Alkanes

Alkanes are saturated hydrocarbons that exhibit single covalent bonds between carbon atoms. This structural characteristic imparts high stability, making alkanes a fundamental topic in organic chemistry.

Key Definitions

• Alkane: Saturated hydrocarbons distinguished by the presence of single covalent bonds.

  infoNote

  • Alkane: Hydrocarbons characterised solely by single carbon-carbon bonds.

• Saturated Hydrocarbons:

  • Hydrocarbons composed exclusively of single bonds, maximising hydrogen atom content per carbon atom.

Introduction to Alkanes

• Alkanes follow the general formula $C_nH_{2n+2}$.
• They play a vital role in fuel production, offering significant energy release upon combustion.
• Alkanes are essential building blocks in organic chemistry, with properties varying based on chain length and branching.

Physical Characteristics

• Alkanes are colourless and odourless.
• Small alkanes (e.g., methane, ethane) are gases, medium-sized alkanes (e.g., hexane, heptane) are liquids, while larger alkanes (e.g., decane) are solids.

Role in Fuel Production

• Common reaction: $\text{Alkane} + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O} + \text{energy}$
• Fuels such as petrol and diesel are derived from alkanes.

Industrial Importance

• Alkanes are modified to produce compounds like ethylene.
• Catalytic techniques enhance combustion productivity, although they release carbon dioxide, leading to efforts to reduce environmental impact.

Bonding in Alkanes

• Alkanes contain only single bonds, rendering them saturated and stable.
• Sigma bonds (σ) are formed through sp³ hybridisation, contributing to the structure's stability and resulting in a tetrahedral geometry.

• Intramolecular and Intermolecular Forces:
  • Intramolecular bonds maintain molecular structure.
  • Weak intermolecular van der Waals forces influence physical properties such as boiling points.

Physical Properties

• Molecular size affects boiling/melting points; larger molecules possess higher points due to increased van der Waals forces.
• Branching decreases boiling/melting points.

Volatility and Density

• Volatility decreases as molecular mass increases.
• Density generally rises with chain length due to more compact molecular packing.

Functional Groups in Alkanes

• Alkanes Lack Functional Groups:
  • Their simple structures contribute to stability.
  • They primarily undergo combustion and substitution reactions.

Formulae

• Structural Formula:
  • Demonstrates atom arrangement.
• Molecular Formula:
  • Lists atom quantities.
• Formulating involves determining the root name and employing $C_nH_{2n+2}$ to represent hydrogen content.

Model Construction

• Models visually depict three-dimensional atomic interactions.
• Ball-and-stick models, though potentially misrepresenting bond angles, facilitate comprehension.

Practice Problems and Solutions:

Problem 1: Identify and Draw

Task: Illustrate pentane's structural and molecular formulas.

Solution:

• Molecular formula: C₅H₁₂
• Structural formula: CH₃-CH₂-CH₂-CH₂-CH₃
• Ensure all carbon atoms have four bonds each to satisfy valency

Problem 2: Boiling Points

Task: Compare the boiling points of butane and 2-methylpropane.

Solution:

• Butane (linear structure) has a higher boiling point than 2-methylpropane (branched structure)
• This is because the linear structure allows for stronger van der Waals forces between molecules, requiring more energy to separate

Problem 3: Combustion Reaction

Task: Formulate the balanced combustion equation for hexane.

Solution:

• Unbalanced: C₆H₁₄ + O₂ → CO₂ + H₂O
• Balanced: C₆H₁₄ + 9½O₂ → 6CO₂ + 7H₂O
• Alternatively: 2C₆H₁₄ + 19O₂ → 12CO₂ + 14H₂O (with whole numbers)

Problem 4: Combustion Types in Octane

Task: Differentiate between complete and incomplete combustion.

Solution:

• Complete combustion: C₈H₁₈ + 12½O₂ → 8CO₂ + 9H₂O
  • Occurs with excess oxygen
  • Produces carbon dioxide and water only
• Incomplete combustion: C₈H₁₈ + limited O₂ → mixture of CO₂, CO, C (soot), H₂O
  • Occurs with insufficient oxygen
  • Produces carbon monoxide, carbon (soot), and/or unburnt fuel in addition to CO₂ and H₂O