Alkanes are saturated hydrocarbons with the general formula C_nH_{2n+2} (for non-cyclic alkanes). Each carbon atom is sp³ hybridized, forming four sigma bonds directed toward the corners of a tetrahedron. The carbon-carbon and carbon-hydrogen bonds are nonpolar, making alkanes hydrophobic and chemically inert under normal conditions. Alkanes form the structural backbone of organic molecules and serve as important fuels, lubricants, and solvents.
IUPAC nomenclature for alkanes identifies the longest continuous carbon chain as the parent name (methane C1, ethane C2, propane C3, butane C4, pentane C5, hexane C6, etc.). Substituents branching from the parent chain are named as alkyl groups (methyl, ethyl, propyl) and are preceded by their position numbers. When multiple substituents are present, they are listed in alphabetical order, and numerical prefixes (di-, tri-, tetra-) indicate multiplicity. The lowest set of locants is assigned to substituents regardless of their identity, following the first point of difference rule.
Physical properties of alkanes follow predictable trends. Boiling points increase with molecular mass due to stronger London dispersion forces; each additional CH₂ group raises the boiling point by approximately 20–30 °C. Branching lowers the boiling point by reducing the molecular surface area available for intermolecular contact. Alkanes are essentially insoluble in water (a polar solvent) but dissolve readily in nonpolar organic solvents. Their density increases with chain length but never exceeds about 0.8 g·mL⁻¹, causing them to float on water — an important property in oil spill behavior.
Conformational analysis of butane illustrates the energetic preferences in carbon-carbon bond rotation. The anti conformation (dihedral angle 180°) is the most stable, with methyl groups maximally separated. The gauche conformation (60°) is less stable by about 3.8 kJ·mol⁻¹ due to steric repulsion. The eclipsed conformations (0° and 120°) are the least stable, with energy barriers of approximately 15–20 kJ·mol⁻¹ above the anti conformation. These conformational preferences influence the three-dimensional structure of larger molecules and are critical in understanding stereochemistry and reaction outcomes.
Cycloalkanes have the general formula C_nH_{2n} (two fewer hydrogen atoms than open-chain alkanes due to ring closure). Baeyer strain theory proposed that deviations from the ideal tetrahedral angle (109.5°) create angle strain, and that larger rings would be strain-free. However, we now recognize three types of ring strain: angle strain (deviation from ideal bond angles), torsional strain (eclipsing interactions), and transannular strain (steric crowding across the ring). Cyclopropane (60° bond angles) has severe angle strain (~115 kJ·mol⁻¹), making it highly reactive. Cyclobutane and cyclopentane have progressively less strain.
Cyclohexane adopts a chair conformation that eliminates both angle and torsional strain, with all C–C–C bond angles at 109.5° and all bonds staggered. The chair conformation has two types of hydrogen positions: axial (parallel to the ring axis, alternating up and down) and equatorial (radiating outward from the ring). Substituents prefer equatorial positions to avoid 1,3-diaxial interactions. The ring flip interconverts axial and equatorial positions through a boat conformation intermediate. This conformational preference is central to understanding the reactivity and stability of substituted cyclohexanes, a topic of fundamental importance in organic chemistry.
