Preparation of alkenes
General methods of preparation of alkenes
Alkenes are usually prepared from either alcohols or haloalkanes (alkyl halides).
By the dehydration of alcohols
Alkenes are obtained by the dehydration of alcohols. The dehydration of alcohols can be affected by two common methods.
• By passing the vapors of an alcohol over heated alumina.
• By heating an alcohol with concentrated mineral acid, such as concentrated H2SO4 or concentrated H3PO4. Anhydrous zinc chloride can also be used as a dehydrating agent.
By passing the vapors of an alcohol over alumina (Al2O3) at 623K (350°C).
The order of the ease of dehydration of alcohols is, tertiary > secondary > primary
Secondary and tertiary alcohols are best dehydrated by dilute sulphuric acid.
By heating an alcohol with concentrated sulphuric acid at 453 K (180°C).
Other dehydrating agents like phosphoric acid and anhydrous zinc chloride may also be used.
Cyclohexanol on dehydration gives cyclohexene.
The loss of water from an alcohol to give an alkene does not occur in just one step; a series of steps are involved in the mechanism of dehydration of alcohols. In the dehydration reaction given above, the following steps are involved.
• First, the acid protonates (adding a proton or H+) the alcohol on the most electronegative atom, namely oxygen. This process is usually reversible.
• In the second step, the protonated alcohol loses water to give a positively charged species known as a carbonium ion or carbocation.
• Finally the carbonium ion loses a proton to give alkene.
The mechanism of dehydration of ethyl alcohol is described below.
From haloalkanes (or, alkyl halides)
Alkenes can be obtained from haloalkanes (alkyl halides). These haloalkanes are usually bromo and iodo and less commonly, chloro derivatives.
By dehydrohalogenation of haloalkanes
Haloalkanes on heating with alcoholic potash loses one molecule of hydrogen halide to give alkene.
If two alkenes may be formed due to dehydrohalogenation of a haloalkane, the one which is most substituted is the main product. For example, dehydrohalogenation of 2-bromobutane gives,
The order of reactivity of haloalkanes in dehydrohalogenation is, Tertiary >Secondary > Primary
Note: Reactions in which a small molecule like H2O or HX is lost are known as elimination reactions.
From vicinal dihaloalkanes
Vicinal dihaloalkanes are those dihalogen derivatives of alkanes in which two halogen atoms are on the adjacent carbon atoms. Alkenes can be obtained from vicinal dihaloalkanes by dehalogenation. When such a dihaloalkane is heated with zinc in methanol, an alkene is formed.
From carboxylates: By Kolbe's electrolytic method
Sodium or potassium salt of a dicarboxylic acid on electrolysis gives an alkene. When an aqueous solution of sodium or potassium salt of a dibasic acid is electrolyzed, an alkene is produced. For example, electrolysis of sodium succinate gives ethene.
2H2O + 2e- â¶ 2OH- + H2(g)
Alkenes can be easily obtained by hydrogenation of alkynes. An alkyne on controlled hydrogenation with hydrogen in the presence of Ni or Pd at 200°C give corresponding alkene.
Ethyne gives ethene on hydrogenation.