INDUSTRIAL PREPARATION OF ALKENES
1. From Alkanes
Alkenes can be obtained from alkanes by the process of cracking. Alkanes for this purpose are obtained from petroleum or from natural gas. Cracking of ethane from petroleum gas yields ethene.
Higher alkanes may also be subjected to cracking to obtain ethene. Here propene is obtained as one of the byproducts
2. From Natural Gas
Alkenes can also be obtained by cracking of natural gas. This method is applied if the natural gas contains appreciable amounts of ethane, propane and butane.
CH3-CH3 à7 CH2 = CH2 + H2
CH3-CR2-CH3 à7 CH2 = CH2 + CH4
CH3-CH2-CH3 à7 C3 – CH = CH2 + H2
CH3-CH2-CH2-CH3 à7 C3 – CH = C2 + CH4
PREPARATION OF ALKENES
1 From Alkyl Halides
Alkenes can be prepared from alkyl halides by treatment with alcoholic solution of caustic potash (KOH) at about 353-363 K. This reaction is known as dehydrohalogenation of alkyl halides.
Similarly, treatment of 1-bromopropane or 2-bromopropane with ethanolic solution of potassium hydroxide produces propene
In case the alkyl halide can undergo dehydrohalogenation in two different ways, the preferred alkene is the one which is more alkylated i.e., which carries more number of alkyl groups attached to double bonded carbon atoms. This generalization is known as SAYTZEFF RULE. For example,
The alkene (I) is trialkylated whereas alkene (II) is monoalkylated. Here, alkene (I) is the major product.
2. From Alcohols
Alkenes can be prepared by dehydration (removal of water molecule) of alcohols. The two common methods for carrying out dehydration are to heat the alcohol with either alumina or a mineral acid such as tetraoxophosphate(V) acid (H3PO) or cone. tetraoxosulphate(VI) acid (H 2SO ). In dehydration reaction OH group is lost from a. carbon while H atom is lost by βcarbon atom creating a double bond between a and 13 carbon atoms. Some examples are: