Preparation of Alkenes

Synthesis of Alkenes in Organic Chemistry

Alkenes are the untrusted hydrocarbon with molecular formula CnH2n, contain at least one double bond or olefinic chemical bond. Laboratory preparation of alkenes done by different synthesis mechanism in organic chemistry, alkene from the dehydration of alcohol or alcohols, elimination of organic compound (ester or xanthates), and the witting reaction are the most common and important chemical reactions for the preparation of alkene for study chemistry.

Reaction and Mechanism of Dehydration of Alcohols

The synthesis of the alkene from alcohol is an important reaction mechanism in organic chemistry. When primary alcohols heated with concentrated sulfuric acid at 1700 to 1800C to produce alkene.

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

But when secondary and tertiary-alcohol are best carried out in the laboratory by using dilute sulfuric acid. Tertiary alcohol can polymerize under the influence of concentrated sulfuric acid.

Organic preparation of alkenes from alcohol dehydration reaction in chemistry

Synthesis of an organic compound from primary alcohol with an acid catalyst gives one alkene. But secondary and tertiary alcohol gives a mixture of alkenes due to the rearrangement of carbocation form by an alcohol dehydration reaction in the laboratory. Rearrangement reaction avoided by dehydration of alcohol over alumina in pyridine.

Saytzeff Formula in Organic Synthesis

Rearrangement often occurs with acid-catalyzed dehydration. Hence all 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 which 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 Mechanism in Alkenes Preparation

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

Laboratory synthesis mechanism of the alkene from organic compound

Halogenoalkanes to Alkene

  • Propene can be synthesis by chemical reaction of propyl bromide with ethanolic potassium hydroxide.


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


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


Laboratory Preparation of Alkene

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.

(C2H5)4N+OH → CH3CH=CH2

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

Witting Reaction for Alkenes Preparation

The witting reagent, alkylidene-triphenyl phosphorane prepared by treating triarylphosphine with alkyl halide in ether solution. Thus the resulting phosphonium salt treated with a strong base such as C6H5Li, BuLi, NaNH2, NaH, C2H5ONa.

Ph3P + CH3Br → Ph3PCH3 → Ph3P=CH2
Ph3P = CH2 + Ph2C=O → Ph2C=CH2 + Ph3P=O

Therefore, Wittig reagent in organic chemistry is an important and useful chemical compound for the synthesis of an alkene by the reaction of aldehyde or ketones with triphenylphosphonium ylide or phosphorane.