Hydrocarbons – JEE Chemistry Detailed Notes
1. Introduction to Hydrocarbons
Hydrocarbons are organic compounds containing only carbon and hydrogen.
They form the backbone of organic chemistry and are classified based on:
- Type of carbon–carbon bond
- Structure (open chain or cyclic)
2. Classification of Hydrocarbons
| Type | General Formula | Bond Type |
|---|---|---|
| Alkanes | $C_nH_{2n+2}$ | Single ($\sigma$) |
| Alkenes | $C_nH_{2n}$ | Double ($\sigma+\pi$) |
| Alkynes | $C_nH_{2n-2}$ | Triple ($\sigma+2\pi$) |
| Aromatic | Variable | Conjugated $\pi$ system |
3. Alkanes (Paraffins)
3.1 Definition
Alkanes are saturated hydrocarbons containing only single bonds.
3.2 Preparation of Alkanes
Wurtz Reaction:
$2R–X + 2Na \rightarrow R–R + 2NaX$
$2R–X + 2Na \rightarrow R–R + 2NaX$
Kolbe’s Electrolysis:
$2RCOO^- \rightarrow R–R + 2CO_2$
$2RCOO^- \rightarrow R–R + 2CO_2$
3.3 Properties of Alkanes
- Non-polar
- Low reactivity
- Insoluble in water
3.4 Chemical Reactions
Combustion:
$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{heat}$
$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{heat}$
Halogenation (Free Radical Substitution):
$CH_4 + Cl_2 \xrightarrow{hv} CH_3Cl + HCl$
$CH_4 + Cl_2 \xrightarrow{hv} CH_3Cl + HCl$
Reactivity order: $3^\circ > 2^\circ > 1^\circ$
4. Alkenes (Olefins)
4.1 Definition
Alkenes are unsaturated hydrocarbons containing one double bond.
4.2 Preparation of Alkenes
Dehydration of Alcohol:
$CH_3CH_2OH \xrightarrow{conc.\ H_2SO_4} CH_2=CH_2 + H_2O$
$CH_3CH_2OH \xrightarrow{conc.\ H_2SO_4} CH_2=CH_2 + H_2O$
4.3 Chemical Reactions
Hydrogenation:
$CH_2=CH_2 + H_2 \xrightarrow{Ni} CH_3–CH_3$
$CH_2=CH_2 + H_2 \xrightarrow{Ni} CH_3–CH_3$
Halogen Addition:
$CH_2=CH_2 + Br_2 \rightarrow CH_2Br–CH_2Br$
$CH_2=CH_2 + Br_2 \rightarrow CH_2Br–CH_2Br$
Decolourisation of bromine water is a test for unsaturation.
4.4 Markovnikov’s Rule
In addition of HX, H adds to carbon having more hydrogens.
Exception: Peroxide effect (only with HBr).
5. Alkynes
5.1 Definition
Alkynes contain at least one triple bond.
5.2 Preparation
$CaC_2 + 2H_2O \rightarrow C_2H_2 + Ca(OH)_2$
5.3 Chemical Reactions
$C_2H_2 + 2H_2 \xrightarrow{Ni} C_2H_6$
Terminal alkynes are acidic due to $sp$ hybridisation.
6. Aromatic Hydrocarbons
6.1 Benzene
Benzene has formula $C_6H_6$ and exhibits aromatic stability.
6.2 Structure of Benzene
Hybrid structure with delocalised $\pi$ electrons.
6.3 Electrophilic Substitution Reactions
Nitration:
$C_6H_6 + HNO_3 \xrightarrow{H_2SO_4} C_6H_5NO_2$
$C_6H_6 + HNO_3 \xrightarrow{H_2SO_4} C_6H_5NO_2$
Halogenation:
$C_6H_6 + Cl_2 \xrightarrow{FeCl_3} C_6H_5Cl$
$C_6H_6 + Cl_2 \xrightarrow{FeCl_3} C_6H_5Cl$
Benzene prefers substitution over addition due to stability.
7. Final JEE Revision Points
- Remember general formulas
- Understand reaction mechanisms
- Memorise exceptions
- Practice conversions
- Focus on aromatic reactions