Allotropes are different structural modifications of an element whereas isomers are chemical compounds that share the same molecular formula but have different structural formulae.
Certain elements can exist in two or more different forms. These forms are called allotropes in which the element's atoms are bonded together in a different manner. For example, dioxygen (O2), ozone (O3), tetraoxygen (O4) and octaoxygen (O8) are allotropes of oxygen. Another example is carbon whose allotropes include graphite and diamond. In short, allotropes contain the same element (the same atoms) that bond together in different ways to produce different molecular structures.
In contrast, isomers are compounds (see Elements vs. Compounds) that share the same molecular formula but have different structural formulas. Isomers do not share their chemical properties unless they belong to the same functional group. For example, propanol has the formula C3H8O (or C3H7OH) and occurs as two isomers: propan-1-ol (n-propyl alcohol; I) and propan-2-ol (isopropyl alcohol; II). The difference between the two isomers lies in the position of the oxygen atom: it is attached to an end carbon in propan-1-ol, and to the center carbon in propan-2-ol. There is a third isomer of C3H8O whose properties are so different that it's not an alcohol (like propanol) but an ether. Called methoxyethane (methyl-ethyl-ether; III), this isomer has an oxygen connected to two carbons rather than to one carbon and one hydrogen.
History of allotropes and isomers
Both allotropy and isomerism were concepts proposed by the Swedish scientist Jöns Jakob Berzelius. He proposed the concept of allotropy in 1841. After the acceptance of Avogadro's hypothesis in 1860 it was understood that elements could exist as polyatomic molecules, and the two allotropes of oxygen were recognized as O2 and O3. In the early 20th century it was recognized that other cases such as carbon were due to differences in crystal structure.
Isomerism was first noticed in 1827, when Friedrich Woehler prepared cyanic acid and noted that although its elemental composition was identical to fulminic acid (prepared by Justus von Liebig the previous year), its properties were quite different. This finding challenged the prevailing chemical understanding of the time, which held that chemical compounds could be different only when they had different elemental compositions. After additional discoveries of the same sort were made, such as Woehler's 1828 discovery that urea had the same atomic composition as the chemically distinct ammonium cyanate, Jöns Jakob Berzelius introduced the term isomerism to describe the phenomenon.
Types of Isomers
Different classes of isomers include stereoisomers, enantiomers and geometrical isomers.
- Structural isomers - In structural isomers, the atoms and functional groups are joined together in different ways. Types of structural isomers include:
- chain isomerism - hydrocarbon chains have variable amounts of branching
- position isomerism - deals with the position of a functional group on a chain
- functional group isomerism - one functional group is split up into different ones.
- skeletal isomers - the main carbon chain is different between the two isomers.
- Tautomers - structural isomers of the same chemical substance that spontaneously interconvert with each other.
- Stereoisomers - In stereoisomers the bond structure is the same, but the geometrical positioning of atoms and functional groups in space is different. Types of stereoisomers include:
- enantiomers - different isomers are non-superimposable mirror-images of each other
- diastereomers - isomers are not mirro images of each other