Chemical structures

A chemical structure (or structural formula) can be used to check that a chemical is what the name says, or to identify different isomers which may have different chemical properties.

A chemical structure shows the layout of a molecule, including the position of the elements and type of bonding.  There is more than one convention for showing a chemical structure, and also specific conventions for some types of molecule.  A good summary is given at .

The structure of ethanol can be displayed in several ways:

Ethanol 2-D structure (click on image to enlarge it).

Ethanol 3-D structure (click on image to enlarge it).

Ethanol skeletal formula – all C atoms / ions are shown on a line, and H atoms / ions are not shown (click on image to enlarge it).

Ethanol skeletal formula used at ECHA – the end groups are shown, and the mid-point carbon atom/ ion is shown as a line, and its hydrogen atoms/ ions are not shown (click on image to enlarge it).

For REACH-registered substances, chemical structures are included in the REACH dossiers at ECHA.  The Classification and Labelling Inventory may show both the molecular formulae and structure of a substance (on the Infocard).

Chemical structures can be derived from IUPAC names, where given, and IUPAC names can be derived from chemical structures.

The CAS number can be used to view a chemical structure, either officially, through a subscription to the Chemical Abstracts Service, e.g. ; or through searching for the CAS number on the internet.  If the substance is hazardous, it may come up on a number of regulatory databases. such as the Pubchem database in the USA, .

Pubchem itself uses the chemical formula and drawn structures, and also several other structural descriptors, such as:

  • SMILES/SMARTS – Simplified Molecular Input Line Entry System, a chemical structure line notation (a typographical method using printable characters) for entering and representing molecules. SMARTS is SMILES ARbitrary Target Specification, a chemical structure query line notation which is a subset of SMILES
  • InChI – IUPAC International Chemical Identifier, a chemical structure line notation.
  • CID — PubChem Compound Identification, a non-zero integer PubChem accession identifier for a unique chemical structure.

For more details on the structure conventions used in PubChem, see .

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