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Alkenes

Preparation of Alkenes in Organic Chemistry

Preparation of Alkenes or olefins in Organic Chemistry Laboratory synthesis

Preparation of alkenes in the laboratory done by different synthesis mechanism in organic chemistry, alkene from the dehydration of alcohol or alcohols, elimination of organic compound (ester or xanthates). The witting reaction is the most common and important chemical reactions for the preparation of alkene for learning chemistry. Alkene or olefin is the untrusted hydrocarbon with molecular formula CnH2n, contain at least one double bond or olefinic chemical bond obtained by creaking natural petroleum products.

Dehydration of Alcohols to Alkenes

The synthesis or preparation of the alkenes from alcohol is an important reaction mechanism in organic chemistry. When primary alcohol heated with concentrated sulfuric acid at 170° to 180°C to produce alkenes.

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

The preparation of alkenes from 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.

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

Saytzeff Formula in Organic Synthesis

Rearrangement in the preparation of alkenes 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

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

Elimination Reaction in Alkenes Preparation

Most eliminations in alkenes preparation occur by polar mechanism, whereas cyclic eliminations are uni-molecular non-polar chemical 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 equilibrium reactions show a negative entropy of activation in chemistry.

Halogenoalkanes to Alkene

Alkenes like Propene can be synthesis by 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 chemical elements zinc dust and methanol produce alkenes.

CH3CH2CHBr2 → CH3CH=CH2

Zinc dust and methanol also dehalogenation of 1,2- di halogen derivative of alkanes uses for the preparation of alkenes. Therefore propene is prepared from propene dibromide.

CH3CHBrCH2Br+Zn → CH3CH=CH2

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 produced alkenes.

(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 an alkyl halide in ether solution. 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 carbon chemistry 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.