3.2..2 HYDROCARBONS (ALKANES)

What are the two main methods for producing alkanes, and what are their key conditions?

• Alkanes are saturated hydrocarbons with single C-C bonds. They are produced through:

  • Hydrogenation: Addition of hydrogen to alkenes, requiring heat, H₂ gas, and a Pt/Ni catalyst.

  • Cracking: Breaking long-chain hydrocarbons into smaller molecules, requiring heat with an Al₂O₃ catalyst in oxygen-free conditions.

What is hydrogenation, and how does it produce alkanes?

• Definition: Addition of hydrogen to unsaturated alkenes with C=C bonds to form saturated alkanes.

• Mechanism:

  • Reactants: Alkene + H₂ gas.

  • Conditions: Heat and a finely divided Pt/Ni catalyst (high surface area increases reaction rate).

• Example:

  • Reaction: Butene + H₂ → Butane (with Pt/Ni catalyst).

• Applications:

  • Used in the food industry to partially hydrogenate vegetable oils to make margarine (raises melting point).

Details of Hydrogenation Reaction

• Exothermic Reaction: Releases energy as heat.

• Industrial Relevance: Straightens unsaturated hydrocarbon chains in oils for food production.

• Tips:

  • Catalyst efficiency is critical—finely divided Pt/Ni catalysts provide high surface area.

Cracking of Hydrocarbons

• Definition: Breaking large hydrocarbons into smaller alkanes and alkenes.

• Mechanism:

  • Reactants: Large hydrocarbon chains (e.g., decane).

  • Conditions: Heat in steel chambers with Al₂O₃ catalyst (oxygen-free to prevent combustion).

• Example:

  • Reaction: Decane → Octane + Ethene.

• Products:

  • Smaller alkanes: Fuel-grade hydrocarbons.

  • Alkenes: Used in polymer production.

Cracking Process Characteristics

• Endothermic Reaction: Requires heat input.

• Applications:

  • Converts less useful hydrocarbons in crude oil to valuable products.

• Key Notes:

  • Oxygen exclusion prevents formation of CO₂ and H₂O.

Comparison of Hydrogenation and Cracking

• Hydrogenation:

  • Type: Addition reaction.

  • Energy: Exothermic.

  • Uses: Food industry, fuel refinement.

• Cracking:

  • Type: Thermal decomposition.

  • Energy: Endothermic.

  • Uses: Fuel production, chemical synthesis.

What is complete combustion, and what are its products?

Complete combustion occurs when alkanes are burnt in an excess of oxygen. During this reaction, all carbon and hydrogen are fully oxidized, producing:

• Carbon dioxide (CO₂)

• Water (H₂O)

Example: Complete combustion of octane: C₈H₁₈ + 12.5O₂ → 8CO₂ + 9H₂O

Key Features:

• Requires plenty of oxygen.

• Releases a significant amount of energy, making alkanes useful as fuels.

What is incomplete combustion, and what are its risks?

Incomplete combustion occurs when alkanes are burnt in a limited oxygen supply. This reaction partially oxidizes carbon, producing:

• Carbon monoxide (CO): A toxic gas.

• Water (H₂O) Example: Incomplete combustion of octane: C₈H₁₈ + 8.5O₂ → 8CO + 9H₂O

Key Risks of Carbon Monoxide:

• Binds to haemoglobin, preventing oxygen transport in the blood.

• Causes symptoms like dizziness and loss of consciousness; prolonged exposure can be fatal.

• Odourless and hard to detect.

Occurrence: Common in car engines due to limited oxygen.

What is free-radical substitution, and what is needed for the reaction to occur?

Free-radical substitution is a reaction in which a hydrogen atom in an alkane is substituted by a halogen (chlorine/bromine).

• Requirement: Ultraviolet light (UV light) is essential for initiating the reaction, as alkanes are generally unreactive.

• Example: When bromine is mixed with an alkane and exposed to UV light, the bromine's colour disappears, indicating the reaction has occurred. Key Reaction Steps:

1. Initiation: Formation of halogen radicals.

2. Propagation: Chain reaction producing products and regenerating radicals.

3. Termination: Radicals combine to end the reaction.

What happens during the initiation step of free-radical substitution?

The covalent bond in a halogen molecule (Cl-Cl or Br-Br) is broken by UV light energy, causing homolytic fission.

• Reaction: Cl₂ → 2Cl•

• Homolytic Fission: Each halogen atom takes one electron from the bond, forming two highly reactive radicals.

• Key Feature: The radicals formed will initiate further reactions by attacking alkanes.

Describe the two main reactions in the propagation step of free-radical substitution.

