What are alkenes?
Alkenes are unsaturated hydrocarbon or organic compounds that contain one double bond between two successive carbon atoms. They have the general molecular formal CnH2n. The double bond in alkenes is known as the olefinic or ethylenic chemical bond.
They are also called olefin. The olefin name of alkenes comes from ethylene because ethylene is called olefiant gas or oil forming gas. It is formed oily liquids when treated with chlorine or bromine.
Preparation of alkenes
A number of alkenes like ethylene, propylene, and butylene can be prepared by the creaking of petroleum.
- For the industrial production of lower alkenes, the most suitable starting material is gas oil.
- For the production of higher alkenes, we used paraffin wax.
Some important laboratory preparation methods of alkenes are given below the picture.
In the laboratory, alkenes can be prepared from different organic substances like alkanes, alcohol, ester, aldehyde, etc. In organic chemistry, it can be prepared in the laboratory by following synthesis methods,
- Dehydration of alcohol
- Pyrolysis of ester
- Cope reaction
- Alkyl halide to alkene
- From Grignard reagent
- Wittig reaction
Dehydration of alcohol
Dehydration of alcohol is an important method for the preparation of alkenes.
- The primary alcohol is heated with concentrated sulfuric acid at 170 °C to 180 °C to produce alkenes.
- Dehydration of secondary and tertiary alcohol is best carried out in the laboratory by using dilute sulfuric acid.
- Tertiary alcohol can polymerize under the influence of the concentrated sulfuric acid solution.
Dehydration of primary alcohol with an acid catalyst normally gives one alkene.
But secondary and tertiary alcohol gives a mixture of alkenes due to the rearrangement of the carbocation. However, rearrangement can be avoided by dehydration of alcohol over alumina in pyridine at 350 °C.
Saytzeff rule
Rearrangement in the preparation of alkene 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 is stated in two ways
- The predominant product is the most substituted alkene which is the one carrying the largest number of alkyl substituents.
- Hydrogen is eliminated preferentially from the carbon atom which is joined to the least number of the hydrogen atom.
Pyrolysis of ester
Most eliminations occur by polar mechanisms. The cyclic eliminations are uni-molecular non-polar chemical kinetics that takes place in one step.
When the compound is subjected to pyrolysis, the reaction proceeds via a cyclic transition state. This organic mechanism is supported by the fact that these chemical equilibrium reactions show a negative entropy of activation.
Cope reaction
Cope reaction is used for the preparation of alkenes by heating amine oxide at about 150 °C. The reaction was also carried out in presence of dimethyl sulfoxide or tetrahydrofuran at room temperature.
Alkyl halide to alkene
Alkene like propene can be synthesized by the chemical reaction of propyl bromide with ethanolic potassium hydroxide.
CH3CH2CH2Br + KOH → CH3CH＝CH2
Dehalogenation of 1,1 – di-halogen derivatives of alkanes by means of zinc dust and methanol produce alkenes.
CH3CH2CHBr2 → CH3CH＝CH2
Zinc dust and methanol are also dehalogenations of 1,2- di halogen derivatives of alkanes uses for the preparation of alkenes. Therefore propene is prepared from propylene dibromide.
CH3CHBrCH2Br + Zn → CH3CH＝CH2
From Grignard reagent
Boord has prepared alkenes by conversion of an aldehyde into its chloro-ether. This chloro-ether reacts with bromine followed by the Grignard reagent. Finally, the product treated with zinc in n-butanol produced alkenes.
This method is very useful for the preparation of alkene with a definite structure. An interesting point about it is the replacement of the ∝-chlorine atom by bromine when the ɑ-chloro ether bromination is in the β-position.
Wittig reaction
How to prepare wittig reagent?
The Wittig reagent, alkylidene-triphenyl phosphorane prepared by treating triarylphosphine with an alkyl halide in ether solution. The resulting phosphonium salt is treated with a strong base such as C6H5Li, BuLi, NaNH2, NaH, and C2H5ONa.
Preparation of alkenes by Wittig reaction
Wittig reagent is an important and useful chemical compound for the synthesis or preparation of alkenes by the reaction of aldehyde or ketones with triphenylphosphonium ylide or phosphorane molecule.
