Preparation and Properties of Alkanes

Theory

Methods of preparation:

1. From alkenes and alkynes (by hydrogenation): 

Catalyst :

 2. From alkyl Halides (by reduction):

Catalyst :

Mechanism:

(a) Alkyl halides can also be reduced to alkane by H₂/Pd or LiAlH₄ or H₂/Ni.
(b) Reduction is due to the electron transfer from the metal to the substrate (R- X)
(c) If any alkyl halide is asked, the H-atom of any carbon atom of given alkane is removed by halogen atom.

3. From alkyl halide (By Wurtz reaction): A solution of alkyl halide in ether on heating with sodium gives alkane.

(a) Methane can not be prepared by this method. The alkane produced is higher and symmetrical i.e. it contains double the number of carbon atoms present in the alkyl halide taken and thus Wurtz reaction is not suitable for the synthesis of alkanes containing odd number of carbon atoms.
(b) Two different alkyl halides, on Wurtz raction gives all possible alkanes. The seperation of mixture into individual members is not easy because their B.P. are near to each other.
(c) This reaction generally fails with tertiary alkyl halide.

Mechanism:

4. Corey-House Synthesis: This method is suitable for the preparation of both symmetrical and unsymmetrical alkanes (R—R').

(i)

(ii)

(iii)

5. From Carboxylic Acid (By decarboxylation): Saturated monocarboxylic acid salt of sodium or potassium on dry distillation with soda lime gives alkane.

(a) The process of elimination of carbon dioxide from carboxylic acid is called decarboxylation.
(b) Replacement of -COOH by hydrogen is known as decarboxylation. Thus, the alkane formed always contains one carbon atom less than the original acid.
(c) This reaction is employed for stepping down a homologous series.
(d) Decarboxylation of sodium formate gives H₂.
Mechanism:

6. From alkanol, alkanals, Alkanone and alkanoic acid (By reduction) : The reduction of either of the above in presence of red P and HI gives corresponding alkane. 

(i)

(ii)

(iii)

(iv)

Physical Properties :

1. State: C₁ to C₄: Gas,
   C₅ to C₁₇: Colourless liquids and above C17 are Waxy solids.
2. Density : The density of alkanes increases with increase in molecular weight and becomes constant at 0.8 g/mL. Thus all alkanes are lighter than water.
3. Solubility : Alkanes being non polar and thus insoluble in water but soluble in non-polar solvents. Example : C₆H₆, CCl₄, ether etc. The solubility of alkanes decreases with increase in molecular weight. Liquid alkanes are themselves good non-polar solvents.
4. Boiling point ∝ molecular weight (for n-alkanes). Pentane < Hexane < Heptane. Also  boiling pointof n–Pentane > Isopentane > neopentane.
   
5. Melting point : M.P. of alkanes do not show regular trend. Alkanes with even number of carbon atoms have higher M.P. than their alkanes of odd number of carbon atoms.

Chemical Properties :

Oxidation reactions :
(i) Complete oxidation or combustion : Burn readily with non-luminous flame in presence of air or oxygen to give CO₂ and water with evolution of heat. Therefore, alkanes are used as fuels.

(ii) Incomplete oxidation : In limited supply of air gives carbon black and CO.

(iii) Catalytic oxidation : 

(a) Alkanes are easily converted to alcohols and aldehydes under controlled catalytic oxidation.

(b) Tertiary alkanes are oxidized to give tertiary alcohols by KMnO₄.

Substitution Reactions: Substitution reaction in alkanes shows free radical mechanism. They give following substitution reaction.

(i) Halogenation : Replacement of H-atom by halogen atom. Halogenation is made on exposure to (halogen + alkane) mixture to UV or at elevated temperature. 

The reactivity order for halogens shows the order. F > Cl > Br > I

Reactivity order of hydrogen atom in alkane is Tertiary C – H > Sec. C – H > primary C – H

Chlorination of methane:  The monochloro derivative of alkane is obtained by taking alkane in large excess.
When chlorine is in excess, a mixture of mono, di, tri, tetra and perchloro derivatives is obtained.

Mechanism of

(ii) Nitration :  At ordinary temperature, alkanes do not react with HNO3. But reacts with vapours of concentrated HNO₃ at 450 ^oC.

Mechanism: Free radical mechanism:

Step 1: 

Step 2:

Step 3:

(iii) Sulphonation : Replacement of H atom of alkane by –SO3H is known sulphonation. Alkane react with fuming H2SO4 or oleum (H2S2O7). The branched lower alkanes and higher alkanes react to give alkane sulphonic acid.

The reactivity order for sulphonation is 3^o H > 2^o H > 1^o H.

Isomerization : Unbranched chain alkanes on heating with AlCl₃ + HCl / 200^oC are converted in to branched chain alkanes.

Pyrolysis or Cracking or thermal decomposition : When alkanes are heated to 500-7000C they are decomposed in to lower hydrocarbon. This decomposition is called pyrolysis. In petroleum industry it is also termed as cracking. Cracking is used for the manufacture of petrol, petrol gas/oil gas etc.