Alkanes: Structure, Classification, and Nomenclature

• Alkanes are the simplest, least reactive hydrocarbons in the homologous series of organic compounds
• Alkanes are saturated hydrocarbons
  • Contain only C-H and C-C single (σ) bonds
  • Have a maximum of 4 σ bonds per carbon
• All C atoms in alkanes:
  • Are sp³ hybridized
  • Have a tetrahedral geometry
• Alkanes are classified as:
  • Acyclic (aliphatic, straight and branched chains)
  • Cyclic (closed rings)

Acyclic (Non-Cyclic) Alkanes

• General formula: CnH2n+2
• Have zig-zag chains of carbon atoms
• Can be straight or branched chains.
• Examples:
  • Methane (CH4)
  • Butane (CH3CH2CH2CH3)
  • 3-methylpentane (CH3CH2CH(CH3)CH2CH3)

Cyclic Alkanes

• Also known as cycloalkanes
• General formula: CnH2n
• Share same general formula as alkenes
• Examples:
  • Cyclopropane (C3H6)
  • Cyclobutane (C4H8)
  • Cyclopentane (C5H10)
  • Cyclohexane (C6H12)

Nomenclature of Branched Chain Isomers (IUPAC System)

• Alkane names end with '-ane'
• When branching occurs, side chains are considered substituents.
• IUPAC name consists of a parent (or stem), a suffix, and prefixes (as needed).

IUPAC components when naming organic compounds:

• Prefix(es): Substituents
• Parent: Longest continuous carbon chain bearing the principal (most important) functional group
• Suffix: Principal functional group
• To name an organic compound:
  • Identify the longest continuous carbon chain for the parent name.
  • Number carbon atoms in the longest chain nearer to the first branch point.
  • Identify and label numbers on the substituents; If two or more substituents are present, cite alphabetically
  • Number prefixes like di-, tri-, and tetra- are ignored when alphabetizing

Isomerism

• Alkanes with 4+ carbon atoms exhibit structural isomerism due to branching of hydrocarbon chains
• Optical isomerism can also occur in higher members of alkane series

Physical Properties of Alkanes

• Composed of C and H atoms with similar electronegativities
• C-H bonds: considered non-polar with a very small dipole moment
• Alkane molecules are held together by weak instantaneous dipole-induced dipole interactions

Boiling and Melting Points

• Increase with the number of carbon atoms (Mr increase) as the size of the electron cloud increases
• Branched-chain alkanes tend to have lower boiling points than straight-chain alkanes
• The greater the branching, the lower the boiling point

Solubility

• Alkanes are insoluble in polar solvents (e.g., water)
• Alkanes are soluble in non-polar solvents (e.g., CCl4, benzene)

Density

• Increases down the homologous series
  • Larger Mr and stronger instantaneous dipole-induced dipole interactions bring molecules closer
  • Larger mass and smaller volume results in higher density
• Density decreases with more branching
  • Poorer packing (branched isomers occupy bigger space than straight chain)

Viscosity

• Measure of internal resistance to flow
• Formed by friction between neighboring molecules
• Viscosity of liquid alkanes increases with increasing Mr due to strength of intermolecular instantaneous dipole-induced dipole interactions.

Reactivity

• Alkanes are generally unreactive/chemically inert towards most chemical reagents
• Low chemical reactivity due to:
  • C-H bonds being non-polar (C and H have similar electronegativity)
  • C-C and C-H bonds being relatively strong
    • No electron-rich or electron-poor centers to attract reactants
• Alkanes react with oxygen (combustion) and halogens (substitution)

Combustion

• Alkanes burn exothermically when ignited

• Limited Oxygen:

  • Incomplete combustion occurs to produce CO and/or C (soot) and H2O

• Excess Oxygen:

  • Complete combustion occurs to produce CO2 and H2O,
  • e.g., CH4 + 2O2 → CO2 + 2H2O (ΔH = -890 kJ mol⁻¹)

• General equation for the combustion of hydrocarbon:

  • CxHy + (x + y/4) O2 → xCO2 + (y/2) H2O

• Alkanes used as fuels (methane, propane, butane) due to their ease of burning; highly exothermic

• Readily burn in the gaseous state, so solids/liquids must be vaporized first

• Higher members of alkanes have lower volatility and burn less readily, producing sooty flames

Free Radical Substitution (FRS)

• Alkanes react with halogens (Cl2(g) or Br2(l)) with presence of UV light or at high temperatures to produce halogenoalkanes, which involves free radicals
• Substitution occurs when one or more hydrogen atoms are replaced by halogen atoms

Free-Radical Substitution Mechanism

• Involves 3 elementary steps:
  • Initiation
  • Propagation
  • Termination

Example: Chlorination of Methane (CH4 + Cl2 → CH3Cl + HCl)

