Lecture on Hydrocarbons

Alkanes

Combustion

• Alkanes burn readily in oxygen, highly exothermic.
• Used as fuels.
• Complete Combustion:
  • Example: ( C_2H_6(g) + 3.5O_2(g) \rightarrow 2CO_2(g) + 3H_2O(l) )
  • Products: CO2 and H2O.

Chemical Reactivity

• Alkanes have low reactivity due to strong C-C and C-H bonds and lack of polar side groups.
• React mainly in combustion.
• Can react with halogens under UV light.

Synthesis of Chloroalkanes

• Reaction with bromine/chlorine in UV light leads to substitution of hydrogen.
• Mechanism: Free Radical Substitution
  • Step 1: Initiation
    • Cl2 → 2Cl• (Requires UV light)
    • Homolytic fission forming free radicals.
  • Step 2: Propagation
    • Free radicals react to form products and regenerate radicals.
  • Step 3: Termination
    • Collision of free radicals terminates chain reaction.

Fractional Distillation

Industrial Process

• Oil pre-heated, passed into column.
• Fractions condense at different heights based on boiling point.
• Temperature decreases upwards in the column.

Laboratory Process

• Heat mixture, vapors pass up column.
• Lower boiling point vapors condense at top.
• Fractional distillation separates based on boiling points.

Cracking

Types of Cracking

• Thermal Cracking: High pressure, high temperature; produces alkenes.
• Catalytic Cracking: Low pressure, high temperature, zeolite catalyst; produces motor fuels.

Alkenes

Structure

• Unsaturated hydrocarbons with (C=C) double bond.
• Reactively due to low bond enthalpy of (\pi)-bond.

Reactions

• Hydrogenation: Alkenes to alkanes with hydrogen, nickel catalyst.
• Hydration: Alkenes to alcohols with steam and acid catalyst.
• Electrophilic Addition: Reaction with bromine, formation of carbocation intermediates.

Aromatic Hydrocarbons (Arenes)

Benzene Structure

• Planar molecule, ring of six carbon atoms with delocalized electrons.
• More stable than expected due to delocalization energy.

Reactions

• Halogenation: Electrophilic substitution with halogen carrier catalyst.
• Nitration: Electrophilic substitution, formation of nitrobenzene.
• Friedel-Crafts Reactions: Acylation and alkylation forming phenyl ketones and alkylbenzenes.

Hydrogenation

• Benzene to cyclohexane with hydrogen, nickel catalyst.

Environmental Concerns

• Combustion of hydrocarbons contributes to pollution and greenhouse gases.
• Catalytic converters reduce harmful emissions.

Note on Polymers

• Addition polymers from alkenes are unreactive due to strong bonds.
• Waste polymers can be incinerated for energy, but may produce toxic byproducts.

Summary

• Alkanes and alkenes have distinct chemical properties and reactions.
• Fractional distillation and cracking are key industrial processes.
• Benzene's stability and reactions differ from alkenes due to delocalization.