=CH2 + H2SO4 CH3-CH2-HSO4
CH3-CH=CH2 H2SO4 CH3-CH(HSO4)-CH3
5. Addition of water:
CH3-CH=CH2 + H2O CH3-CH(OH)CH3
6. Oxidation:
(a) CH2=CH2 + H2O + O dil. KMnO4 /273K CH2(OH)CH2(OH)
(b) CH3-CH=CH2 + H2O + O dil. KMnO4 /273K CH3-CH(OH)CH2(OH)
(c) (CH3)2C=CH2 KMnO4 /H + (CH3)2C=O + CO2 + H2O
(d) CH3-CH=CH-CH3 KMnO4 /H + 2CH3-COOH
7. Ozonolysis:
CH3-CH=CH2 + O3 Zn/H2O CH3-CHO + H-CHO
CH3-C(CH3)=CH2 + O3 Zn/H2O (CH3)2C=O + H-CHO
8. Polymerisation:
n(CH2=CH2) High temp, pressure/catalyst -(CH2-CH2)-n