Overview
This lecture covers hydrocarbons, focusing on alkanes and alkenes, their sources, properties, reactions, industrial processes, environmental impacts, and key organic chemistry mechanisms relevant to A-Level exams.
Introduction to Hydrocarbons
- Hydrocarbons are compounds made only of carbon and hydrogen.
- They are central to organic chemistry and connect with other organic topics.
- Alkanes are saturated hydrocarbons (contain only single bonds), with the general formula CnH2n+2.
- Cycloalkanes are ring-shaped saturated hydrocarbons, with the general formula CnH2n.
- Understanding basic organic chemistry (from Topic 13) is important, as later topics build on these concepts.
Sources and Fractional Distillation of Alkanes
- Alkanes are mainly found in crude oil and natural gas, which are extracted from the ground or under the sea.
- Crude oil is a mixture of different hydrocarbons, which are separated by fractional distillation based on their boiling points.
- The process involves vaporizing crude oil and passing it through a fractionating column with a temperature gradient (hot at the bottom, cooler at the top).
- Longer-chain hydrocarbons (higher boiling points) condense and are collected at the bottom; shorter-chain hydrocarbons (lower boiling points) rise and condense higher up.
- Main products (fractions) include:
- Natural gas (top of the column, shortest chains)
- Petrol (used in cars)
- Kerosene (jet fuel)
- Diesel oil (trains, some vehicles)
- Fuel oil (ships)
- Bitumen (bottom, used for tarmac and roofing)
- The petrochemical industry relies on these fractions for a wide range of products.
Cracking of Hydrocarbons
- Cracking is used to break down long-chain hydrocarbons into shorter, more useful ones to meet demand.
- Two main types:
- Thermal cracking: Uses high temperature and pressure (about 70 atm), mainly produces alkenes (useful for making plastics and polymers).
- Catalytic cracking: Uses a zeolite catalyst at lower temperature (about 450°C) and slight pressure, mainly produces aromatic hydrocarbons (used in fuels).
- Cracking helps convert less useful heavy fractions (like bitumen and fuel oil) into high-demand products (like petrol and alkenes).
- The process can produce both alkanes and alkenes, depending on the method used.
Combustion of Alkanes
- Alkanes are widely used as fuels because they release large amounts of energy when burned.
- Complete combustion (plentiful oxygen): Produces carbon dioxide (CO₂) and water (H₂O).
- Example: Butane + O₂ → CO₂ + H₂O (balanced equations required for exams)
- Incomplete combustion (limited oxygen): Produces carbon monoxide (CO, toxic), carbon (soot), and water.
- Example: Butane + O₂ → CO + H₂O or C (soot) + H₂O
- Carbon monoxide is dangerous because it binds to hemoglobin, preventing oxygen transport in the body.
- Soot can cause breathing problems and engine issues.
- Incomplete combustion is more likely when oxygen supply is limited.
Environmental Impacts and Pollutant Removal
- Burning hydrocarbons produces pollutants:
- Nitrogen oxides (NO, NO₂) from high-temperature reactions between nitrogen and oxygen in air.
- Sulfur dioxide (SO₂) from sulfur impurities in fuels.
- Unburnt hydrocarbons and particulates (soot).
- Environmental effects include:
- Photochemical smog (from ozone, unburnt hydrocarbons, and nitrogen oxides)
- Acid rain (from SO₂ and nitrogen oxides reacting with water in the atmosphere)
- Damage to plants, aquatic life, buildings, and human health.
- Pollution control methods:
- Catalytic converters in cars convert CO and NO into CO₂ and N₂ using platinum, palladium, and rhodium alloys.
- Flue gas scrubbing in industry uses calcium carbonate or calcium oxide (alkali) to neutralize acidic gases.
- Infrared spectroscopy is used to monitor pollutant levels by detecting specific molecular bonds.
- Congestion charging zones in cities help reduce pollution in high-traffic areas.
Halogenation of Alkanes
- Alkanes react with halogens (e.g., chlorine) via a free radical substitution mechanism, which requires UV light.
