Reactions of Alkenes
Addition of Halogens to Alkenes
When halogens such as bromine, chlorine are added to alkenes, vicinal dihalides are formed. Iodine does not show addition reaction under normal conditions while fluorine reacts with alkenes too rapidly to be controlled in the laboratory.
Bromine test : The addition of bromine to unsaturated compounds in an organic solvent such as CCl4 results in the disappearance of reddish-orange colour of bromine. Because of this reason, this reaction is used to test unsaturation.
Addition of hydrogen halides to Alkenes
The addition of hydrogen halides (HCl, HBr, HI) to alkenes yields alkyl halides (haloalkanes). The order of reactivity of the hydrogen halides is HCl > HBr > HI.
Case of unsymmetrical alkenes : When halogen halide reacts with unsymmetrical alkenes, two types of products are formed. Let us take the example of propene to understand the concept :
Markovnikov's rule states that in the addition reactions of unsymmetrical alkenes, the negative part of reagent gets attached to the carbon having lesser number of hydrogen atoms. Let us apply Markovnikov's rule to the above reaction :
Anti Markovnikov's rule
When HBr is added to unsymmetrical alkenes such as propene in the presence of peroxide, the negative part gets attached to the carbon which posses more number of hydrogen atoms. Since the rule is contrary to the Markovnikov's rule, it is called Anti Markovnikov's rule. This rule is also termed as peroxide effect or Kharash effect.
Addition of Sulphuric Acid to Alkenes
The addition of cold conc. sulphuric acid to alkenes produces alkyl hydrogen sulphate. Unsymmetrical alkenes follow Markovnikov's rule.
The OSO2OH in the above reaction can alternatively be written as HSO4. For example :
C2H5OSO2OH = C2H5HSO4
Addition of Water to Alkenes
Alkenes react with water in the presence of mineral acids to form alcohols in accordance with the Markonikov's rule. Since a molecule of water is added in this reaction, this reaction is termed as hydration of alkenes.
Oxidation of Alkenes
(i) Alkenes are readily oxidised by cold, dilute, aqueous solution of potassium permanganate (Baeyer's reagent) to give vicinal glycols.
(ii) Acidic potassium permanganate or acidic potassium dichromate (K2Cr2O7) are used to oxidise alkenes to ketones and/or carboxylic acids depending upon the nature of alkene.
Ozonolysis of Alkenes
Ozonolysis of alkenes involves the addition of an ozone molecule (O3) to alkene to form ozonide which are reduced with zinc dust and water to give smaller molecules.
Cycloalkenes also undergo ozonolysis reaction. For example, cyclopropene on reductive ozonolysis gives propane-1,3-dial.
Notice that in the above reaction only one product is formed. Now it is your turn to answer a question : How does a compound containing two double bonds undergo ozonolysis?
What will be the products formed when buta-1,3-diene undergoes ozonolysis?
Ozonolysis is highly useful in detecting the position of the double bond in alkenes because two different alkenes always give different products.
Polymerisation of Alkenes
In poymerisation, a large number of simple molecules, known as monomers, combine to form a big molecule which is known as polymer. For example : Ethene on heating to 473 K under a pressure of 1500 atm and in the presence of a trace of oxygen gives a low density polythene.
Other alkenes also udergo polymerisation.
Revision includes earlier concepts
Let's do some revision. The reactions used in the answers are the ones that we have learned so far. To find the answer, all you need to do is click the 'Answer' button; however, it is not recommended. Try to put some effort before clicking the answer button.
An alkene on ozonolysis gives ethanal and propanal. Can you tell the name of the alkene?
Conversion : Prepare bromopropane from 2-chloropropane.
Remember these reactions. We will need them to do conversions in later chapters.