ALKENES

  • Alkenes are unsaturated hydrocarbons containing at least one double bond.
  • They are also known as olefins (oil forming) since the first member, ethylene or ethene (C2H4) was found to form an oily liquid on reaction with chlorine.
  • The general formula for alkenes is CnH2nIsomerism in alkenes
  • Alkenes show structural and stereo isomerism.
  • The important structural isomerisms shown by alkenes are chain isomerism and position isomerism.
  • The stereo isomerism shown by alkenes is geometrical isomerism.

Geometrical Isomerism

  • The isomerism arising due to the difference in the spatial arrangement of atoms around carbon-carbon double bond is called geometrical isomerism.
  • Such isomers are called geometrical isomers. It is a type of stereo isomerism.
  • Geometrical isomerism arising due to the restricted rotation about carbon-carbon double bond.
  • There are two types of geometrical isomers – cis isomer and trans isomer.
  • Isomer in which identical atoms or groups are on the same side of the double bond is called cis isomer.
  • If the identical groups or atoms are on the opposite side of the double bond, it is called trans isomer.

Compounds with formula YX C = C XY can show geometrical isomerism as follows:

e.g. But-2-ene

Due to different arrangement of atoms or groups in space, these isomers differ in their physical properties like melting point, boiling point, dipole moment, solubility etc

Cis form of alkene is found to be more polar than the trans form. In the case of solids, the trans isomer has higher melting point than the cis form.

PREPARATION OF ALKENES

1. From alkynes:

Alkynes on partial reduction with dihydrogen in the presence of palladised charcoal partially deactivated with sulphur compounds or quinoline give alkenes.

  • Partially deactivated palladised charcoal is known as Lindlar’s catalyst. Alkenes thus obtained are having cis geometry.

If we use sodium in liquid ammonia as the reducing agent, we get trans alkene

2. From Alkyl halides:

  • Alkyl halides (R-X) on heating with alcoholic potash, eliminate one molecule of hydrogen halide to form alkenes.
  • This reaction is known as dehydrohalogenation i.e., removal of hydrogen halide.
  • Since hydrogen atom is eliminated from the β carbon atom, the reaction is also known as β-elimination reaction.

CH3-CH2X → CH2 = CH2 + HX

CH3-CH2-CH2Br → CH3-CH = CH2 + HBr

3. From vicinal dihalides:

  • Dihalides in which two halogen atoms are attached to two adjacent carbon atoms are known as vicinal (vic) dihalides.
  • Vicinal dihalides on treatment with zinc metal lose a molecule of ZnX2 to form an alkene.
  • This reaction is known as dehalogenation (i.e. elimination of halogen molecule).

4. From alcohols:

  • Alcohols when heated with concentrated sulphuric acid undergo dehydration (elimination of water molecule) to form alkenes.
  • This reaction is also the example of β-elimination reaction since –OH group is eliminated from the β-carbon atom.

CH3-CH2-OH → CH2 = CH2 + H2O

Chemical Properties

1.Addition of hydrogen:

Alkenes add hydrogen in presence of finely divided Ni, Pd or Pt to form alkanes.

CH2 = CH2 + H2 → CH3-CH3

2. Addition of halogen:

Alkenes add halogen (Cl2 orBr2) to form vicinal dihalides.

CH2 = CH+ X2 → CH2X-CH2X

When Br2 dissolved in CCl4 (carbon tetrachloride) is added to unsaturated compounds (alkenes or alkynes), the reddish orange colour of bromine solution is discharged. This reaction is used as a test for unsaturation.

CH2 = CH2 + Br2 → CH2Br-CH2Br

3. Addition of hydrogen halide:

Alkenes add hydrogen halide to form alkyl halides.

Addition of HBr to unsymmetrical alkenes (Markovnokov’s Rule)

When HBr is added to propene, we get 2 products – 1-bromopropane and 2-bromopropane.

Here the major product is determined by a rule called Markovnikov rule (Markownikoff’s rule). The rule states that “when an unsymmetrical reagent is added to an unsymmetrical alkene, the negative part of the addendum (adding molecule) gets attached to the carbon containing lesser number of hydrogen atoms” (less hydrogenated C atom).

Thus in the above  reaction 2-bromopropane is the major product.

In the presence of peroxide, addition of HBr to unsymmetrical alkenes takes place against Markovnikov rule. This is known as peroxide or Kharash effect or anti-Markovnikov addition reaction.

e.g.

4. Addition of water:

Alkenes add water in presence of a few drops of concentrated sulphuric acid to form alcohols. The reaction occurs according to the Markovnikov rule.

5. Oxidation:

(i) When oxidised using cold and dilute aqueous solution of potassium permanganate (KMnO4) [commonly called Baeyer’s reagent], alkenes give vicinal glycols.

When KMnO4 is added to alkene, its pink colour gets discharged. So this reaction is also used as a test for unsaturation.

(ii) Acidic KMnO4 or acidic potassium dichromate (K2Cr2O7) oxidises alkenes to ketone or acids depending on the nature of the alkene.

6. Ozonolysis: Alkenes add ozone to form an ozonide which on hydrolysis in presence of Zn to form carbonyl compounds (Aldehydes or ketones).

7. Polymerisation:

  • The complex organic molecules formed by the combination of simple molecules are called polymers (macromolecules) and the reaction is called polymerisation reaction.
  • The simple molecule from which a polymer is formed is called monomer.