Chemical Properties of Alkenes

What are Alkenes

Alkenes consist of a large number of loosely held pi bonds. Due to the presence of such bonds they exhibit addition reactions. In addition reaction, the electrophiles (electron seeking species) are added to the double bonded compounds which result in the formation of the additional product. Under special conditions, in some cases, it has the ability to undergo free radical substitution reactions also. Ozonolysis reaction and oxidation reaction is also prominently exhibited it.

Reactions of alkenes with different compounds

Addition of dihydrogen

Alkanes are formed on addition of one molecule of dihydrogen to alkenes in the presence of different types of catalysts such as palladium, nickel or platinum.

Addition of halogens

Vicinal dihalides are the products formed when alkenes react with halogens such as bromine, chlorine. Iodine is an exception as it does not exhibit addition reaction under normal conditions.Bromine solution loses its reddish-orange color in carbon tetrachloride solution due to the addition of bromine to an unsaturated site. Hence this reaction is used as a test for the presence of unsaturation in a solution. The reaction of alkenes with halogens is an example of the electrophilic addition reaction.

Chemical Properties of Alkenes
Addition of hydrogen halides

Alkyl halides are formed when hydrogen halides react with it.The reactivity exhibited by HI is more than HBr and the reactivity of HBr is more than HCl. The addition reaction is possible to both symmetrical and unsymmetrical alkenes. Addition reaction of hydrogen halides to unsymmetrical alkenes takes place in accordance with the following rules:

  • Markovnikov rule
  • Anti-Markovnikov rule

As per Markovnikov rule when unsymmetric alkene is added to hydrogen halide, halogen gets attached to the carbon having less number of hydrogen.

Chemical Properties of Alkenes

As per anti-Markovnikov rule unsymmetric alkenes add hydrogen halide in the presence of peroxide.The reverse of Markovnikov rule occurs which results in the attachment of halogen to a carbon containing more number of hydrogen.

Chemical Properties of Alkenes
Addition of water

It is made to react with water in the presence of few drops of sulphuric acid.This results in the formation of alcohols as per Markovnikov rule.

Chemical Properties of Alkenes

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Practise This Question

Read the following questions and answer as per the directions given below:

Statement – I: The first ionization energy of Be is greater than that of B.

Statement – II: 2p-orbital is lower in energy than 2s.