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CBSE Class 11th Chemistry Notes for Chapter 13 Hydrocarbons

                        UNIT 13 Hydrocarbons
Compounds consisting of carbon and hydrogen only are known as hydrocarbons.
These are saturated open chain hydrocarbons containing carbon-carbon single bond.
Physical properties:

  • Alkanes are almost non-polar molecules.
  • They are colourless and odourless.
  • Boiling points of alkanes increases with an increase in molecular mass.

Chemical properties:

  • Substitution reaction: One or more hydrogen atoms of alkanes can be replaced by halogens, nitro group and sulphonic acid group.
  • Combustion: Alkanes on heating in the presence of air or dioxygen are completely oxidized to carbon dioxide and water with the evolution of large amount of heat.
  • Controlled oxidation: Alkanes on heating with a regulated supply of dioxygen or air at high pressure and in the presence of suitable catalysts give a variety of oxidation products.
  • Isomerisation: n-alkanes on heating in the presence of anhydrous aluminium chloride and hydrogen chloride gas isomerise to branched chain alkanes.
  • Reaction with steam: Methane reacts with steam to form carbon monoxide and dihydrogen.
  • Pyrolysis: Higher alkanes on heating to higher temperature decompose into lower alkanes, alkenes, etc.

Alkenes are unsaturated hydrocarbons containing at least one double bond.
Isomerism: Alkenes show both structural and geometrical isomerism
Physical properties:

  • Alkenes resemble alkanes in physical properties.
  • They are, however, different in types of isomerism and difference in polar nature.
  • Ethene is a colourless gas with a faint sweet smell. All other alkenes are colourless and odourless.

Chemical properties:

  • Addition of dihydrogen: Alkenes add up one molecule of dihydrogen gas in the presence of finely divided nickel, palladium or platinum to form alkanes.
  • Addition of halogens: Halogens like bromine or chlorine add up to alkene to from vicinal dihalides.
  • Addition of hydrogen halides: Hydrogen halides add up to alkenes to form alkyl halides.
  • Addition of sulphuric acid: Cold concentrated sulphuric acid adds to alkenes in accordance with Markovnikov rule to form alkyl hydrogen sulphate by the electrophilic addition reaction.
  • Addition of water: In the presence of a few drops of concentrated sulphuric acid alkenes react with water to form alcohols.
  • Oxidation: Alkenes on oxidation produce vicinal glycols.
  • Ozonolysis: Ozonolysis of alkenes gives ozonide.

Alkynes are unsaturated hydrocarbons which contain at least one triple bond between two carbon atoms.
Physical properties:

  • Physical properties of alkynes follow the same trend of alkenes and alkynes.
  • All alkynes are colourless.
  • Ethyne has a characteristic odour but other members are odourless.
  • Alkynes are weakly polar in nature.
  • They are lighter than water and immiscible with water but soluble in organic solvents like ethers, carbon tetrachloride and benzene.

These hydrocarbons are also known as arenes. Most of these compounds possess a pleasant odour and hence called aromatic.
Most of these compounds contain benzene ring. Benzene ring is highly unsaturated. Aromatic compounds containing benzene ring are known as benzenoids and those not containing a benzene ring are known as non-benzenoids.
Physical properties

  • Aromatic hydrocarbons are non-polar molecules.
  • These are usually colourless liquids or solids with a characteristic aroma.
  • They are immiscible with water.

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