Definitions [1]
Definition: Alkanes
Alkanes are hydrocarbons in which all the linkages between the carbon atoms are single covalent bonds.
Key Points
Key Points: Alkanes
- General formula: CₙH₂ₙ₊₂ (where n = number of carbon atoms)
- Suffix used for IUPAC naming: –ane
- e.g., Methane (CH₄), Ethane (C₂H₆), Propane (C₃H₈)
- Alkanes exhibit chain isomerism due to absence of any functional group and the possibility of more than one chain type for the same molecular formula
- e.g., C₅H₁₂ forms n-pentane, neo-pentane, and iso-pentane
Conformations in Alkanes:
Conformation = spatial arrangement of atoms that can be interconverted by rotation about a C–C single bond. Also called conformers or rotamers.
For ethane, two extreme conformations exist:
| Conformation | Description | Energy | Stability |
|---|---|---|---|
| Eclipsed | Hydrogen atoms on adjacent carbons are as close as possible | Maximum energy | Highly unstable |
| Staggered | Hydrogen atoms are as far apart as possible | Minimum energy | Highly stable |
Skew conformation is an intermediate between eclipsed and staggered conformations. Both are represented by Sawhorse and Newman projections.
Sawhorse formula of ethane
Newmann projection formula ethane
Key Points: Occurrence of Methane and Ethane
- Source: Methane forms in marshes, coal mines, and during cellulose fermentation.
- Greenhouse Gas: It traps 20× more heat than CO₂ and stays in the atmosphere for ~10 years.
- Uses & Emission: Comes from landfills, farming, fossil fuels, and can be used as clean energy.
Key Points: Physical Properties of Methane and Ethane
- Physical State: Methane and ethane are colourless, odourless gases.
- Boiling & Melting Points: Methane (b.p. -162 °C, m.p. -183 °C), Ethane (b.p. -89 °C, m.p. -172 °C).
- Solubility: Both are insoluble in water but dissolve well in organic solvents.
Key Points: Chemical Properties of Methane and Ethane
- Complete Combustion: Alkanes burn in air to give CO₂ and H₂O with a bluish, non-sooty flame.
- Incomplete Combustion: In limited air, alkanes form CO or soot (C) along with water.
- Substitution Reaction: In the presence of sunlight, alkanes react with halogens (Cl₂, Br₂, I₂) to form alkyl halides.
- Catalytic Oxidation: Alkanes form alcohols, aldehydes, or acids using catalysts like Cu, MoO₃, or Pt under specific conditions.
- Slow Oxidation: With oxidising agents (e.g., K₂Cr₂O₇), alkanes oxidise stepwise to alcohols, aldehydes, acids, and finally CO₂ + H₂O.
Key Points: Uses of Methane and Ethane
- Methane is a source of carbon monoxide and hydrogen and is also used as a domestic fuel.
- Methane helps in the preparation of compounds like ethyne, methanol, chloromethane, and carbon tetrachloride.
- Halothane (CF₃CHBrCl) has replaced chloroform for anaesthetic use due to its safer properties.
- Ethane is used to produce ethene, ethanol, acetaldehyde, and acetic acid.
- Ethane is used to form ethyl chloride (for tetraethyllead) and acts as a fuel and dry-cleaning solvent.
Important Questions [1]
Concepts [10]
- Alkanes
- Isomerism in Alkenes
- Occurrence of Methane and Ethane
- Structure of Methane
- Laboratory Preparation of Methane
- Laboratory Preparation of Ethane
- Other Methods of Preparation of Methane and Ethane
- Physical Properties of Methane and Ethane
- Chemical Properties of Methane and Ethane
- Uses of Methane and Ethane
