General Methods for the Preparation of Alkenes

General Methods of Preparation of Alkenes: 

1. From Alcohol : 

Primary alcohols when heated with conc. H₂SO₄ at 170° to 180°C to produce alkenes.

C₂H₅OH + H⁺ → C₂H₅OH₂⁺ → H₂O + C₂H₅⁺ → CH₂=CH₂ + H⁺

Secondary and tertiary-alcohol is best carried out by using dil H2SO₄.

However tertiary alcohol can polimerise under influence of conc. 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. 
                                             dil H₂SO₄                         

CH₃–CH₂–CH(CH₃)–OH → CH₃–CH₂–CH=CH₂ + CH₃–CH=CH–CH₃

                                      Al₂O₃ in Pyridine

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 undergoes methyl or hydride ion 1,2 shift.

Examples: 
Me₂CHCH₂OH→ Me₂C=CH₂ + CH₂=CHCH₂Me + MeCH=CHMe

Dehydration of Primary Alcohol

Me₃CCH(OH)CH₃→ Me₃CCH=CH + CH₂=CMeCHMe₂ + Me₂C=CMe₂

Dehydration of Secondary Alcohol

Me₃CCH₂OH → CH₂ = C(Me)CH₂Me + Me₂C = CHMe

Dehydration of Primary Alcohol

In each case, the major product is in accordance with Saytzeff's Rule. 

Saytzeff's Rule:

This rule may be states 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. 

Convert (i) MeCH₂CH₂OH → MeCH(OH)Me

(ii) Me₂CHCH₂OH → Me₃COH(iii) Me₂CHCH(OH)Me → Me₂C(OH)CH₂Me  

Conversation MeCH₂CH₂OH → MeCH(OH)Me

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

R₂CHCH₂OH ⟶ RCH₂CH₂–O–CS–SNa ⟶ RCH=CH₂

       NaOH                                           200°C

Reaction Mechanism:

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

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 + ZnBr₂

Zinc dust and methanol also dehaloginate 1,2-dihalogen derivative of alkanes, as for example propene from propene dibromide .

CH₃CHBrCH₂Br+Zn → CH₃CH＝CH₂ + ZnBr₂

4.By Heating of Quaternary ammonium hydroxide:

As for example , 

(C₂H₅)₄N + OH⁻ → C₂H₄ + (C₂H₅)₃N+H₂O

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

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.

Preparation of Wittig Reagent

Mechanism:

The reaction probably proceeds by the nucleophilic attack of the ylid on the carbonyl carbon. The dipolar complex (betain) so formed decomposes to olefine and triphenylphosphine oxide via a four-centered transition state.

Reaction Mechanism of Wittig Reaction