During propagation, radicals react with alkanes to create a chain reaction:

1. Step 1: A halogen radical (e.g., Cl•) reacts with the alkane, breaking a C-H bond and producing an alkyl radical. Example: CH₄ + Cl• → •CH₃ + HCl

2. Step 2: The alkyl radical reacts with a halogen molecule, forming a halogenoalkane and regenerating the halogen radical. Example: •CH₃ + Cl₂ → CH₃Cl + Cl•

Key Feature: The regenerated halogen radical continues the chain reaction, repeating the cycle.

What occurs during the termination step of free-radical substitution?

In termination, two radicals combine to form a stable product, ending the chain reaction. Examples of Termination Reactions:

1. Methyl radical + Chlorine radical: •CH₃ + Cl• → CH₃Cl

2. Methyl radical + Methyl radical: •CH₃ + •CH₃ → CH₃CH₃

3. Chlorine radical + Chlorine radical: Cl• + Cl• → Cl₂

Key Feature: The removal of radicals stops the reaction.

What happens if there is excess halogen present during free-radical substitution?

Excess halogen leads to multiple substitutions, replacing all hydrogens in the alkane with halogens. Example of Multiple Substitutions with Methane:

1. First Substitution: CH₄ → CH₃Cl CH₃Cl + Cl• → •CH₂Cl + HCl •CH₂Cl + Cl₂ → CH₂Cl₂ + Cl•

2. Second Substitution: CH₂Cl₂ → CHCl₃ CHCl₃ + Cl• → •CCl₃ + HCl •CCl₃ + Cl₂ → CCl₄ + Cl•

Key Note: Multiple substitution makes free-radical substitution unsuitable for preparing specific halogenoalkanes, as mixtures of products are formed.

How are arrows used to depict the free-radical substitution mechanism?

• Initiation: Uses half-headed (fish-hook) arrows to show the movement of a single electron.

• Propagation: Continues with fish-hook arrows showing radical generation and product formation.

• Termination: Fish-hook arrows depict radicals combining to form stable molecules.

Exam Tip: Avoid using equations that reform the original halogen (e.g., Cl₂ → 2Cl• → Cl₂) when asked about the termination step, as these may be ignored in mark schemes.

What is crude oil, and how is it processed to obtain useful fractions?

Crude oil is a mixture of hydrocarbons, including alkanes, cycloalkanes, and arenes (compounds with a benzene ring).

• Extraction: Obtained from the earth through drilling.

• Transportation: Taken to oil refineries.

• Fractional Distillation: A process in which hydrocarbons are separated based on boiling points, producing fractions with similar boiling points.

• High Demand Fractions: Smaller hydrocarbon fractions (e.g., gasoline) are in high demand compared to heavier fractions.

What is cracking, and why is it performed on heavier crude oil fractions?

Cracking is the process of breaking larger, heavier hydrocarbons into smaller, more useful alkanes and alkenes of lower relative formula mass (Mr). Purpose:

• Converts excess heavy crude oil fractions into smaller hydrocarbons in high demand. Process:

1. Large hydrocarbon molecules are fed into a steel chamber.

2. Heated to high temperatures.

3. Passed over an aluminium oxide (Al₂O₃) catalyst.

4. Oxygen is excluded to prevent combustion into CO₂ and H₂O.

What are the products of cracking, and what are their uses?

Cracking produces:

1. Alkanes: Low-molecular mass hydrocarbons, highly useful as fuels due to their energy release during combustion.

   • Example: Octane, used in gasoline.

2. Alkenes: Reactive hydrocarbons with double bonds, used as feedstock in chemical industries.

   • Example: Ethene, used as a monomer in polymerisation reactions to create plastics.

Why are alkenes valuable in the chemical industry

Alkenes are more reactive than alkanes due to their double bonds, making them versatile starting compounds for creating new products. Uses:

• Polymerisation: Alkenes act as monomers to form polymers (e.g., plastics).

• Chemical Reactions: Alkenes undergo various reactions to create valuable compounds for industrial applications.

What are the key features of the industrial cracking process?

• Environment: Steel chambers heated to high temperatures.

• Catalyst: Aluminium oxide (Al₂O₃) accelerates the reaction.

• Oxygen Exclusion: Prevents combustion, ensuring hydrocarbons break into alkanes and alkenes rather than burning into carbon dioxide and water.

• Example: Cracking decane to produce octane and ethene.

What are the advantages of cracking heavier crude oil fractions?

• Increased Fuel Supply: Produces low-molecular mass alkanes like gasoline, which are in high demand.

• Versatile Raw Materials: Generates alkenes for industrial processes such as polymerisation.

• Economic Benefits: Utilises heavier fractions that would otherwise be less useful, maximising resource efficiency.

Why are alkanes generally unreactive?

Alkanes are unreactive due to:

1. Strong C-H and C-C Bonds: These covalent bonds require a lot of energy to break, making alkanes stable.

2. Lack of Polarity: The electronegativities of carbon and hydrogen are almost the same, resulting in nonpolar bonds.

How does bond strength contribute to the unreactivity of alkanes?

