Methods of preparation of alkenes

Preparation of alkenes from alcohols

Primary alcohols when heated with concentrated sulfuric acid 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 sulfuric acid. Tertiary alcohol can polymerize under the influence of concentrated sulfuric acid.

Acid-catalyzed dehydration of primary alcohol gives 1-alkenes. Secondary and tertiary alcohol gives a mixture of alkenes due to the rearrangement of carbocation form by an alcohol dehydration reaction.

CH₃–CH₂–CH(CH₃)–OH

CH₃–CH₂–CH=CH₂ + CH₃–CH=CH–CH₃

Rearrangement reaction avoided by dehydration of alcohol over alumina in pyridine.

Preparation of alkenes from alcohols
Alkenes from alcohols

Saytzeff rule simple definition

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. The major product is in accordance with Saytzeff’s rule.

This rule may be stated in two ways

  1. The predominant product is the most substituted alkene that is the one carrying the largest number of alkyl substituents.
  2. Hydrogen is eliminated preferentially from the carbon atom joined to the least number of hydrogen atoms.

Elimination reaction of alkenes

Most eliminations occur by polar mechanism, whereas cyclic eliminations are uni-molecular non-polar reactions which take place in one step. When the compound is subjected to pyrolysis, and proceed via a cyclic transition state.

This mechanism is supported by the fact that these chemical reactions show a negative entropy of activation.

Pyrolysis of ester reaction, the ester usually acetate.

R₂CHCH₂OCOMe ⟶ RCH=CH₂ + HOCOMe

What type of reaction is Halogenoalkanes to alkene?

Propene can be prepared from propyl bromide by the action of ethanolic potassium hydroxide.

CH₃CH₂CH₂Br + KOH → CH₃CH=CH₂

Dehalogenation of 1,1 – di-halogen derivatives of alkanes by means of zinc dust and methanol produces alkenes.

CH₃CH₂CHBr₂ → CH₃CH=CH₂

Zinc dust and methanol also dehalogenation of 1,2- di halogen derivative of alkanes. Propene from propene dibromide.

CH₃CHBrCH₂Br+Zn → CH₃CH=CH₂

Heating of quaternary ammonium hydroxide salts

(C₂H₅)₄N⁺ OH⁻ → CH₃CH=CH₂

Boord has prepared alkenes by conversion of an aldehyde into its chloro – ether. This chloro – ether reacts with bromine followed by a Grignard reagent and finally treating the product with zinc and n-butanol.

This method is very useful for preparing alkenes of definite structure. An interesting point about it is the replacement of the ∝-chlorine atom by bromine when the ɑ-chloro ether undergoes bromination in the β-position.

Triphenylphosphonium ylide

The witting reagent, alkylidene-triphenyl phosphorane prepared by treating triarylphosphine usually the latter with an alkyl halide in ether solution.

The resulting phosphonium salt is treated with a strong base such as C₆H₅Li, BuLi, NaNH₂, NaH, C₂H₅ONa.

Ph₃P + CH₃Br

Ph₃PCH₃⁺Br⁻ + PhLi

Ph₃P=CH₂

Wittig reaction affords an important and useful method for the synthesis of alkenes by the treatment of aldehyde or ketones with triphenylphosphonium ylide or phosphorane.

Ph₃P = CH₂ + Ph₂C=O

Ph₂C = CH₂ + Ph₃P = O

Triphenylphosphonium ylide
Triphenylphosphonium ylide