Properties of Alkanes

Alkanes: Key Features

Alkanes are fundamental, fully saturated hydrocarbons composed exclusively of carbon (C) and hydrogen (H) atoms. They are noted for having C–C and C–H sigma bonds, signifying stability and saturation.

• General Formula: $C_nH_{2n+2}$:
  • $n$ represents the number of carbon atoms.
  • Alkanes are saturated, indicating they contain only single covalent bonds with hydrogen.

Homologous Series in Organic Chemistry

• Homologous Series:
  • A series of compounds where each differs by a CH₂ group.
  • This variability influences properties such as boiling points and solubility.

Structural and Chemical Properties

Basic Nomenclature of Alkanes Using IUPAC Rules

• Naming Alkanes:

  • Names are derived from the number of carbon atoms with specific prefixes followed by '-ane'.

  Number of Carbons	Name
  1	Methane
  2	Ethane
  3	Propane
  10	Decane

Structural Diagrams

• Visualising Alkanes:

  • Diagrams depict linear versus branched configurations, affirming saturation.

  

  

  

sp³ Hybridisation

• In sp³ hybridisation, one s orbital and three p orbitals in a carbon atom merge to form four equivalent sp³ hybrid orbitals.
  • Example: Methane (CH₄) articulates four strong C–H sigma bonds through sp³ orbitals.

Physical Properties of Alkanes

1. Overview of Physical Properties

Alkanes exhibit distinctive physical properties:

• Boiling Point: The temperature at which a liquid transitions into a gas.
• Melting Point: The temperature at which a solid transitions into a liquid.
• Solubility: The capacity to dissolve in a solvent.
• Viscosity: Resistance to flow in a fluid.
• Density: Mass per unit volume.

2. Boiling and Melting Points

• Alkanes are influenced by dispersion forces, which grow with molecular weight.
• Branching: Reduces surface area contact, resulting in decreased boiling points compared to linear configurations.

3. Solubility

• Alkanes dissolve in non-polar solvents due to their non-polar nature but do not dissolve in polar solvents like water.

4. Viscosity and Density

• Viscosity increases with longer carbon chains.
• Density is influenced by branching; generally, more carbon atoms mean higher density.

Intermolecular Forces

Dispersion Forces (London Forces)

• Dispersion Forces: Arise from transient polarisation, which induces temporary dipoles.
• Their strength increases with more electrons and larger molecules.
• They are fundamental in influencing the physical properties of alkanes.

Chemical Reactions and Inertness of Alkanes

• Alkanes are chemically stable due to strong sigma (σ) bonds between carbon and hydrogen atoms, marking them as saturated hydrocarbons.

Common Reactions of Alkanes

Combustion

• Combustion Reaction: Alkanes react with oxygen to yield carbon dioxide and water, with the release of energy.

Example:

• Methane: $\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$

Substitution Reactions

• Occur when an atom in a compound is replaced by another atom, often a halogen.

Homologous Series and Trends

Trends in Physical Properties

• Boiling Points & Melting Points increase with molecular weight.
• Density rises with increasing molecular mass.
• Branching Influence:
  • Branched isomers exhibit lower boiling points due to reduced surface area.

Trends in Chemical Properties

• Chemical Inertness: Alkanes demonstrate stability against many reagents.

Glossary

• Dispersion Forces: Weak forces resulting from temporary shifts in electron density.
• Reactivity: The capability to participate in chemical reactions with other substances.