Methods of preparation of alkenes

Preparation of alkene from alcohol

    Primary alcohols when heated with concentration H₂SO₄ at 170° to 180°C to produce alkenes.
C₂H₅OH + H⁺

C₂H₅OH₂⁺

H₂O + C₂H₅⁺

CH₂=CH₂ + H⁺
    Secondary and tertiary-alcohol are best carried out by using dilute H₂SO₄.
    However tertiary alcohol can polymerize under the influence of concentration H₂SO₄. Acid-catalyzed dehydration of primary alcohol gives 1-alkenes but secondary and tertiary alcohol gives a mixture of alkenes due to rearrangement of alcohol intermediate.
    Rearrangement however avoided by dehydration of alcohol over alumina in pyridine.
CH₃–CH₂–CH(CH₃)–OH

CH₃–CH₂–CH=CH₂ + CH₃–CH=CH–CH₃

CH₃–CH₂–CH(CH₃)–OH → CH₃–CH₂–CH=CH₂
    Rearrangement often occurs with acid-catalyzed dehydration. All the three types of alcohol may behave in this way via a carbonium ion that may undergo methyl or hydride ion 1,2 shift.
Preparation of alkenes
Preparation of alkenes
In each case, the major product is in accordance with Saytzeff's Rule

Saytzeff's Rule:

This rule may be stated in two ways: The predominant product is the most substituted alkene that is the one carrying the largest number of alkyl substituents.
Or, hydrogen is eliminated preferentially from the carbon atom joined to the least number of hydrogen atoms. 
  • Problem:
Convert (i) MeCH₂CH₂OH  MeCH(OH)Me
(ii) Me₂CHCH₂OH Me₃COH(iii) Me₂CHCH(OH)Me Me₂C(OH)CH₂Me  
Dehydration of Alcohol
Conversation MeCH₂CH₂OH → MeCH(OH)Me
Dehydration of Alcohol
Conversation Me₂CHCH₂OH → Me₃COH
Conversation Me₂CHCH(OH)Me  Me₂C(OH)CH₂Me

2. By Cyclic Elimination: 


Most eliminations occur by polar mechanism, whereas cyclic eliminations are uni-molecular non-polar reactions which take place in one step. Most occur when the compound is subjected to pyrolysis, and proceed via a cyclic transition state.This mechanism is supported by the fact that these reactions show a negative entropy of activation. 

(a) Pyrolysis of Ester:


    500°C                    

R₂CHCH₂OCOMe RCH=CH₂ + HOCOMe

Mechanism:
Reaction Mechanism Of Pyrolysis of Easter
(b) Pyrolysis of Xanthates: This is known as Tschugaev Reaction.


   CS₂                                                MeI
RCHCH₂OH RCH₂CH₂–O–CS–SNa RCH=CH₂
       NaOH                                           200°C
Reaction Mechanism:
Methods of Preparation of alkenes
Reaction Mechanism of Pyrolysis of Xanthate

(c) Cope Reaction:
The reaction in which alkenes are formed when amine oxides are heated.

150°C         

RCH₂CH₂NOMe₂   RCH=CH₂ + Me₂NOH
Methods of Preparation of alkenes
Reaction Mechanism of Cope Reaction

3. By the action of ethanolic potassium hydroxide on alkyl halides:

    As for example, propene from propyl bromide.

    CH₃CH₂CH₂Br + KOH CH₃CH=CH₂
    Dehalogination: 

    CH₃CH₂CHBr₂  CH₃CH=CH₂
    Zinc dust and methanol also dehaloginate 1,2-dihalogen derivative of alkanes, as for example propene from propene dibromide .
    CH₃CHBrCH₂Br+Zn  CH₃CH=CH₂
    4.By Heating of Quaternary ammonium hydroxide:
    As for example , 
    (C₂H₅)₄N + OH⁻  CH₃CH=CH₂
    5. Boord Synthesis:
    Boord have prepared alkenes by conversion of an aldehyde into its chloro-eather, treating this with bromine followed by a Grignard reagent and finally treating the product with zinc and n-butanol.
    RCH₂CHO RCH=CH₂
    Methods of Preparation of alkenes
    General Method of Boord Synthesis
    This method is very useful for preparing alkenes of definite structure, and an interesting point about it is the replacement of the ∝-chlorine atom by bromine when the ∝-chloro eather undergoes bromination in the β-position.

    6. The Wittig Reaction:


      Wittig reaction affords an important and useful method for the synthesis of alkenes by the treatment of aldehyde or ketones with alkylidenetriphynylphosphorane (Ph₃P=CR₂) or simply known as phosphorane. 
      Ph₃P = CH₂ + Ph₂C=O Ph₂C=CH₂ + Ph₃P = O
    1,1-Diphenyl Triphenyl and ethylene Phosphonium oxide
    The witting reagent, alkylidenetriphenylphosphorane, is prepared by treating triarylphosphine usually the latter with an alkyl halide in eather solution. The resulting phosphonium salt is treated with strong base (such as C₆H₅Li, BuLi, NaNH₂, NaH, C₂H₅ONa etc.) which removes a halacid to give the reagent, methylenetriphenyl phosphorane.
    Methods of Preparation of alkenes
    Preparation of Wittig Reagent

    Mechanism:
    The reaction probably proceeds by the nucleophilic attack of the yield on the carbonyl carbon. The dipolar complex (betain) so formed decomposes to olefine and triphenylphosphine oxide via a four-centered transition state.
    Methods of Preparation of alkenes
    Reaction Mechanism of Wittig Reaction