Preparation of Alkenes


Alkenes are the hydrocarbons that contain a double bond between any two adjacent or adjoining carbon atoms. The structure of alkene is such that the number of hydrogen atoms is twice the number of carbon atoms. Alkenes form a homologous series, hence, the general formula of an alkene is stated as CnH2n. The simplest alkene which has one double bond in its structure is ethene, C2H4. Alkenes have important industrial uses, and also play an important role in our everyday lives. 

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There are various methods to prepare an alkene. The most commonly used ones are explained below. 

Methods Of Preparation Of Alkenes

Preparation Of Alkenes From Alkynes 

Alkynes are hydrocarbons containing a triple bond between any two adjoining carbon atoms. Alkenes can be prepared from alkynes by carrying out hydrogenation in the presence of palletised charcoal. The charcoal which is used in this reaction has been moderately deactivated. Lindlar catalyst is palladium on calcium carbonate which has been deactivated by lead acetate to stop further hydrogenation. These alkenes will have a cis- form. To obtain a trans- alkene, these alkynes should be made to react with sodium in liquid ammonia. 

                             CH \[\equiv\] CH + H2 -------> CH2=CH2 (Δ, Lindlar catalyst/ Pd/C) 

                              Ethyne                           Ethene

                              RC \[\equiv\] CH + H2  --------->. RCH=CH2 (Pd/C)

                               Alkyne                           Alkene

                              CH3-C \[\equiv\] CH + H2 ------------>  CH3-CH=CH(Pd/C)

                              Propyne                                   Propene

Preparation of Alkenes From Alkyl Halides

In order to form alkenes from alkyl halides, dehydrohalogenation is performed. Alkyl halides have to be heated in the presence of alcoholic KOH. Alcoholic KOH is obtained when potassium hydroxide is dissolved in alcohol. As the reaction takes place, one molecule of halogen acid is eliminated, and a double bond is formed. The rate at which the reaction will proceed will be determined by the alkyl group and the attached halogen group.  This reaction is also called beta elimination, as the halo group is removed from the alpha carbon while the hydrogen atom is removed from the beta carbon. 

                                  CH3 – CH2X ------> CH2=CH2 (alcoholic KOH,   -HX)

                                   (X = Cl, Br, I)               (ethene)

Preparation of Alkenes From Vicinal Dihalides

In vicinal halides, two halogen groups are attached to two adjoining carbon atoms in a compound.  In germinal dihalides, the two halogens are attached to the same carbon atom.

When this dihalide undergoes a reaction with zinc or sodium iodide in acetone, the halogen groups attached to form a compound with zinc or sodium, leading to the formation of a double bond between those two carbon atoms. This reaction is called dehalogenation. 

CH2Br - CH2Br  + NaI  --------->  CH2=CH2 +I2 + 2NaBr ( acetone)

CH2Br - CH2Br +Zn  ----------> CH2=CH2  + ZnBr2  

Preparation of Alkenes From Alcohols

When alcohol undergoes a reaction with concentrated sulphuric acid, a water molecule gets removed, leading to the formation of a double bond, hence an alkene. As a molecule of water is eliminated in the presence of an acid, the reaction is called acidic dehydration of alcohol. Concentrated sulphuric acid is used as the dehydrating agent. The elimination of one –OH group and one hydrogen atom from the beta carbon constitute a molecule of water. 

CH3-CH2-OH    ----------->  CH2=CH2 +H2O   ( Concentrated H2SO, Δ)

  (ethanol)                             (ethene)

Fun Facts about Alkenes:

  1. Alkenes can exist in all three states: solid, liquid, and gas. The first three alkenes are gaseous, the next fourteen are in the liquid state. As the molar mass increases further, they exist in the solid-state. 

  2. Alkenes are not soluble in water due to the existence of weak van der Waal forces.

  3. Alkenes, however, are soluble in organic solvents such as benzene and acetone. 

  4. Higher the molar mass of a given alkene, greater will be its boiling point. Hence, the boiling points of higher alkenes are quite high. 

  5. The functional group attached to the alkene is the determinant of its polarity. 

  6. Being unsaturated in nature, alkenes are highly reactive compounds. Most of these reactions will take place at the site of the double bond, that it at the two carbon atoms between which the double bond is placed. They can easily undergo addition and oxidation reactions. 

FAQ (Frequently Asked Questions)

1. What are the Everyday Life Applications of the Preparation of Alkenes?

Alkenes exist in nature in the form of many products. The double bond of alkenes (C=C) is rich in electrons, therefore nucleophilic. Alkenes are found in terpenes, which have an important role to play in the lives of insects and plants. This is contrary to the C=O bond, which is deficient in electrons. C=O bond is found in proteins, fatty acids, and carbohydrates.

Alkenes are an important product of the petroleum industry. The manufacturing sector of our economy is largely dependent on the reactions of alkenes. Plastics, such as polystyrene are obtained from petroleum are a result of reactions of alkenes. This is why alkenes are also referred to as olefins.  The materials so manufactured are in wide use because of their properties such as low weight, durability, strength, and flexibility.

2. What is the Saytzeff Rule in the Formation of Alkenes?

When a dehydrohalogenation reaction takes place during the formation of an alkene, in certain cases, more than one product may be possible. In such a case, the preferred product will be the alkene that will have a greater number of alkyl groups attached to the carbon atoms which are linked by a double bond. Saytzeff rule determines the favourable alkene to be formed.

For example, in the dehydrohalogenation of 2-bromobutane, two products can be expected- But-1-ene and But-2-ene. Out of these two, but-2-ene will be the major product, constituting around 80% as it is more substituted, therefore more stable. 

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