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  • March 25, 2023
  • 29
  • 2022/2023
  • Summary
  • Secondary school
  • 12th Grade
  • Chemistry
  • 5
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anuragji229
Chapter - 13
Hydrocarbons
FAST TRACK : QUICK REVISION
Hydrocarbons are the organic compounds containing carbon and hydrogen only.
Depending upon the types of carbon-carbon bonds present, hydrocarbons can
be classified into three categories- (i) Saturated (ii) Unsaturated (iii) Aromatic
hydrocarbons.
Saturated hydrocarbons contain carbon-carbon muiltiple bonds–double bonds,
triple bonds or both.
ALKANES : Saturated open chain hydrocarbons containing carbon-carbon single
bonds. These are inert under normal conditions i.e. do not react with acids, bases
and other reagents. Alkanes exhibit Chain isomerism, Position isomerism and
conformational isomerism.
General methods of preparation of alkanes :
1. From Unsaturated hydrocarbons : By hydrogenation in the presence of
platinium, palladium or nickel as catalyst.
General Chemical Equation :
Ni
R – CH = CH2 + H2  R – CH2 – CH3
[Where R is H or Alkyl group]
Ni
e.g. CH2 = CH2 + H2  CH3 – CH3
Ethene Ethane
Ni Ni
CH3 –C  CH + H2  CH3 –CH =CH2 + H2  CH3 –CH2 –CH3
Propyne Propene Propane

2. From alkyl halides : on reduction with Zinc and dilute hydrochloric acid
Zn, H+
CH3 – Cl + H2 CH4+ HCl
Chloromethane Methane
+
Zn, H
C2 H5 – Cl + H2 C2H6 + HCl
Chloroethane Ethane

, Zn, H+
CH CH CH Cl + H CH CH CH + HCl
3 2 2 2 3 2 3
1-Chloropropane Propane

3. From alkyl halides by Wurtz reaction : Reaction of alkyl halide with
sodium in dry ether, useful only for the preparation of symmetrical alkanes.
CH CH Br + 2Na + Br CH CH Dry Ether CH CH CH CH
3 2 2 3 3 2 2 3
Bromoethane Butane
Dry Ether
CH CH CH Br+ 2Na+Br CH CH CH CH CH CH CH CH CH
3 2 2 2 2 3 3 2 2 2 2 3
1-Bromopropane n–Hexane
CH CH Br + 2Na + Br CH CH Dry Ether
3 2 2 3 CH3CH 2CH2CH 3
1- Bromopropane 1–Bromoethane n–Pentane (desired)

4. From Carboxylic acids : By decarboxylation with soda lime

CH COONa + NaOH  CH + Na CO
3 CaO 4 2 3
Sodium acetate Sodium Methane Sodium
Hydroxide

5. By Kolbe's electrolytic method : Electrolysis of an aqueous solution sodium
or potassium salt of carboxylic acid. Alkane containing even number of
carbon atoms is formed at anode.
Electrolysis
2CH3COO Na + 2H2O CH3 – CH 3+ 2CO 2+ H 2+ 2NaOH
At Anode : (Oxidation)
O –2e– O
2CH – C – O– 2CH – C – O– — 2CH • + 2CO
3 3 3 2

2CH3 — CH 3– CH 3
At Cathode : (Reduction)
2H2O — 2OH– + 2H+
2H+ + 2e– — H2

Physical Properties of alkanes :
1. Boiling point of alkanes decreases on branching due to decrease in surface
ara of molecule with branching which decreases magnitude of van der Waal's
forces of attraction.
2. Alkanes being non-polar in nature are soluble in non-polar solvents.




264 Chemistry Class XI

, Chemical properties of Alkanes :
• Alkanes undergo substitution reactions.
e.g., Halogenation, Nitration, Sulphonation.
Halogenation : For example Chlorination of methane
CH4 + Cl2 CH3Cl + HCl
Chloromethane

CH3Cl + Cl2 CH2Cl2 + HCl
Dichloromethane

CH2Cl2 + Cl2 CHCl3 + HCl
Trichloromethane

CHCl3 + Cl2 CCl4 + HCl
Tetrachloromethane

Rate of reaction of alkanes with halogens is F2 > Cl2 > Br2 > I2. Rate of
replacement of hydrogen in alkanes is 3° > 2° > 1°. Fluorination is too
violent to be controlled. Iodination is reversible and it is therefore carried
out in the presence of oxidising agent like HNO3.

Mechanism of halogenation : Free radical mechanism

Cl – Cl Cl• + •Cl Initiation

Cl• + CH4 H – Cl + •CH3 Propagation

•CH3 + Cl – Cl CH3 Cl + •Cl Propagation

Cl• + •CH3 CH3 Cl Termination


Combustion : Complete combustion gives carbon dioxide and water.
CH4 + 2O2 CO2 + 2H2O

Isomerisation :
CH3
CH3 CH 2CH 2CH 3 AlCl3 + HCl CH3CHCH 3
200° C, 35 atm
n-Butane Isobutane




Hydrocarbons 265

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