Molecular Formulae of Organic Molecules

From school-level chemistry upwards students and scientists use formulae to represent chemicals (substances). Molecules are the chemical building-blocks of compounds. In many cases, and especially when representing inorganic compounds or ionic solids or solutions, this is straighforward.

Organic molecules may be more complicated than inorganic molecules, hence there are several different types of formulae of organic molecules.

The simplest type of formula is the molecular formula, which also provides the least information about the molecule.

What do Molecular Formulae look like ?

Molecular formulae will be familiar to most students from the way they are used to representing simple chemical compounds in school-level inorganic chemistry.

A molecular formula just states the numbers of each type of atom present in a molecule, without providing any information about the way those atoms are arranged or joined together.

Examples of molecular formulae of organic compounds:

However, because they do not include details of the bonding within the molecule, molecular formulae are rarely sufficient in organic chemistry.

When molecular structures are used in organic chemistry it tends to be to describe the reactions of relatively simple molecules, e.g. combustion of hydrocarbons.

Where the purpose is obviously to describe a simple reaction rather than to consider the organic molecule itself in any detail.

Information omitted (not included) in Molecular Formulae of Organic Molecules:

When comparing the different types of formulae used to describe organic molecules it is useful to be able to list the information missing from the molecular formulae of organic compounds. This includes:

  • How the atoms are arranged, e.g. in approx. straight lines or in circles or 'rings'.
  • Which atoms are connected to which other atoms - as there may be several possible arrangements.
  • Which type of bonds are present in the molecule,
    e.g. C4H8 is the molecular formula of both butene (a linear alkene that includes one double bond), and cyclobutane (a cycloalkane that does not include any double or triple bonds, but only single bonds).
  • Information about branches
    e.g. C5H12 is the molecular formula of the alkanes pentane (no branches), methylbutane (one branch), and dimethylpropane (2 branches).
  • 3D Structure - which can be shown in various ways by use of diagrams (sketches of 3D molecules), physical models and computer software

General (Molecular) Formulae for types, or 'groups' of organic compounds

Descriptions of individual organic molecules by their molecular formulae alone is often insufficient because the same molecular formula applies to several different molecules.

General Molecular Formulae of groups of similar organic moleclues, which are often called "homolgous series of [name of group of compounds]", e.g. the homologous series of linear alkynes, are useful.

Some such general formulae are listed in the following table with links to pages about each type of series.

Name of Series

General chemical formula*


Alkanes

CnH2n+2

Alkenes

CnH2n

Alkynes

CnH2n-2

1-bromoalkanes
(e.g. of Haloalkanes)

CnH2n+1Br

Cycloalkanes

CnH2n

1-alcohols
(e.g. of Alcohols)

CnH2n+1OH = CnH2n+2O

Aldehydes

CnH2nO

Carboxylic Acids

CnH2nO2

Acid Chlorides

CnH2n-1OCl

Amines

CnH2n+1NH2 = CnH2n+3N

Amides

CnH2n-1ONH2 = CnH2n+1ON

Nitriles

CnH2n-3N

The formulae in the table above describe the relationship between members of the series and, when 'n' is subsituted for a specific value, they also describe the numbers of each type of atom in specific members of the series,

e.g. in the case of the alkane butane n = 4, so each molecule of butane includes:

  • 4 atoms of carbon,
  • and ( 2 x 4 ) + 2 = 10 atoms of hydrogen.

To read more about groups, or 'series' of organic compounds see the page about homologous series.

Other Types of Formulae used to describe Organic Molecules include fully displayed formulae, simplified displayed formulae (also known as semi-displayed formulae), sketched structural formulae, 3D Models of organic molecules, and skeletal formulae of organic molecules.

See also How to draw Skeletal Formulae of Organic Molecules.

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