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ALKANES

Methods of preparation of Alkanes:

1. From unsaturated hydrocarbons: By hydrogenation of alkenes and alkynes.
R-CH=CH2 + H2 Ni/Pt/Pd R-CH2-CH3
2. From Alkyl halides:
(a)By reduction of alkyl halide with Zn and dilute HCl:
R-X + H2 Zn/dil. HCl R-H + HX
(b)By Wurtz reaction:
R-X + 2 Na + R-X dry ether R-R + 2NaX
3. From Carboxylic acids:
(a)By Decarboxylation:
R-COONa + NaOH CaO/heat R-H + Na2CO3
(b)By Kolbe’s Electrolytic method:
2 R-COONa + 2H2O electrolysis R-R + 2 CO2 + H2 + 2 NaOH
Physical properties:
(i)Greater the branching of the chain, lower is the boiling point.
(ii)Alkanes with even no. of carbon atoms have higher melting point than the next lower and
next higher alkanes having odd no.of carbon atoms.
Chemical properties:
1. Substitution reaction: Halogenation
CH4 Cl2/hν CH3-Cl Cl2/hν CH2Cl2 Cl2/hν CHCl3 Cl2/hν CCl4
2. Combustion:
CH4 + 2 O2 CO2 + 2 H2O
3. Controlled oxidation:

CH4 + 2 O2 Cu/ 523 K/ 100 atm 2CH3OH
CH4 + O2 Mo2O3 /heat H-CHO + H2O

2 CH3-CH3 + 3O2 (CH3-COO)2Mn /heat 2 CH3-COOH + 2 H2O
(CH3)3CH KMnO4/oxidation (CH3)3C-OH

4. Isomerisation:

CH3CH2CH2CH2CH2CH3 Anhy. AlCl3/HCl CH3CH(CH3)CH2CH2CH3 + CH3CH2CH(CH3)CH2CH3
5. Aromatisation:
CH3CH2CH2CH2CH2CH3 Cr2O3/V2O5/Mo2O3, 773K, 10-20 atm C6H6
6. Reaction with steam:
CH4 + H2O Ni/heat CO + 3 H2
7. Pyrolysis: C6H12 + H2

C12H26 773K C4H8 + C2H6
C 3H6 + C2H4 + CH4
Conformations: Spatial arrangement of atoms which can be converted into one another
by rotation around C-C single bond are called conformations or conformers or rotamers.
Torsional strain: The rotation around C-C single bond is hindered by a weak repulsive
interaction between the adjacent bonds which is called torsional strain.
Conformations of Ethane: Staggered is more stable than eclipsed conformation.
Eclipsed conformation: H-atroms attached to 2 C-atoms are very closed to each other.
Staggered conformation: H-atroms attached to 2 C-atoms are far apart from each other.