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Preparation of alkenes

  1. Dehydrohalogenation of haloalkanes: The reaction of a haloalkane with an alcoholic solution of potassium hydroxide at high temperature forms an alkene with the elimination of a molecule of hydrogen. The hydrogen of an alkyl halide which is eliminated comes from a beta-carbon and the halogen comes from an alpha-carbon. Since in this reaction the hydrogen is lost from beta-carbon, therefore the reaction is classified as beta-elimination.
     
    Description: 45331.png
     
    Description: 45340.png
     
    Description: 45348.png
     
    However, if the size of base is increased, it finds it relatively easier to abstract proton from a less-substituted b-carbon atom than from more substituted b-carbon atom of alkyl halide. Therefore, less stable alkene becomes the major product. This is known as Hoffmann’s rule. For example,
     
    Description: 45361.png
  2. Dehalogenation of vicinal dihalides:
     
    Description: 45371.png
     
    Description: 45379.png
  3. Cleavage of ethers:
     
    Description: 45389.png
  4. Pyrolysis of esters: Thermal cleavage of an ester, usually acetate, involves the formations of a six-membered ring as the transition state, leading to the elimination of acid leaving behind alkene.
     
    Description: 45397.png
     
    As a direct consequence of cyclic transition state, both the leaving groups, namely proton and carboxylate ion, are in the cis position. This is an example of cis elimination.
  5. Partial reduction of alkynes: Reduction of alkyne to alkenes is brought about by any one of the following reducing agents.
    1. Alkali metal dissolved in liquid ammonia.
    2. Hydrogen in the presence of palladium poisoned with BaSO4 or CaCO3 along with quinolone (Lindlar’s catalyst).
    3. Hydrogen in the presence of Ni2B (nickel boride).
       
      Description: 45415.png
       
      Alkali metal dissolved in liquid ammonia produces nearly 100% trans alkene by the following mechanism:
       
      Na + liq. NH3 Na+ + es (solvated electron)
       
      Description: 45427.png
  6. Hoffmann degradation method: Alkenes can be prepared by heating quaternary ammonium hydroxide under reduced pressure at a temperature between 100°C and 200°C.
     
    Description: 45437.png
  7. Wittig reaction:
     
    Description: 45445.png
     
    Description: 45455.png
     
    Description: 45464.png
     
    (R, R, R, and R”' may be hydrogen or any alkyl group)

Chemical properties of the alkenes

  1. Hydrogenation: Alkenes are readily hydrogenated under pressure in the presence of a catalyst.
     
    Description: 45475.png
     
    One molecule of hydrogen is adsorbed for each double bond present in the unsaturated compound. The rate of hydrogenation of alkenes at room temperature and atmospheric pressure is
     
    – CH = CH2>–CH = CH– or a ring double bond
     
    Alkenes of the type R2C = CR2 or R2C = CHR are difficult to hydrogenate under the above reaction conditions.
  2. Addition of hydrogen halides: Hydrogen halides (HCl, HBr, and HI) add to the double bond of alkenes.
     
    Description: 45485.png
     
    Mechanism for the addition of hydrogen halide to an alkene involves the following two steps:
     
    Step 1: Description: 45493.png
     
    Step 2: Description: 45502.png
     
    The addition of HBr to some alkenes gives a mixture of the expected alkyl bromide and an isomer formed by rearrangement.
     
    Description: 45510.png
     
    Description: 45520.png
  3. Addition of hydrogen bromide in presence of peroxide
     
    Chain initiation step:
     
    R – O – O – R 2RO. (–O–O–bond is weak)
     
    RO + H – Br RO – H + Br

    Chain propagation step
     
    Description: 45540.png
     
    The initiation is by Br, as hydrogen abstraction by RO from HBr is energetically much more favorable than the alternative of bromine abstraction to form ROBr + H+. The alternative addition of Br to (1) to form MeCH(Br)CH2 (4) does not occur, as secondary radical Description: 45543.png is more stable than primary radical.
     
    HBr is the only one of the four hydrogen halides that will add readily to alkenes via a radical pathway. The reason for this is reflected in the ΔH values (kJ/mol) for the two steps of the chain reaction for addition of HX to CH2 = CH2. For example,
     
      (1) X + CH2 = CH2 (2) XCH2CH2 + HX
    H – F
    H – Cl
    H – Br
    H – I
    –188
    –109
    –21
    +29
    +155
    +21
    –46
    –113
     
    Only for HBr, both the chain steps are exothermic, while for HF the second step is highly endothermic, reflecting the strength of the H – F bond and the difficulty of breaking it. For HCl, it is again the second step that is endothermic (though not to such a great extent) while for HI it is the first step that is endothermic, reflecting the fact that the energy gained in forming the weak I – C bond is not as much as that is lost in breaking the C = C π-bond. Thus only a few radical additions of HCl are known, but the reactions are not very rapid and the reaction chains are short at ordinary temperatures.
  4. Addition of water
     
    Acid-catalyzed hydration
     
    Description: 45558.png
     
    Oxymercurationdemercuration
     
    Description: 45566.png
     
    Hydroborationoxidation
     
    2R – CH = CH2 + B2H6 2RCH2CH2BH2
     
    R CH2CH2BH2 + R – CH = CH2 (R CH2CH2)2BH
     
    (R CH2CH2)2BH + R – CH = CH2 (RCH2CH2)3B
     
    Oxidation of trialkylborane is carried out by alkaline H2O2 which results in the formation of alcohol as if water has been added to alkene according to anti-Markovnikov’s rule.
     
    H – O – O – H + OH H – O – O + H2O
     
    Description: 45592.png
     
    Description: 45603.png
     
    Description: 45616.png
     
    Description: 45626.png
  5. Addition of bromine and chlorine
     
    Mechanism
     
    Description: 45634.png
     
    When cyclopentene reacts with bromine in CCl4, anti-addition occurs and the products of the reaction are trans-1,2-dibromocyclopentane enantiomers (as a racemate).
     
    When cis-2-butene adds bromine, the product is a racemic form of 2-3-dibromobutane. When trans-2-butene adds bromine, the product is the meso compound.
     
    Description: 45578.png
     
    Description: 45644.png
  6. Oxidative cleavage by hot alkaline KMnO4
     
    Description: 45654.png
  7. Hydroxylation
     
    Syn hydroxylation
     
    Description: 45663.png
     
    Anti-hydroxylation
     
    Description: 45673.png
  8. Ozonolysis
     
    Description: 45682.png
     
    Zn + H2O2ZnO + H2O
  9. Substitution reactions at allylic position
     
    Description: 45691.png




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