Alkanes

Definition and General Characteristics

Basic Definition

Alkanes: hydrocarbons containing only single C–C and C–H bonds. Saturated: no double/triple bonds. General formula: C_nH_2n+2. Also called paraffins.

Classification

Linear (normal) alkanes: straight chains. Branched alkanes: contain alkyl substituents. Cycloalkanes: cyclic, different formula (C_nH_2n).

Functional Group Status

Alkanes lack functional groups with heteroatoms. Considered as reference hydrocarbons. Chemically inert under normal conditions but undergo combustion and radical substitution.

"Alkanes form the backbone of organic chemistry, providing a stable framework for functionalization." -- J. March, Advanced Organic Chemistry

Molecular Structure and Bonding

Hybridization and Geometry

Carbon atoms: sp³ hybridized. Geometry: tetrahedral, bond angle ~109.5°. Bonds: strong σ C–C and C–H bonds.

Bond Strength and Stability

C–C bond energy ~348 kJ/mol. C–H bond energy ~412 kJ/mol. High bond dissociation energy confers thermal stability.

Conformation and Rotation

Free rotation around C–C sigma bonds. Conformers: staggered (lowest energy), eclipsed (highest energy). Influences physical and chemical properties.

Example: Ethane conformationsStaggered: Dihedral angle 60°, lowest energyEclipsed: Dihedral angle 0°, torsional strain present

Nomenclature of Alkanes

IUPAC Naming Rules

Longest continuous carbon chain: parent name. Number chain to give substituents lowest possible numbers. Alphabetical order for substituents. Use prefixes: methyl-, ethyl-, propyl- etc.

Common Names vs Systematic Names

Common names based on trivial names (e.g., isobutane). Systematic names preferred for clarity in complex molecules.

Examples

CH₄: Methane; C₂H₆: Ethane; C₄H₁₀: Butane (n-butane and isobutane).

Physical Properties

State and Appearance

Lower alkanes (C1-C4): gases at room temperature. Medium alkanes (C5-C17): liquids. Higher alkanes: waxy solids.

Boiling and Melting Points

Increase with molecular weight. Branched alkanes have lower boiling points than straight chains due to decreased surface area and weaker van der Waals forces.

Solubility

Nonpolar molecules: insoluble in water. Soluble in nonpolar solvents (hexane, benzene).

Isomerism in Alkanes

Structural Isomerism

Different connectivity of atoms. Number of isomers increases exponentially with carbon number. Examples: n-butane vs isobutane.

Stereoisomerism

Alkanes generally lack stereocenters. No cis-trans isomerism in open-chain alkanes due to free rotation.

Conformational Isomerism

Rotation about C–C bonds creates conformers. Energy barriers low, conformers rapidly interconvert at room temperature.

Number of isomers for selected alkanes:C4H10: 2 isomersC5H12: 3 isomersC6H14: 5 isomersC7H16: 9 isomers

Natural and Synthetic Sources

Natural Occurrence

Found in natural gas, crude oil, coal deposits. Produced biogenically via anaerobic decay.

Petroleum Refining

Alkanes separated by fractional distillation. Catalytic cracking converts heavy alkanes to lighter fractions.

Synthetic Production

Wurtz reaction: coupling alkyl halides using sodium. Fischer-Tropsch process: catalytically converts CO and H₂ to alkanes.

Preparation Methods

Laboratory Synthesis

Reduction of alkyl halides with zinc and acid. Wurtz coupling for symmetrical alkanes. Catalytic hydrogenation of alkenes.

Industrial Methods

Fischer-Tropsch synthesis using cobalt/iron catalysts. Hydrocracking: breaks heavier hydrocarbons into smaller alkanes.

Key Reactions

Radical halogenation intermediates enable functional group transformations. Corey-House synthesis produces substituted alkanes.

Chemical Reactions

Combustion

Complete combustion: alkane + O₂ → CO₂ + H₂O + energy. Highly exothermic, basis for fuel use.

Free Radical Halogenation

Initiation: homolytic cleavage of X₂. Propagation: H abstraction and halogen radical formation. Termination: radical recombination.

Cracking and Reforming

Thermal or catalytic cracking breaks long chains. Reforming rearranges alkanes to branched/cyclic forms to improve fuel quality.

General halogenation mechanism:Initiation: X2 → 2 X•Propagation:RH + X• → R• + HXR• + X2 → RX + X•Termination: R• + X• → RX

Combustion and Energy Content

Energy Yield

High energy density: ~47-48 MJ/kg. Energy increases with chain length. Used as fuels in heating, engines.

Environmental Considerations

Incomplete combustion produces CO, soot. Releases CO₂, contributing to greenhouse effect.

Heat of Combustion Data

Industrial and Laboratory Applications

Fuel Industry

Primary components of gasoline, diesel, LPG. Combustion properties dictate engine performance.

Chemical Feedstock

Precursors for synthesis of halogenated compounds, polymers, lubricants, solvents.

Laboratory Uses

Nonpolar solvents, calibration standards in GC. Model compounds in mechanistic organic studies.

Environmental Impact

Greenhouse Gas Emissions

CO₂ from alkane combustion significant greenhouse contributor. Methane itself is a potent greenhouse gas.

Pollution and Toxicity

Incomplete combustion releases CO and particulate matter. Volatile organic compounds contribute to smog formation.

Mitigation Strategies

Improved combustion efficiency, catalytic converters, shift to alternative energy sources.

Spectroscopic Identification

Infrared (IR) Spectroscopy

C–H stretching vibrations: 2850–2960 cm^-1. Absence of peaks for double/triple bonds confirms saturation.

NMR Spectroscopy

Proton NMR: signals at 0.5–2.0 ppm typical for alkyl protons. Carbon NMR: sp³ carbon signals 5–50 ppm.

Mass Spectrometry

Fragmentation patterns show successive loss of CH₃ or CH₂ units. Molecular ion peak corresponds to molecular weight.

References

• Clayden, J., Greeves, N., Warren, S., Wothers, P. Organic Chemistry, 2nd ed.; Oxford University Press: 2012; pp 50-85.
• March, J. Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 4th ed.; Wiley: 1992; pp 10-25.
• Smith, M. B. March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 7th ed.; Wiley: 2013; pp 100-130.
• Vogel, A. I. Vogel’s Textbook of Practical Organic Chemistry, 5th ed.; Longman: 1989; pp 120-135.
• McMurry, J. Organic Chemistry, 9th ed.; Cengage Learning: 2015; pp 200-230.