Hydrocarbons are organic compounds composed exclusively of carbon and hydrogen atoms. Alkanes represent the simplest members of this chemical family, serving as the structural backbone for much of organic chemistry. They are widely used in fuels and solvents. Examining the characteristics that all alkanes share reveals a consistent set of properties defining the entire class.
Defined by Single Bonds and Saturation
The defining structural property of every alkane is its nature as a saturated hydrocarbon. This means the molecule contains only single covalent bonds between carbon and hydrogen atoms, utilizing the maximum possible number of hydrogen atoms. Each carbon atom forms four single bonds. The geometry around each carbon is tetrahedral, with bond angles of approximately 109.5 degrees.
The saturation of the carbon chain establishes a consistent mathematical relationship for the entire series. All non-cyclic alkanes conform to the general molecular formula CnH2n+2, where n represents the number of carbon atoms. This formula is a direct consequence of the single-bond structure, where the number of hydrogen atoms is always twice the number of carbons plus two.
Universally Low Chemical Reactivity
A universal characteristic of all alkanes is their low chemical reactivity under standard laboratory conditions. This inertness stems from the inherent strength of the C-C and C-H single bonds. Breaking these bonds requires significant energy, making alkanes resistant to many chemical changes.
Alkanes were historically referred to as paraffins, meaning “little affinity.” They do not readily react with common chemical agents such as acids, bases, or oxidizing and reducing agents. This lack of reactivity contrasts sharply with other hydrocarbons, like alkenes and alkynes, which are much more prone to addition reactions.
The two main reactions alkanes reliably undergo are combustion and halogenation. Combustion is a reaction with oxygen that releases large amounts of energy, making alkanes important fuels. Halogenation is a substitution reaction where a hydrogen atom is replaced by a halogen, typically requiring high heat or ultraviolet light.
Non-Polar Nature and Physical State
All alkanes are characterized by their non-polar nature, which governs their physical properties and interactions with other substances. The electronegativity difference between carbon and hydrogen is very small, meaning electrons in the C-H bond are shared almost equally. This minimal polarity results in a molecule with a zero or near-zero net dipole moment.
Because of their non-polar structure, alkanes interact primarily through weak London dispersion forces. The strength of these forces increases predictably as the molecule’s size and surface area increase. This explains the consistent trend in physical state: the smallest alkanes are gases, followed by liquids of moderate size, and eventually solids for the longest chains.
The non-polar character means that alkanes are universally hydrophobic and insoluble in polar solvents like water. Following the principle of “like dissolves like,” alkanes are miscible with other non-polar organic solvents. This property makes them effective solvents for substances such as fats, oils, and waxes.