ALKANES

Alkanes are saturated open chain hydrocarbons containing carbon-carbon single bonds. They form a homologous

series. Their general molecular formula is CnH2n+2. In alkanes, all the C atoms are sp3 hybridised. So each C atom has a regular tetrahedral shape.

Alkanes do not react with acids, bases and other reagents under normal conditions. So they are also called paraffins. (In Latin paraffin means little affinity).

Preparation of alkanes

1. From unsaturated hydrocarbons: Alkenes and alkynes add Hydrogen in presence of finely divided catalysts like Ni, Pd or Pt to form alkanes. This process is called hydrogenation.

2. From alkyl halides:

a) Alkyl halides on reduction with zinc and dil. HCl, we get alkanes

b) Wurtz reaction:

Alkyl halides react with metallic sodium in dry ether to form alkanes. This reaction is known as Wurtz reaction. The alkane formed contains double the number of C atoms than that present in the alkyl halide. So this method is used for the preparation of alkanes with even number of carbon atoms.

R-X + 2Na + X-R → R-R + 2NaX

When two different alkyl halides are used, we get a mixture of alkanes.

3. From carboxylic acids:

a) Decarboxylation: Sodium salt of carboxylic acids (R-COONa) on heating with soda lime (a mixture of NaOH

and CaO), we get an alkane containing one carbon atom less than that of the carboxylic acid. This process is known as decarboxylation, since it involves the elimination of a CO2 molecule from the carboxylic acid.

b) Kolbe’s Electrolytic method: An aqueous solution of sodium or potassium salt of a carboxylic acid on electrolysis gives alkane containing even number of carbon atoms.

Physical Properties

  • Boiling point of alkanes increase with increase of molecular mass (or with number of C atoms).
  • This is because in alkanes there is only a weak van der Waal’s force of attraction between different molecules.
  • As the molecular size increases, the surface area increases and hence van der Waal’s force increases.
  • So the boiling point increases.
  • The b.p of isomeric alkanes decreases with branching.
  • As the branching increases, the molecule attains the shape of a sphere.
  • So the surface area decreases and hence the b.p.

Chemical Properties

1. Substitution reaction

These are reactions in which hydrogen atom of an alkane is replaced by other atoms or atom groups.

E.g. when an alkane is treated with halogen in the presence of diffused sunlight or uv light, we get haloalkane. This reaction is known as halogenations.

Mechanism

Halogenations takes place by free radical chain mechanism and it involves three steps – initiation, propagation and termination.

i) Initiation step: The reaction is initiated by the homolysis of chlorine molecule in presence of sunlight.

ii) Propagation step: The chlorine free radical attacks the methane molecule and form methyl free radical and HCl.

The methyl radical then attacks the second Cl2 molecule to form CH3Cl and Chlorine free radical.

The above two steps repeat and thus the reaction propagates.

iii) Termination step: The reaction stops after some time due to any one of the following reactions:

2. Combustion (Oxidation):

On combustion in presence of air or oxygen, alkanes give CO2, H2O and large amount of heat.

CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) + heat

The general combustion equation for any alkane is:

CnH2n+2 + (3n+1)/2 O2 → nCO2 + (n+1) H2O

Incomplete combustion of alkanes with insufficient amount of air or O2 gives carbon black.

3. Controlled Oxidation:

Alkanes on heating with O2 at high pressure and in presence of suitable catalysts to form different compounds

4. Isomerisation: n-Alkanes on heating in the presence of anhydrous aluminium chloride and hydrogen chloride gas isomerise to branched chain alkanes.

Aromatization: n-Alkanes having six or more carbon atoms on heating to 773K at 10-20 atmospheric pressure in the presence of oxides of vanadium, molybdenum or chromium supported over alumina, we get aromatic compounds. This reaction is known as aromatization or reforming.

5. Pyrolysis: Alkanes having six or more carbon atoms on heating at higher temperature decompose to form lower alkanes, alkenes etc. This reaction is known as pyrolysis.

Conformations of Alkanes

  • Alkanes contain carbon-carbon sigma (σ) bonds.
  • Since, electron distribution of the sigma bonds is symmetrical around the bond axis, rotation around C–C bond is allowed.
  • This rotation changes the spatial arrangements of atoms attached to C atoms.
  • These different spatial arrangements of atoms arising due to free rotation around a C-C single bond are called conformations or conformers or rotamers.

Conformations of ethane

  • Ethane contains a C-C σ bond and each carbon atom contains three hydrogen atoms. Due to free rotation of C atoms around the single bond, the spatial arrangement of hydrogen atoms attached to the C atoms change. Thus ethane can show an infinite number of conformational isomers.
  • If we fix one carbon atom and rotate the other, there arise two extreme cases called eclipsed and staggered conformations.
  • In eclipsed conformation, the hydrogen atoms attached to each carbon atoms are closed together as possible. Or, here the hydrogen atoms of the 2nd carbon atoms are exactly behind that of the first.
  • In staggered conformation, the hydrogen atoms are far apart as possible. Any conformations between eclipsed and staggered conformations are called skew conformations.
  • Staggered conformation is stabler than eclipsed form. This is because in staggered form, the electron clouds of carbon-hydrogen bonds are very far apart.
  • So there is minimum repulsive forces, minimum energy and maximum stability. But in eclipsed form, the electron clouds are close to each other. So the repulsion is maximum and the stability is minimum.

Eclipsed and staggered conformations can be represented by Sawhorse and Newman projection formulas.

1. Sawhorse projections:

  • Here the molecule is viewed along the molecular axis. C–C bond is denoted by a longer straight line.
  • The front carbon is shown at the lower end of the line and the back carbon is shown at the upper end.
  • Each carbon has three lines attached to it corresponding to three hydrogen atoms.

Sawhorse projections of eclipsed and staggered conformations of ethane are as follows:

2. Newman projections:

  • Here the molecule is viewed at the C–C bond head on (i.e. from the front side).
  • The front carbon atom is represented by a point.
  • Three hydrogen atoms attached to this carbon atom are shown by three lines drawn at an angle of 120° to each other.
  • The back carbon atom is represented by a circle and the three hydrogen atoms are shown attached to it are denoted by shorter lines drawn at an angle of 120° to each other.

The Newman’s projections for eclipsed and staggered conformations of ethane are as follows: