Naming Alkanes

Definition of Alkanes:

Alkanes are hydrocarbons because they consist only of carbon and hydrogen atoms.

They combine in proportions according to the general formula for alkanes:

C_nH_2n+2

where the letter n represents the number of carbon atoms in each molecule of the compound.

The three main types of alkanes are linear alkanes, branched alkanes and cyclic alkanes (also called cycloalkanes). Linear alkanes are the simplest.

Linear Alkanes:

The first ten members of the homologous series of linear alkanes are shown below.

Why stop at ten carbon atoms (in the chain)? It doesn't. There are more similar linear alkanes.

*The synonyms indicated for compounds listed above are just some examples of alternative names found online and believed to have been used to refer to the substance indicated. They may include common (non-scientific) names and trade names specific to particular suppliers. The purpose of these lists is to give a general indication of the range of names by which alkanes are and have been known. Knowledge of these alternative names is not usually required for GCSE, A-Level Chemistry, or other school exams.

In the case of the names of alkanes beginning with n- , the n- part is included to specify the linear (as opposed to a branched or cyclic) form of that particular alkane. In some cases the atoms are arranged in different ways, hence the alkane can exist in the forms of several different structural isomers. Some examples of structural isomers are shown below.

Branched Alkanes

Alkane molecules that include more than 3 carbon atoms can be arranged in more than one way. The simplest example is butane, which can take either of two different forms called structural isomers:

The 2 Structural Isomers of Butane

It is useful to be aware of structural isomers of organic molecules as the result of exchanging the positions of certain atoms and / or groups because this concept applies to many different types of organic molecules, not just to alkanes.

Another note about these diagrams is that, because they are very simple representations of the arrangements of atoms within molecules, the lengths and thicknesses of the lines representing chemical bonds do not generally signify anything about the bonds they represent. Although it is a good idea to draw these lines as consistently as possible, it is not unusual to need to vary the lengths slightly in order to fit everything in to the space available.

The 3 Structural Isomers of Pentane

Pentane (C₅H₁₂) has 5 carbon atoms per molecule. In organic chemistry generally, the larger the molecule, the greater the number of different ways in which the atoms can be arranged. The 3 structural isomers of pentane are shown.

* In reality molecules occupy 3-dimensional space so 2-dimensional representations such as those above are simplifications. The type of representations shown here are sufficient to distinguish between linear- and the different types of branched- alkanes. The molecular diagrams above are intended to represent only which atoms are linked (bonded) with which other atoms and not to indicate the positions, e.g. angles, between atoms as they occur in nature. Such simplification is necessary in order to describe and explain increasingly complex organic structures.

As indicated in the table of linear alkanes further up this page, molecules of alkanes that have more carbon atoms per molecule can exist in many more different isomeric forms. Introductory courses, such as GCSE Chemistry, do not usually require detailed knowledge of all the isomers of the larger alkanes but it can be useful to work out and draw the molecular structures of some them to aid understanding and memory of the concept of structural isomerism.

More about alkanes generally (Revision Notes):

• Alkanes form a homologous series of organic compounds (meaning that all alkanes can be described by a single general chemical formula, specifically C_nH_2n+2 in the case of alkanes) in which the members of the series differ by a constant relative molecular mass of 14, which is the molecular mass of one carbon atom and two hydrogen atoms.
• Alkanes are saturated compounds, meaning that the atoms that form the molecules of alkane compounds are linked exclusively by single bonds.
• There is no limit to the number of carbon atoms that can be linked together in such a way as to form a linear alkane.
• The conditions that alkane molecules must satisfy are that they are:
  1. hydrocarbons (only contain atoms of carbon and hydrogen),
  2. saturated (only contain single bonds; so include no double- or triple- carbon bonds),
• Saturated oils and waxes are examples of larger alkanes in which the number of carbon atoms in the carbon chain, which is sometimes called a 'carbon backbone', is greater than 10.
• Alkanes are not very reactive and have little biological activity.
  However, alkanes are sometimes described in terms of being a 'molecular tree' i.e. a basic structure, onto which more complex and biologically active atoms or functional groups of atoms can be attached, the simplest example being halogenoalkanes also known as haloalkanes.

See also: Boiling Points of Alkanes

Note: This is one of many pages about types of organic compounds in our Organic Chemistry Section. Similar and related pages describe hydrocarbons, alkanes, haloalkanes, alkenes, alkynes, cycloalkanes, aromatic hydrocarbons (arenes), alcohols, esters, ethers, aldehydes, ketones, carboxylic acids, acid chlorides, acid anhydrides, azo dyes, amines, amides, nitriles, amino acids (chemistry), peptides, proteins (chemistry), polypeptides and others. If you need further information ask your chemistry tutor.