Definition
Alkanes are organic compounds composed exclusively of carbon and hydrogen atoms. They belong to the hydrocarbons and contain only C-C and C-H bonds. Alkanes are not aromatic and saturated. They are referred to as aliphatic compounds. The general formula of acyclic alkanes is C
n
H
2n+2
. The simplest alkanes are the linear alkanes with one (methane), two (ethane), three (propane) or four (butane) carbon atoms. With increasing size, the number of possible constitutional isomers increases sharply. Alkanes can be unbranched (n-alkanes) or branched. Cycloalkanes exist as rings, for example cylopentane or cyclohexane. Unsaturated alkanes with double bonds are called alkenes and those with triple bonds are called alkynes.
Representatives (selection)
The name of the substituents is listed in parentheses. n-alkanes:
- Methane (methyl)
- Ethane (Ethyl)
- Propane (Propyl)
- Butane (Butyl)
- Pentane (Pentyl)
- Hexane (Hexyl)
- Heptane (Heptyl)
- Octane (Octyl)
- Nonane (Nonyl)
- Decane (Decyl)
- Undecane (Undecyl)
- Dodecane (Dodecyl)
Cycloalkanes:
- Cyclpropane (cyclopropyl)
- Cyclobutane (cyclobutyl)
- Cyclohexane (Cyclohexyl)
- Cycloheptane (cycloheptyl)
- Etc.
Nomenclature
Alkanes carry the suffix -an, alkenes the suffix -en, and alkynes the suffix -in. Cycloalkanes are preceded by the prefix cyclo-. For the exact procedure concerning the nomenclature, we refer to the literature.
Properties
- Because they lack electronegative atoms such as oxygen and nitrogen, alkanes do not form hydrogen bonds and have a low boiling point. This is in contrast to alcohols or amines.
- Alkanes with a low molecular mass are present in gaseous form. Higher alkanes are liquid, semi-solid and solid.
- With increasing molecular mass, the boiling and melting points increase. The reason lies in the increasing Van der Waals forces.
- Alkanes are hydrophobic (apolar) and therefore insoluble in water.
- Liquid alkanes have a density < 1 g/cm
3
(typically 0.7 g/cm
3
) and therefore float on the water. - Alkanes can be flammable and explosive.
Reactions
Due to the lack of functional groups, alkanes are relatively unreactive. Combustion of alkanes:
- CH
4
(methane) + 2 O
2
(oxygen) CO
2
(carbon dioxide) + 2 H
2
O (water)
Other reactions:
- Halogenations and other substitutions
- Cracking (splitting)
In pharmacy
Numerous active pharmaceutical ingredients and excipients are substituted alkanes. They play an important role as “backbones” and as substituents. They are also used as solvents and extractants. Rubbing alcohol, petrolatum and kerosenes are made of alkanes.
Adverse effects
Alkanes are the main components of fossil fuels such as petroleum, natural gas and corresponding products such as gasoline and diesel. Their combustion leads to the formation of carbon dioxide (CO
2
), which is mainly responsible for global warming.
