Chapter 15: Hydrocarbons

Overview of Hydrocarbons

• Hydrocarbons are compounds primarily consisting of carbon (C) and hydrogen (H).

The Homologous Group of Alkanes

• Alkanes are saturated hydrocarbons following the general formula:
  C_nH_2n+2

• They consist of sp3 hybridized carbon with single bonds.

• Found in crude oil, a mixture that must undergo fractioning to separate individual components.

Crude Oil Processing

• Crude oil is separated into fractions having similar boiling points using a fractioning column:

  • Superheating converts crude oil to gases.

  • Condensation/boiling points are used for separation.

  • Smaller hydrocarbons are more volatile.

  • Used for fuels and plastics

Reactivity of Alkanes

• Alkanes are non-polar due to similar electronegativity between C and H, making them unreactive:

  • No attraction to electrophiles/nucleophiles.

  • Only react through:

  1. Combustion (complete and incomplete)

  2. Free-Radical Substitution with Halogens in Sunlight

Combustion of Alkanes

• Complete Combustion: Excess oxygen results in full oxidation.

  • Reaction:
    2C₈H₁₈ + 25O₂ —> 16CO₂ + 18H₂O

• Incomplete Combustion: Limited oxygen leads to carbon monoxide generation.

  • Reaction:
    2C₈H₁₈ + 17O₂ —> 16CO + 18H₂O

Dangers of Fossil Fuel Combustion

• Carbon Monoxide (CO):

  • Binds to hemoglobin, reducing oxygen transport.

  • Odorless and potentially fatal in poorly ventilated spaces.

  • CO₂ from complete combustion results in greenhouse gas

• Nitrogen Oxides (NO & NO2): Produced from high combustion temperatures, contributing to acid rain.

  • Conversions:
    N₂(g) + O₂(g) <—> 2NO(g)
    2NO(g) + O₂(g) <—> 2NO₂(g)

• Unburnt Hydrocarbons: Result from incomplete combustion, contributing to photochemical smog.

Catalytic Converters

• Utilized in exhaust systems to reduce emissions:

  1. Oxidation of CO to CO2.

  2. Reduction of NO to N2.

  3. Oxidation of unburnt hydrocarbons to CO₂ and water.

• Reduces number of gases released by cars but doesn’t eliminate CO₂ production

Free Radical Substitution

• Involves the substitution of a hydrogen atom in alkanes by halogen atoms under UV light.

• Color —> colorless

• Three steps:

  1. Initiation: Halogen bond cleavage forms free radicals.

  2. Propagation: Free radicals react with alkanes producing new radicals.

  3. Termination: Free radicals combine to form stable products.

The Alkenes

• Alkenes are unsaturated hydrocarbons with the general formula:
  C_nH_2n

• They contain at least one carbon-carbon double bond (sp2 hybridized).

Cracking Alkenes

• Alkenes are produced from cracking larger alkanes.

• Heating and passing alkanes over aluminum oxide catalysts without oxygen breaks them down.

Addition Reactions of Alkenes

• Occur via electrophilic addition, breaking the double bond to add new atoms/groups.

Major and Minor Products

• Asymmetric addition can yield different amounts of products:

  • Major product has more alkyl substituents around the double bond due to carbocation stability.

Oxidation of Alkenes

• Two types of oxidations with potassium permanganate:

  1. Cold Dilute: Forms diols and indicates presence of double bonds.

  2. Hot Concentrated: Oxidizes further to produce carboxylic acids, aldehydes, or ketones depending on carbon types.

Addition Polymerization

• Alkenes can undergo polymerization to form long chains (polymers), e.g., poly(alkenes).

• The repeat unit is denoted in brackets: CnH{2n} .

• Plastics made from these polymers are non-biodegradable and create environmental issues.

Exam Preparation Strategies for Addition Polymers

• Be prepared for two question types:

  1. Deducing repeat units from given alkenes.

  2. Identifying monomers in polymer sections.

• Apply the steps clearly - visualize double bond changes for constructing or deconstructing polymer structures.