• Initiation:
  • A brief exposure to high temperature/UV light cleaves Cl–Cl bond homolytically, producing low concentration of free Cl radicals (atoms)
  • Cl2 → 2Cl∙
• Propagation:
  • Chain reaction sequence where ∙Cl radical reacts with CH4, abstracts a hydrogen to produce ∙CH3; this methyl radical reacts with Cl2 to give CH3Cl and regenerate ∙Cl
  • Cl∙ + CH4 → ∙CH3 + HCl
  • ∙CH3 + Cl2 → Cl∙ + CH3Cl
• Termination
  • Occurs when two free radicals combine to form a stable product; free radicals are consumed, not generated
  • Occurs towards the end of the reaction where the collision between radicals is more likely as decreasing number of reactant alkane molecules becomes increasingly difficult

Key Concepts for FRS

• Cl–Cl bond breaks preferentially because it requires less energy
• No H∙ radicals (atoms) formed during propagation as it is energetically unfavorable

Propagation Reaction

• Does not stop at monosubstitution
• CH3Cl formed can undergo further substitution, yielding CH2Cl2, CHCl3, and CCl4
• FRS does not give a single product but a mixture of products
• Limited synthetic utility due to product multiplicity

Multiple Substitutions Minimized When

• Excess alkane is used, monosubstituted alkane (e.g., CH3Cl) predominates
  • Higher probability for Cl radicals to collide with CH4 molecules
• When excess Cl2 is used, more highly substituted products (CH2Cl2, CHCl3, CCl4) are produced

Conditions for FRS with Different Halogens

• Reactivity of halogens with alkanes increases thus (reverse of X–X bond energies): I2 < Br2 < Cl2 < F2
  • F2: reaction proceeds explosively even at r.t.p.
  • Cl2/Br2: reaction occurs at 250 – 400°C or in UV light
  • I2: least reactive; reaction is so slow that it is not carried out

Example FRS Reactions

• Chlorination is exothermic (-119 kJ/mol)
• Iodination is endothermic (+22 kJ/mol and much less likely to synthesize iodoalkanes

Expected Alkyne Monosubstitution Yield

• In higher members of alkanes, substitution of hydrogen atoms at different carbon atoms leads to different products

Factors affecting Expected/Theoretical Yield

• Equal chance of each H atom being substituted
• Theoretical ratio is based on the # of interchangable H atoms
  • e.g., Butane produces 1-chlorobutane and 2-chlorobutane at ~ 60% : 40%, in accordane with # hydrogens available across the molecule
• Rate of substitution depends on the position of the atoms, with the stabilities as radicals increasing: methyl < primary < secondary < tertiary

Alkyl Groups

• Are weakly electron-donating/releasing
• Stabilize alkyl radicals by shifting electron density towards electron-deficient carbon atom that bears the lone electron
• More alkyl groups: more stable

Petroleum Environmental Consequences

• Pollutants produced during combustion: Smog, Carbon Monoxide, Nitrogen Oxides (NOx)

Smog

• Unburnt hydrocarbons mixed with sunlight
• Can cause lung damage

Carbon Monoxide (CO)

• Formed from incomplete combustion
• Binds more strongly to haemoglobin than O2, reducing the oxygen-carrying ability of blood cells; Fatal

Nitrogen Oxides (NOx, nitrox)

• Formed because high temperatures in car engines break N≡N bonds in nitrogen, which then react with oxygen to form NO and NO2, contributing to:
• Formation of smog (PAN) that is harmful to plants and humans
• Peroxyacetyl nitrate (PAN) forms via Hydrocarbons + O2 + NO2 + light
• Formation of ozone at lower atmospheres to high concentrations, which causes respiratory problems.
  • NO2 → NO + O; O + O2 → O3
• NO2 is a catalyst for the oxidation of SO2 to SO3, which dissolves in rainwater to cause acid rain (H2SO4)
  • Overall Eqn: SO2 + ½ O2 → SO3

Catalytic Converters

• Used in motor vehicles to reduce pollution
• Converts harmful C, CO, NO, NO2, and unburnt hydrocarbons to less harmful ones like CO2, H2O, N2, and O2
• Redox reactions occur
  • hydrocarbons + oxides of nitrogen → carbon dioxide + water + nitrogen
  • 2CO + 2NO → 2CO2 + N2 (CO is oxidised to CO2; NO is reduced to N2)
  • 2NO2 + 2C → 2CO2 + N2 (C is oxidised to CO2; NO2 is reduced to N2)

Greenhouse gases

• The disruption to the Earth's climate equilibrium by increased concentrations of greenhouse gases can lead to increase in global average surface temperatures, known as the enhanced greenhouse effect.
• Key greenhouse gases emitted in significant quantities include:

  • Carbon dioxide(CO2)

    • Accounts for around 75% of impact on of emissions; -Key source is from burning of fossil fuels

  • Methane (CH4)

    • Accounts for around 14% of current emissions and impact on greenhouse-gas
    • Key sources include agriculture such as livestock and rice fields, fossil fuel extraction, decay of organic waste in landfill sites.