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Hydrocarbons



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Chapter 13 : Hydrocarbons



13.1 Classification arrow_upward


  • Hydrocarbons are of different types depending upon the types of carbon-carbon bonds present, they can be classified into three main categories:
    • Saturated
    • Unsaturated
    • Aromatic Hydrocarbon

    13.2 Alkanes arrow_upward


  • Alkanes are saturated open chain hydrocarbons containing carbon - carbon single bonds.
  • Methane (CH4 ) is the first member of this family. Methane is a gas found in coal mines and marshy places.

  • 13.2.1 Nomenclature and Isomerism

  • First three alkanes– methane, ethane and propane have only one structure but higher alkanes can have more than one structure.
  • Structures for C4 H10 . Four carbon atoms of C4 H10 can be joined either in a continuous chain or with a branched chain.

  • 13.2.3 Preparation

  • From Alkyl Halides: Alkyl halides (except fluorides) on reduction with zinc and dilute hydrochloric acid give alkanes.
  •                                                             Chloromethane                         Methane

                                                                Chloroethane                                Ethane

                                                            1-Chloropropane                            Propane

                                    

  • From carboxylic acid: Sodium salts of carboxylic acids on heating with soda lime (mixture of sodium hydroxide and calcium oxide) give alkanes containing one carbon atom less than the carboxylic acid. This process of elimination of carbon dioxide from a carboxylic acid is known as decarboxylation.

  • 13.2.3Properties

    Physical Properties
  • Alkanes are almost non-polar molecules because of the covalent nature of C-C and C-H bonds and due to very little difference of electronegativity between carbon and hydrogen atoms.
  • Chemical Properties
  • Some chemical properties are:
  • 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.

  • 13.2.4 Conformations

  • Alkanes contain carbon-carbon sigma bonds.
  • Spatial arrangements of atoms which can be converted into one another by rotation around a C-C single bond are called conformations.
  • Ethane molecule (C2 H6 ) contains a carbon – carbon single bond with each carbon atom attached to three hydrogen atoms.
  • Conformation in which hydrogen atoms attached to two carbons are as closed together as possible is called eclipsed. And it’s represented by:
    • Sawhorse projections
    • Newman projections


    13.3 Alkenes arrow_upward


  • Alkenes are unsaturated hydrocarbons containing at least one double bond.



  • 13.3.1 Structure of Double bond

  • Carbon-carbon double bond in alkenes consists of one strong sigma (ó) bond (bond enthalpy about 397 kJ mol–1) due to head-on overlapping of sp2 hybridized orbitals and one weak pi (ð) bond (bond enthalpy about 284 kJ mol–1) obtained by lateral or sideways overlapping of the two 2p orbitals of the two carbon atoms.
  • Orbital diagrams of ethene molecule.

  • 13.3.2Nomenclature

  • The suffix ‘ene’ replaces ‘ane of alkanes. It may be remembered that first member of alkene series is: CH2 (replacing n by 1 in CnH2 n) known as methene but has a very short life.

  • 13.3.3 Isomerism

  • Alkenes show both structural isomerism and geometrical isomerism.
  • Structural Isomerism

    13.3.4 Preparation

  • From Alkyl Halides: Alkyl halides (R-X) on heating with alcoholic potash (potassium hydroxide dissolved in alcohol say, ethanol) eliminate one molecule of halogen acid to form alkenes. This reaction is known as dehydrohalogenation.




  • 13.3.5 Properties

    Physical Properties
  • Ethene is a colourless gas with a faint sweet smell.
  • Chemical Properties
  • Addition Reaction of HBr to Unsymmetrical Alkenes:
  •                        

  • Hydrogen bromide provides an electrophile. H+ . which attacks the double bond to form carbocation.
  • The secondary carbocation (b) is more stable than the primary carbocation (a); therefore, the former predominates because it is formed at a faster rate.
  • The carbocation (b) is attacked by Br– ion to form the product as

  • 13.4 Alkynes arrow_upward


  • Alkynes are also unsaturated hydrocarbons. They contain at least one triple bond between two carbon atoms.

  • 13.4.1 Nomenclature and Isomerism

  • Alkanes replacing ‘ane’ by the suffix ‘yne’.
  • There are two possible structures for butyne – (i) but-1-yne and (ii) but-2-yne. Since these two compounds differ in their structures due to the position of the triple bond, they are known as position isomers.

  • 13.4.2 Structure of Triple Bond




    13.4.3 Preparation

  • From Calcium Carbide: Ethyne is prepared by treating calcium carbide with water. Calcium carbide is prepared by heating quick lime with Coke.
  •                                                       Calcium carbide

     + 2


    13.4.4 Properties

  • Some properties of alkynes are:
    • Physical properties
    • Chemical properties
    Physical Properties
  • All alkynes are colourless. Ethyene has characteristic odour. Other members are odourless.
  • Chemical Properties
  • Addition of Dihydrogen
  • Addition of Halogens
  • Cyclic Polymerization: Ethyne on passing through red hot iron tube at 873K undergoes cyclic polymerization.

  • 13.5Aromatic Hydrocarbon arrow_upward


  • These hydrocarbons are also known as ‘arenes’. Aromatic compounds containing benzene ring are known as benzenoids and those not containing a benzene ring are known as non-benzenoids.



  • 13.5.1 Nomenclature and Isomerism

  • Three different position isomers are possible. The 1, 2 or 1, 6 is known as the ortho (o–), the 1, 3 or 1, 5 as meta (m–) and the 1, 4 as para (p–) disubstituted compounds.

  • 13.5.2 Structure of Benzene

  • The molecular formula of benzene is C6 H6 .
  • The Kekulé structure indicates the possibility of two isomeric 1, 2-dibromobenzenes.

  • 13.5.3 Aromaticity

  • Benzene was considered as parent ‘aromatic’ compound. And possessing following characteristics:
    • Planarity
    • Complete delocalisation of the π electron
    • Presence of(4n+2) π electron in the ring.

    13.5.4 Preparation of Benzene

  • Reduction of phenol: Phenol is reduced to benzene by passing its vapours over heated zinc dust.

  • 13.5.5 Physical Properties

  • Aromatic hydrocarbons are non- polar molecules and are usually colourless liquids or solids with a characteristic aroma.



  • 13.5.6 Chemical Properties

  • Nitration: Benzene is heated with a mixture of concentrated nitric acid and concentrated sulphuric acid.
  • Friedel-Crafts Alkylation Reaction: benzene is treated with an alkyl halide in the presence of anhydrous aluminium chloride, alkylbenzene is formed.



  • 13.5.6 Directive influence of a functional group in monosubstituted benzene

  • Ortho and para products or meta product is predominantly formed. It has also been observed that this behaviour depends on the nature of the substituent already present in the benzene ring and not on the nature of the entering group.

  • 13.6 Carcinogenicity and Toxicity arrow_upward


  • Benzene and polynuclear hydrocarbons containing more than two benzene rings fused together are toxic and said to possess cancer producing (carcinogenic) property.
  • Some of the carcinogenic hydrocarbons are given:
    • 1, 2-Benzanthracene
    • 3-Methylcholanthrene
    • 1, 2-Benzpyrene


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