Alcohols & Fuels

Definition and Structure

• Primary Alcohols: Organic compounds wherein the hydroxyl group (-OH) is bonded to a primary carbon atom (R-CH₂OH).

  

• Secondary Alcohols: The hydroxyl group is linked to a carbon atom that connects to two other carbon atoms. The formula is R₁-CHOH-R₂.

  

• Fossil Fuels: Formed from ancient organic matter subjected to heat and pressure, including coal and oil.

• Biofuels: Produced from recent organic material or by-products, such as ethanol and biodiesel.

Oxidation Process

Primary Alcohols

• Conversion to Aldehydes and Acids:

  • Oxidation steps:
    • To Aldehydes: $\text{RCH}_2\text{OH} + [O] \rightarrow \text{RCHO} + \text{H}_2\text{O}$
    • To Carboxylic Acids: $\text{RCHO} + [O] \rightarrow \text{RCOOH}$

• Oxidising Agents:

  • K₂Cr₂O₇: Effective but toxic. KMnO₄ offers a safer alternative.

Secondary Alcohols

• Conversion to Ketones:
  • Key equation: $R_1\text{-CHOH-}R_2 + [O] \rightarrow R_1\text{-CO-}R_2 + \text{H}_2\text{O}$

• Practical Example: Propan-2-ol is oxidised to propanone.

Testing for Aldehydes

• Tollens' Test: A silver mirror suggests the presence of aldehydes.
• Fehling's Solution: Displays a colour change from blue to red when aldehydes are present.

Environmental Impacts of Fuels

• CO₂ Emissions:

  • Biofuels: Feature lower net emissions, offset by the CO₂ absorbed during the growth phase.

• Sulphur Dioxide and Nitrogen Oxides:

  • Biofuels: Generate less pollution than fossil fuels, thus helping to reduce acid rain.

Energy Content

• Energy Density:

  • Fossil Fuels: Possess a higher energy density, making them efficient for industrial applications.
  • Biofuels: Have a lower energy density, necessitating larger volumes.

• Production Methods:

  • Fossil Fuels: Extraction involves mining and refinement.
  • Biofuels: Processed through fermentation and transesterification.

Economic and Sustainability Considerations

Market Dynamics

• Fossil Fuels: Prices are subject to global political influences.

• Biofuels: Fluctuations depend on agricultural yields.

Land Use and Food Security

• The production of biofuels can potentially limit land availability for food crops.

Chemical Properties of Ethanol

• Structure:

  • Formula: C₂H₅OH
  • Comprised of 2 carbon atoms, 5 hydrogen atoms, and 1 hydroxyl group.

  

Combustion Reaction

• Ethanol's Reaction With Oxygen:
  • $\text{C}_2\text{H}_5\text{OH} + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O}$

Benefits and Limitations

Environmental Advantages

• Lower Emissions:
  • Ethanol contributes fewer greenhouse gases to the atmosphere.

Limitations

• Energy Density: Inferior to that of petrol, resulting in less efficient engines.
• Production Challenges: Involves significant costs and land usage.

Practice Questions

1. Outline the oxidation process for propan-1-ol. Solution: Propan-1-ol (CH₃CH₂CH₂OH) undergoes oxidation first to form propanal (CH₃CH₂CHO), and with further oxidation, it forms propanoic acid (CH₃CH₂COOH). The reaction requires an oxidising agent like K₂Cr₂O₇ in acidic conditions.

2. What are the essential conditions for oxidising secondary alcohols? Solution: Secondary alcohols require an oxidising agent (typically K₂Cr₂O₇ or KMnO₄), acidic conditions (usually dilute H₂SO₄), and heating under reflux to ensure complete oxidation to ketones.

3. Compare the fuel efficiency of ethanol with petrol. Solution: Ethanol has approximately 70% of the energy content of petrol, making it less fuel-efficient. This means vehicles need more ethanol than petrol to travel the same distance. However, ethanol produces fewer harmful emissions, offering environmental benefits despite its lower energy density.