Isomer is the term used to describe two or more chemical compounds which can be represented by the same chemical formula. There are two main types of isomers: structural isomers which differ from one another by the attachment of atoms on the molecule; and stereoisomers which differ from on another by the location of the atoms in space.
Chemical compounds can be represented by a formula which qualitatively and quantitatively describes its component elements. For example, the formula for water is H2O, which indicates that the compound contains two hydrogen atoms attached to one oxygen atom. In the early 1800s two chemists, Friedreich Wohler and Justus Liebig, realized that two chemical compounds might have the same elemental composition yet differ in the order in which the atoms were linked together. Therefore, it is possible that a given chemical formula may describe more than one compound. For example, propyl alcohol and isopropyl alcohol both are represented by the same formula ([CH3]2CHOH), but they are different compounds with different properties depending on whether the alcohol group (also known as the hydroxyl group) is located on a terminal (end) carbon atom or on the middle carbon atom. This form of isomerism is known as positional, or structural, isomerism. Positional isomerism occurs because the various sites where groups are attached are not equivalent. This principle is demonstrated by the molecule known as benzene which consists six carbon atoms arranged in a ring. These carbon atoms provide benzene with six different positions where other chemical groups can be substituted for hydrogen atoms. A substituted benzene ring, such as toluene, can accept another substituent on any of the other five carbon atoms; but because two pairs are equivalent, there are only three possible isomers. These are designated as ortho, meta, and para.
Chain isomerism, another type of structural isomerism, occurs among chemical compounds known as alkanes, which consist of chains of carbon atoms. These carbon atom chains can be configured as either a straight or branched chain with exactly the same overall chemical formula. These different structural configurations are isomers of each other. Although the properties of isomers of a given formula are similar, the compounds are nonetheless distinct. Similarly, the location of the double bond in alkenes and the triple bond in alkynes determines another form of positional isomerism.
Isomers may also be stereoisomers which differ from one another by spacial position of their atoms. There are two subcategories of stereoisomers, geometric isomers and optical isomers. Both geometric and optical isomers occur in molecules in which the atoms are attached in the same order but have different spatial relationships. For example, picture a chain of carbon atoms which has two hydroxyl groups attached to the first two positions. Furthermore, assume there is a double bond present between the 1 and 2 carbons. The hydroxyl groups can be attached to the same side of this double bond, (e.g., both on the "top" or both on the "bottom"). Or they may be oriented on opposite sides such that one resides on top and the other on the bottom. If the groups are aligned on the same side of the double bond the compound is said to be a cis isomer (from the Latin "on this side"); if they are on opposite sides it is trans (from Latin "across.") In certain compounds, the atoms are free to rotate about this double bond, which gives rise to multiple isomeric configurations. If the mirror images of these different configurations are superimposable, the isomers are said to be geometric. If the mirror images are nonsuperimposable they are said to be optical. Optical isomers are distinguishable by the way they interact with a beam of polarized light. Such isomers are the subject of the branch of chemistry known as stereochemistry.
Stereochemistry is the study of the spatial arrangement of atoms in molecules and the effect of their orientation on the physical and chemical properties of those compounds. However, the three dimensional nature of this spacial orientation was not really understood until 1871 when two independent chemists, Hendricus van't Hoff and Joseph Achille le Bel, proposed their theories of stereoisomerism, or how isomers are structured.
The study of isomers provides information which is useful in improving the efficiency of the reactions or in the search for new types of reactions or chemical species. Through evaluation of isomeric compounds, chemists gain useful information about chemical reactions and learn how certain bonds are broken and formed or what kinds of intermediates are involved.