The ability for carbon atoms to easily create bonds means that there are tens of millions of possible compounds in organic chemistry. Because it is impossible to individually name all these compounds, systematic nomenclature systems have been devised such as the one by IUPAC (International Union of Pure and Applied Chemistry) which is explained here. Even though these systematic naming schemes exist, common names are still very much in use.
What are alkanes?
Alkanes are saturated carbon chains (which means there are only single bonds and zero double or triple bonds) where each carbon is surrounded by four other atoms which are only carbon or hydrogens. The general formula of alkanes is CnH2n+2 and can be branched. Cycloalkanes (which have a general formula of CnH2n) are alkanes with one or more rings and are generally regarded as a separate class of molecules.
Alkanes are hydrophobic, fatty compounds. At room temperature methane - octane exist as gases, nonane - hexadecane exist as viscous liquids and even "higher" alkanes are solid. The higher alkanes are used as lubricants, water repellants (against rust), in candles and in fuels. Lower alkanes are also used in fuels. Alkanes are obtained through oil and gas drilling and can be seperated from one another by destillation.
Nomenclature of alkanes
Depending on the number of carbon atoms, the alkane will receive a name based on a Greek/Latin root. The names of the first twelve non-branched alkanes are as follows:
As can be seen from the structural formula of dodecane, it is one long non-branched chain of carbons.
Branches & substituents
When talking about branched carbon chains, the primary characteristic to determine is the longest possible chain. This chain is used to determine the "root" of the name of the molecule. When, for example, the longest chain consists of eight carbon atoms the molecule will be an octane. After that the names of the substituents are determined. A substituent consisting of one carbon atom is called a methyl-group, two carbon atoms is an ethyl, three a propyl, and so on. A propane with a methyl substituent will be named methylpropane. These alkane substituents are also commonly referred to as alkyl groups.
The position of the substituent is indicated by a number based on it's position of the main chain, where the lowest number is preferred (see for example 2-methylbutane). When an alkane has multiple substituents of the same type, all of these are numbered and a prefix is added based on the amount of the same substituents (di-, tri-, tetra-, etc.), see for example 2,3-dimethylbutane.
When an alkane has different substituents, these substituents are ordered alphabetically and the first substituent is used in determining the lowest position. In this ordering prefixes such as di- and tri- are ignored, but prefixes such as iso- (see below) are included. See, for example, the compound 5-ethyl-2,11-dimethyldodecane.
Alkylgroups that have their own substituents or that are not directly connected to the main chain are called complex substituents. The names of complex substituents are always between brackets with their position on the main chain before the opening bracket. The ways of naming the same compound can differ quite a bit and are not adressed in this course (for example, the names (2-methylpropan-2-yl)cyclohexane, (2-methyl-2-propanyl)cyclohexane and (1,1-dimethylethyl)cyclohexane all refer to the same compound).
Rings
Alkanes that contain a ring will be prefixed with cyclo-. A ring consisting of three carbon atoms is called cyclopropane, a ring with four carbons is cyclobutane, five is cyclopentane, and so on. Substituents and their positions are determined using the same rules as alkanes (see 1-ethyl-3-methylcyclohexane). The rules for multiple or fused rings are beyond the scope of this course (see bicyclo[2.2.1]heptane or camphor). When a ring has only one substituent the position is usually omitted.
Common names
Non-branched alkane carbon chains are usually prefixed by n-to indicate that it is one lineair chain. See, for example, n-octane, where this prefix is in cursive. Some other commonly used prefixes for branched carbohydrates are: iso, sec and tert. See, for example, the next common names and their associated systematic names.
- isobutane: methylpropane
- sec-butylacetate: butan-2-yl ethanoate
- neopentane: 2,2-dimethylpropane
- tert-butanol: 2-methylpropan-2-ol
Methyl, ethyl, propyl and butyl substituents are also commonly shorted using Me, Et, Pr and Bu respectively. These shorthands are sometimes also combined using prefixes like iso, sec and tert which creates, for example, iPr (isopropyl) or tBu (tert-butyl).
Practice