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12.2.1. Alkanes

Alkanes (methane and its homologs) have a common formula CnH2n+2. The first four hydrocarbons are called methane, ethane, propane, butane. The names of the higher terms of this series consist of a Greek number and suffixe -ane. The names of alkanes are the basis of IUPAC.

Rules of systematic nomenclature:

  1. The rule of the main chain.

    The main chain is selected using the following criteria:

    1. Maximal number of functional substitutes.
    2. Maximal number of multiple bonds.
    3. Maximal extension.
    4. Maximal number of side hydrocarbon groups.

  2. The rule of the least numbers.

    The main chain is numbered from one side to another using arabic numerals. Each substitute gets the number of the main chain carbon atom to which it is connected. The sequence of numbers is selected in the way that the sum of substitutes' numbers is minimal. This ruule is also applied in monocyclic compounds numbering.

  3. The rule of radicals.

    All hydrocarbon side groups are considered as univalent (single bond) radicals. If a side radical has his own side chains, then the rule described above is used to choose an additional main chain. This chain is numbered beginning with a carbon atom connected to the main chain.

  4. Alphabetic order rule.

    The name of the compound begins with the list of substitutes in alphabetic order. The number of teach substitute is preceded by its number in the main chain. If there are several substitutes the prefixes di-, tri-, tetra-, etc. are used. Then goes a hydrocarbon corresponding to the main chain.

Table 12.1 contains the names of the first five hydrocarbons, their radicals, possible isomers and the corresponding formulas. The names of the radicals end with suffix -yl.

Formula Name
hydrocarbon radical hydrocarbon radical
    methane methyl
    ethane ethyl

propane propyl







n-pentane n-pentyl
Table 12.1.
Alkanes of acyclopean series CnH2n+2.

Example. Name all hexane isomers.

  1. n-hexane
  2. 2-methylpentane
  3. 3-methylpentane
  4. 2,3-dimethylbutane
  5. 2,2-dimethylbutane

Example. Name the alkane of the following structure


In this example from two 12-atom chains we chose the one with the least sum of numbers (rule 2).

Using branchy radicals from table 12.2,

Radical Name Radical Name
  isopropyl   isopentyl
  isobutyl   neopentyl
  di-butyl   tri-pentyl
  tri-butyl   isohexyl
Table 12.2.
Branchy radicals' names.

the name of the alkane is somewhat simplier:


When closing the hydrocarbon chain with the loss of two hydrogen atoms, monocycloalkanes are formed with the common formula of CnH2n. Cyclization begins with C3, the names are generated from Cn with "cyclo-" prefix:

cyclopropane,         cyclobutane,         cyclopentane,         cyclohexane,         cyclohexadecane

Polycyclic alkanes.Their names are formed using prefixes bicyclo-, tricyclo-, etc. Bicyclic and tricyclic compounds include two and three molecule cycles respectively. To describe their structure, the number of carbon atoms in every chain connecting knot atoms is indicated in brackets in descending order. Atom names are below ther formulas:

bicyclo-2,2,0-hexane         bicyclo-2,2,1-heptane         bicyclo-2,2,2-heptane         tricyclo-1,1,1-decane

This tricyclic hydrocarbon is usually refered to as adamantane (from "adamant" - diamond). It has a combination of three condenced cyclohexane rings, whose position in the lattice is similar to diamond.

Cyclic hydrocarbons with one common carbon atom are called spirans, for example spiro-5,5-unedecane:

Plain cyclic molecules are instable, therefore conformational isomers are formed. While in conformational isomers the position of atoms in the molecule doesn't depend on orientation, conformational isomers differ between each other by the rotation of atoms or radidcals about frmally single bonds. The structure of the molecule stays the same. The energy of generating a stable conformer is called conformational.

The conformers stay in dynamic equillibrium and turn into each other through instable forms. The instability of plain cycles is caused by the significant deformation of valency angles. Keeping tetrahedral valency angles there are two possible stable configurations for cyclohexane C6H12: the chair-shape (a) and the bath-shape (b):

Figure 12.5.