Alkanes
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Alkanes — Key Facts for SLMC Medical (Sri Lanka)
- Alkanes are saturated hydrocarbons with the general formula CₙH₂ₙ₊₂
- They contain only single bonds (σ-bonds) between carbon atoms
- Methane (CH₄) is the simplest alkane; ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀) follow
- High-yield: Alkanes are chemically inert due to strong C–H and C–C bonds
- ⚡ Exam tip: Combustion reactions of alkanes are frequently tested; methane combustion is especially important
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Alkanes — SLMC Medical (Sri Lanka) Study Guide
What Are Alkanes?
Alkanes belong to the homologous series of saturated hydrocarbons. “Saturated” means they contain the maximum possible number of hydrogen atoms per carbon skeleton — no double or triple bonds exist. Their general formula is CₙH₂ₙ₊₂, where n is the number of carbon atoms.
Nomenclature
Alkanes follow the IUPAC naming system:
- 1 carbon: Methane (CH₄)
- 2 carbons: Ethane (C₂H₆)
- 3 carbons: Propane (C₃H₈)
- 4 carbons: Butane (C₄H₁₀)
- 5 carbons: Pentane (C₅H₁₂)
- 6 carbons: Hexane (C₆H₁₄)
- 7 carbons: Heptane (C₇H₁₆)
- 8 carbons: Octane (C₈H₁₈)
- 9 carbons: Nonane (C₉H₂₀)
- 10 carbons: Decane (C₁₀H₂₂)
For branched-chain alkanes, the longest continuous chain determines the parent name. Number the chain from the end that gives substituents the lowest possible numbers. Substituents use the “-yl” suffix (e.g., methyl = –CH₃, ethyl = –C₂H₅).
Physical Properties
| Property | Trend |
|---|---|
| Boiling point | Increases with molecular size (more carbons → higher BP) |
| Melting point | Increases with molecular size |
| Density | Increases with molecular size (but all less dense than water) |
| Solubility | Insoluble in water; soluble in organic solvents |
| State at STP | C₁–C₄: gases; C₅–C₁₇: liquids; C₁₈+: solids |
Methane is a colorless, odorless gas — the principal component of natural gas. It is flammable and can form explosive mixtures with air (5–15% methane in air is explosive).
Chemical Properties
1. Combustion
Alkanes burn in excess oxygen to produce CO₂ and H₂O, releasing heat:
- CH₄ + 2O₂ → CO₂ + 2H₂O (methane combustion)
- 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O (ethane combustion) In limited O₂, incomplete combustion produces CO (carbon monoxide) or C (soot).
2. Halogenation
Alkanes undergo free-radical substitution with halogens (Cl₂, Br₂) under UV light:
- CH₄ + Cl₂ → CH₃Cl + HCl (one substitution)
- With excess Cl₂: CH₄ + 4Cl₂ → CCl₄ + 4HCl This is a chain reaction involving initiation, propagation, and termination steps.
3. Isomerism
Butane (C₄H₁₀) has two isomers:
- n-Butane: straight chain (CH₃–CH₂–CH₂–CH₃)
- Isobutane: branched chain (CH₃–CH(CH₃)–CH₃)
Pentane (C₅H₁₂) has three isomers. isomerism increases with more carbons.
Key Reactions Summary
| Reaction | Reagent/Condition | Product |
|---|---|---|
| Complete combustion | O₂ (excess), ignition | CO₂ + H₂O |
| Incomplete combustion | O₂ (limited) | CO + H₂O or C + H₂O |
| Chlorination | Cl₂, UV light | Chloroalkane + HCl |
| Bromination | Br₂, UV light | Bromoalkane + HBr |
Clinical Correlation (SLMC Focus)
- Methane is produced by intestinal bacteria during digestion — a common exam fact
- “Fart gas” contains methane (typically 7–10% in human flatus)
- Natural gas (methane) inhalation can cause asphyxiation in enclosed spaces
- Propane and butane are used as compressed fuel gases in medical settings
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Alkanes — Comprehensive SLMC Medical (Sri Lanka) Notes
Electronic Structure and Bonding
Each carbon in an alkane is sp³-hybridized with four equivalent σ-bonds arranged tetrahedrally (109.5° bond angle). The C–H bond length is ~109 pm, and the C–C bond length is ~154 pm. This tetrahedral geometry means there is no charge accumulation anywhere in the molecule — explaining the low reactivity.
Free Radical Mechanism of Halogenation
The chlorination of methane proceeds by a chain mechanism:
Step 1 — Initiation:
- Cl₂ → 2Cl· (UV light dissociates chlorine molecules)
Step 2 — Propagation:
- Cl· + CH₄ → HCl + CH₃· (methyl radical formed)
- CH₃· + Cl₂ → CH₃Cl + Cl· (chain continues)
Step 3 — Termination:
- Cl· + Cl· → Cl₂
- CH₃· + Cl· → CH₃Cl
- CH₃· + CH₃· → C₂H₆
Multiple substitution products can form (CCl₄, CHCl₃, CH₂Cl₂) because the methyl radical can react further with Cl₂.
Cycloalkanes
Cycloalkanes have the formula CₙH₂ₙ (one ring reduces H count by 2 compared to the open-chain alkane):
- Cyclopropane (C₃H₆): highly strained, very reactive
- Cyclobutane (C₄H₈)
- Cyclopentane (C₅H₁₀)
- Cyclohexane (C₆H₁₂): most stable cycloalkane; chair conformation preferred
Baeyer strain theory: Cycloalkanes with angles deviating significantly from 109.5° are strained. Cyclopropane (60°) and cyclobutane (90°) have angle strain.
Natural Sources of Alkanes
- Natural gas: methane (80–90%), ethane, propane
- Petroleum: complex mixture of alkanes separated by fractional distillation
- Coal: contains long-chain alkanes and aromatic compounds
Fractional distillation of petroleum yields:
- Refinery gas: methane, ethane, propane, butane (BP < 30°C)
- Gasoline/Petrol: C₅–C₈ (BP 40–200°C)
- Kerosene: C₉–C₁₅ (BP 175–325°C)
- Diesel: C₁₅–C₁₈
- Lubricating oils, paraffin wax: C₁₈+
Combustion Energetics
Standard enthalpy of combustion (ΔHₒ) values:
- Methane: –890 kJ/mol
- Ethane: –1560 kJ/mol
- Propane: –2220 kJ/mol
- Butane: –2877 kJ/mol
Higher alkanes release more energy per gram, but methane has the highest energy per mole among common fuels.
Environmental and Medical Relevance
- Greenhouse effect: Methane is 25× more potent a greenhouse gas than CO₂ over 100 years (though shorter atmospheric lifetime)
- Fluorinated alkanes (CFCs) were refrigerants that depleted the ozone layer (Montreal Protocol phased them out)
- Halothane (CF₃–CHCl–Br): an alkane derivative used as an inhalation anesthetic — relevant to medical students!
Common SLMC Exam Traps
- Students often confuse alkanes with alkenes — remember: alkane = single bonds only = saturated
- Combustion in limited O₂ produces CO, not just CO₂ — this is a common MCQ trap
- Isomerism questions: always count hydrogens first to determine molecular formula before counting structures
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