Unsaturated Hydrocarbons- Alkene

Hydrocarbons that have double or triple covalent bonds between adjacent carbon atoms are unsaturated hydrocarbons. The term ‘unsaturated’ indicates that more hydrogen atoms can be added to the hydrocarbon to make it saturated (single-bonded hydrocarbons). Alkene and alkyne are unsaturated hydrocarbons.

General methods of preparation:

1. Dehydration of alcohol

i. With conc. sulphuric acid:

ethene from dehydration of alcohol with conc. sulphuric acid

ii. With alumina:

alkene from dehydration of alcohol using alumina

2. Dehydrohalogenation (Elimination):

alkene from dehydrohalogenation

3. Catalytic hydrogenation of ethyne:

alkene from Catalytic hydrogenation of ethyne

Physical Properties
  1. Members having 2-4 carbon are gases, 5-17 carbons are liquid and the remaining are solids.
  2. They are insoluble in water but soluble in non-polar organic solvents.
  3. The boiling point increases with an increase in molecular mass.

Chemical Properties
Addition reactions

1. Addition of hydrogen halide: Alkenes react with hydrogen halide to give haloalkane.

addition of HX to alkene

Markovnikov’s rule:

When an unsymmetrical reagent is added to an unsymmetrical alkene, the positive part of the reagent goes to the unsaturated carbon having a higher number of hydrogen.

markovnikov's rule

Anti-Markovnikov’s rule/peroxide effect/Kharasch effect:

When an unsymmetrical reagent(HBr) is added to an unsymmetrical alkene in presence of organic peroxide, bromine is added to the unsaturated carbon having more number of hydrogen. It proceeds through a free radical addition mechanism.

anti markovnikov rule

2. Addition of Water:

addition of water to alkene

3. Addition of Sulphuric acid:

addition of sulphuric acid to alkene

Oxidation Reaction(Ozonolysis of alkene):

Alkene is treated with ozone in presence of CCl4 to give an ozonide which is decomposed by water and zinc dust to give aldehyde or ketone or both.

ozonolysis of alkene
Reactions Summary
alkene reaction summary
Important Conversions
alkene imp conversions
Some Important Questions
  1. Identify A and B.
alkene qstn
  1. Write short note on:
    a. Markonikov’s rule b. Ozonolysis
  2. An organic compound X on ozonolysis gives Ethanal and propanone. Identify X with the IUPAC name.

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