Hydrocarbons: Physical and Chemical Properties of Alkenes, Mechanism of Electrophilic Addition (For CBSE, ICSE, IAS, NET, NRA 2022)

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Physical Properties of Alkenes

Physical State

Unbranched alkenes containing up to four carbon atoms are gases and containing five to sixteen carbon atoms are liquids and with more than 16 carbon atoms are solids.

Boiling Points

The boiling points of alkenes increase with molecular mass.

The Boiling Points of Alkenes Increase with Molecular Mass
AlkeneEthenePropeneBut-1-enePent-1-eneHex-1-ene
b. p. (K)169226267303337
  • The increase in boiling point is attributed to the van der Waals forces which increases with number of carbon atoms of the alkene.
  • The branched chain alkene have lower boiling points than those of straight chain isomers.

Melting Point

  • In alkenes, there is increase in the melting point with the increase in molecular mass.
  • The cis and trans isomers have different melting points.

For example,

Melting Point

Chemical Properties of Alkenes

Addition Reactions

The chemical reactions in which a molecule adds to another molecule are called an addition reaction.

Addition of Hydrogen

Addition of hydrogen to unsaturated hydrocarbons takes place in the presence of a catalyst like , or .

Addition of Halogens

Halogens on addition to alkenes, form 1,2- dihaloalkanes.

Addition of Halogen Acids (HX)

When halogen acids are added to alkene , hydrogen adds to one carbon atom whereas halogen atom adds to the second carbon atom of the double bond.

  • In unsymmetrical alkenes (which contain unequal number of H- atoms attached to the carbon atoms of the double bonds) , the addition of HX takes place according to the Markownikoff՚s rule.
  • Markownikoff՚s rule - According to this rule, in the addition of halogen acids to unsymmetrical alkenes, the halogen of HX goes to that carbon atom of bond which already has less H-atoms attached to it.

Mechanism of Electrophilic Addition

  • The electron cloud of the pi bond is present above and below the plane of the molecules in alkene.
  • Various electron seeking species and reagents thus react with the alkenes.
  • For example, of can add to the double bond to yield a carbocation.
Mechanism of Electrophilic Addition
  • The carbocation being highly reactive reacts with the halide ion in the second step to yield an alkyl halide.
  • If the starting alkene is unsymmetrical e. g. propene, then in the first step of formation of a carbocation, there are two possibilities of attachment of of which are shown below:
Mechanism of Electrophilic Addition
  • The secondary carbocation (II) is more stable than the primary carbocation (I) .
  • Thus, attack of on the carbocation yields 2-bromopropane as the major product.
Mechanism of Electrophilic Addition

If the addition of is carried out in the presence of peroxides such as benzoyl peroxide, then the reaction takes place contrary to Markownikoff՚s rule. This is also known as Anti-Markownikoff՚s addition or peroxide effect.

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