Structural formula

The structural formula of a chemical compound is a graphical representation of the molecular structure showing how the atoms are arranged. The chemical bonding within the molecule is also shown, either explicitly or implicitly. There are three common representations used in publications, condensed, Lewis type and line-angle formulæ. There are also several formats used for structural representation in chemical databases, such as SMILES, InChI and CML.

Unlike molecular formulae or chemical names, structural formulae provide a very powerful representation of the molecular structure. Chemists nearly always describe a chemical reaction or synthesis using structural formulæ rather than chemical names, because the structural formulæ allow the chemist to visualize the changes that occur.

Many chemical compounds can exist in different isomeric forms which have different structures yet the same overall chemical formula. A structural formula indicates the arrangements of atoms in a way that a chemical formula cannot. A simple example of this may be seen with the hydrocarbon butane, which has the molecular formula C4H10. The four carbons may be arranged in a linear pattern, or in a branched, "T" pattern. The first arrangement is known as orthobutane or n-butane, while the second is isobutane (For further simplification the skeletal formula is used). Their structural formulæ are shown at right:

Common types of structural formula
There are three main types of structural formula in widespread use in the chemical literature.

Condensed formulae
In early organic chemistry publications, where use of graphics was severely limited, a text-based system arose to describe organic structures in a line of text. Although this system tends to break down with complex cyclic compounds such as strychnine, it remains a convenient way to represent simple structures such as ethanol (CH3CH2OH).

Skeletal formulae
Note that for organic compounds, line drawings of structural formula are assumed to have carbon atoms at the vertices and termini of all line segments not marked with the atomic symbol of an element (other than carbon). Each carbon atom is in turn assumed to bear enough hydrogen atoms to give the carbon atom four bonds. Equivalent full and abbreviated forms are shown in the adjacent figures.

A chemical structure can be precisely, uniquely and unambiguously described using IUPAC nomenclature. In the case of isobutane, the IUPAC systematic name is methylpropane.

Multiple planes
When substituents of a molecule exist in different planes, their position can be described using solid and dotted wedges, with the former showing a substituent coming out of the plane, and the latter going into it. This system is useful in describing differences between chiral molecules.