• C-H Bonds: Strong covalent bonds between carbon and hydrogen atoms.

• C-C Bonds: Single bonds between carbon atoms are also strong.

• Result: These bonds are difficult to break unless a significant amount of heat is supplied, reducing alkane reactivity in chemical reactions.

Why are alkanes nonpolar, and how does this affect their reactivity?

• Electronegativity Difference: The difference in electronegativity between carbon and hydrogen is only 0.4 (Pauling Scale).

• Nonpolar Molecules: Electrons in the covalent bonds are shared almost equally, resulting in no partial charges (δ+ or δ-).

• Effect:

  • Alkanes lack electron-deficient areas to attract nucleophiles (negatively charged species).

  • They also lack electron-rich areas to attract electrophiles (positively charged species).

How does ethane illustrate the lack of polarity in alkanes?

• Structure: Ethane (C₂H₆) consists of carbon and hydrogen atoms with similar electronegativities.

• Result: The molecule is nonpolar, with no regions of partial positive or negative charge.

• Reactivity: Ethane, like other alkanes, does not react with polar reagents due to its lack of polarity.

What types of reactions do alkanes undergo despite their unreactivity?

• Combustion Reactions: Alkanes react with oxygen to produce carbon dioxide and water, releasing energy.

• Substitution by Halogens: Alkanes can undergo free-radical substitution reactions with halogens (e.g., chlorine or bromine) under ultraviolet light.

What should you remember about nucleophiles and electrophiles in relation to alkanes?

• Nucleophiles: Negatively charged species attracted to electron-deficient regions.

• Electrophiles: Positively charged species attracted to electron-rich regions.

• Alkanes: Do not attract nucleophiles or electrophiles due to their nonpolar nature and lack of electron-deficient or electron-rich areas.

What are the main pollutants produced from the combustion of alkanes in car engines, and why are they harmful?

The main pollutants are:

1. Carbon Monoxide (CO): Toxic gas formed during incomplete combustion due to limited oxygen.

2. Oxides of Nitrogen (NO/NO₂): Formed at high temperatures and pressures in car engines.

3. Volatile Organic Compounds (VOCs): Unburnt hydrocarbons and their oxides.

Harmful Effects:

• Damage to human health, plant life, and the environment.

• Contribution to smog, acid rain, and global warming.

How is carbon monoxide formed, and what are its environmental and health consequences?

• Formation: Produced during incomplete combustion of alkanes when oxygen is limited. Equation: 2C₈H₁₈ + 17O₂ → 16CO + 18H₂O.

• Health Effects:

  • Toxic and odourless gas.

  • Binds to haemoglobin, preventing oxygen transport in the blood.

  • Causes dizziness, loss of consciousness, and can be fatal.

How are oxides of nitrogen formed, and what are their environmental consequences?

• Formation: High temperatures and pressures in car engines cause nitrogen to react with oxygen. Equations:

  • N₂ + O₂ → 2NO.

  • N₂ + 2O₂ → 2NO₂.

• Environmental Consequences:

  • React with water and oxygen to form nitric acid, causing acid rain.

  • Contribute to photochemical smog by reacting with VOCs to form peroxyacetyl nitrate (PAN).

  • Harmful to lungs, eyes, and plant life.

What are VOCs, and how do they contribute to environmental pollution?

• VOCs: Unburnt hydrocarbons and their oxides released from car engines.

• Formation of PAN: VOCs react with nitrogen oxides in the atmosphere to form peroxyacetyl nitrate (PAN), a major component of photochemical smog.

• Effects of PAN:

  • Harmful to lungs, eyes, and plant life.

  • Contributes to smog formation.

How do catalytic converters reduce pollutants in car exhaust fumes?

Catalytic converters use precious metals (e.g., platinum) coated on a honeycomb structure to provide a large surface area for reactions. Key Reactions:

1. Oxidation of CO to CO₂:

   • 2CO + O₂ → 2CO₂.

   • 2CO + 2NO → 2CO₂ + N₂.

2. Reduction of NO/NO₂ to N₂:

   • 2CO + 2NO → 2CO₂ + N₂.

3. Oxidation of VOCs to CO₂ and H₂O:

   • CₙH₂ₙ₊₂ + (3n+1)[O] → nCO₂ + (n+1)H₂O.

What are the broader environmental impacts of pollutants from car engines?

• Carbon Monoxide: Toxic to humans, interferes with oxygen transport.

• Oxides of Nitrogen:

  • Cause acid rain, which corrodes buildings and harms aquatic and plant life.

  • Contribute to smog, reducing air quality.

• VOCs and PAN:

  • Smog formation harms respiratory health and reduces visibility.

  • Damage to ecosystems and plant life.