- The reaction has three stages:
- Initiation: UV light splits a halogen molecule into radicals.
- Propagation: Radicals react with alkanes to form new radicals and products.
- Termination: Two radicals combine to form a stable molecule, ending the chain reaction.
- Example: Methane + chlorine → chloromethane + HCl (via free radical mechanism)
- Multiple substitutions can occur if excess halogen is present, leading to di-, tri-, or tetra-halogenated products.
Alkenes: Structure and Reactions
- Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond (C=C), with the general formula CnH2n.
- The double bond gives alkenes high electron density, making them more reactive than alkanes.
- Alkenes can be straight-chain or cyclic (cycloalkenes), and may have more than one double bond (dienes).
- The double bond is the site for addition reactions, where new atoms or groups are added to the molecule.
Key Reactions of Alkenes
- Addition of bromine water: Alkenes decolorize bromine water (brown/orange to colorless) due to an addition reaction, which is a test for alkenes.
- Hydration: Alkenes react with steam and an acid catalyst (e.g., phosphoric acid) at high temperature and pressure to form alcohols (e.g., ethene + steam → ethanol). The reaction is reversible, and unreacted alkene is recycled to improve yield.
- Hydrogenation: Alkenes react with hydrogen (H₂) in the presence of a catalyst to form alkanes (e.g., ethene + H₂ → ethane). This process is used to convert unsaturated compounds to saturated ones.
- Addition of hydrogen halides (e.g., HBr): Alkenes react with hydrogen halides to form halogenoalkanes. The major product depends on the stability of the carbocation intermediate, following Markovnikov's rule (hydrogen adds to the carbon with more hydrogens already attached).
- Oxidation with cold, dilute potassium manganate (KMnO₄): Produces diols (compounds with two -OH groups).
- Oxidation with hot, concentrated KMnO₄: Breaks the double bond, forming carbonyl compounds (ketones, aldehydes, or carboxylic acids), depending on the groups attached to the double bond. If both groups are hydrogens, CO₂ and water can be formed.
- These reactions are important for identifying alkenes and for industrial synthesis.
Key Terms & Definitions
- Hydrocarbon: Compound containing only hydrogen and carbon.
- Alkane: Saturated hydrocarbon with only single bonds.
- Alkene: Unsaturated hydrocarbon with at least one double bond.
- Cycloalkane/Cycloalkene: Ring-shaped saturated/unsaturated hydrocarbons.
- Fractional Distillation: Separation of a mixture by boiling point differences.
- Cracking: Breaking large hydrocarbons into smaller, more useful ones.
- Electrophile: Species that accepts an electron pair (electron-loving).
- Free Radical: Atom or molecule with an unpaired electron, highly reactive.
- Photochemical Reaction: Reaction initiated by light (e.g., UV).
- Markovnikov’s Rule: In addition of hydrogen halides to alkenes, the hydrogen attaches to the carbon with more hydrogens already present.
- Carbocation: Positively charged carbon atom (reaction intermediate).
- Diol: Molecule with two hydroxyl (-OH) groups.
- Ketone/Aldehyde: Organic compounds containing a carbonyl group (C=O); ketones have two alkyl groups attached, aldehydes have at least one hydrogen.
- Aromatic Hydrocarbon: Compound containing benzene rings (mainly produced in catalytic cracking).
Action Items / Next Steps
- Review and practice writing balanced equations for both complete and incomplete combustion, as well as key alkene reactions.
- Practice drawing and explaining mechanisms: free radical substitution (alkanes) and electrophilic addition (alkenes), including curly arrow notation.
- Study Topic 13 (Introduction to Organic Chemistry) for foundational concepts, and Topics 17–18 for more detail on oxidation reactions.
- Prepare for exam questions on environmental impacts of hydrocarbon combustion and methods for pollutant removal.
- Use past paper questions to reinforce understanding of mechanisms and environmental chemistry.
- Familiarize yourself with tests for alkenes (e.g., bromine water) and the outcomes of oxidation reactions for different types of alkenes.