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Reactions of Alkenes

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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.

Addition reactions of alkenes : addition of halogens

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 reactions of alkenes : addition of halogens

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.

Addition reactions of alkenes : addition of hydrogen halide
Addition reactions of alkenes : addition of halogen hydride

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 :

Addition reactions of alkenes : addition of unsymmetrical hydrogen halide

Markovnikov's rule

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 :

Markovnikov's rule

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.

Anti-markovnikov's rule

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.

Addition reactions of alkenes : addition of sulphuric acid
Addition reactions of alkenes : addition of sulphuric acid

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.

Addition reactions of alkenes : addition of water

Oxidation of Alkenes

(i) Alkenes are readily oxidised by cold, dilute, aqueous solution of potassium permanganate (Baeyer's reagent) to give vicinal glycols.

Glycol : Glycol is an another term for diol.

Oxidation of alkenes
Oxidation of alkenes

(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.

Oxidation of alkenes using potassium permanganate
Oxidation of alkenes using KMnO4

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.

Ozonolysis of Alkenes
Ozonolysis of Alkenes

Cycloalkenes also undergo ozonolysis reaction. For example, cyclopropene on reductive ozonolysis gives propane-1,3-dial.

reductive ozonolysis of cycloalkenes

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 of compound containing two double bonds

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.

Polymerisation of alkenes

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.

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