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Isomers
In chemistry, isomers are molecules with the same chemical formula and often with the same kinds of chemical bonds between atoms, but in which the atoms are arranged differently (analogous to a chemical anagram). more...
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Many isomers share similar if not identical properties in most chemical contexts. This should not be confused with a nuclear isomer, which involves a nucleus at different states of excitement.
A simple example of isomerism is given by propanol: it has the formula C3H8O (or C3H7OH) and two isomers propan-1-ol (n-propyl alcohol; I) and propan-2-ol (isopropyl alcohol; II)
Note that the position of the oxygen atom differs between the two: it is attached to an end carbon in the first isomer, and to the center carbon in the second. The number of possible isomers increases rapidly as the number of atoms increases; for example the next largest alcohol, named butanol (C4H10O), has four different structural isomers.
In the example above it should also be noted that in both isomers all the bonds are single bonds; there is no type of bond that appears in one isomer and not in the other. Also the number of bonds is the same. From the structures of the two molecules it could be deduced that their chemical stabilities are liable to be identical or nearly so.
There is, however, another isomer of C3H8O which has significantly different properties: methoxyethane (III). Notice that unlike the top two examples, the oxygen is connected to two carbons rather than to one carbon and one hydrogen. As it lacks a hydroxyl group, the above molecule is no longer considered an alcohol but is classified as an ether, and has chemical properties more similar to other ethers than to either of the above alcohol isomers.
Another example of isomers having very different properties can be found in certain xanthines. Theobromine is found in chocolate, but if one of the two methyl groups is moved to a different position on the two-ring core, the isomer is theophylline, used as a bronchodilator.
Allene and propyne are examples of isomers containing different bond types. Allene contains two double bonds, while propyne contains one triple bond.
Read more at Wikipedia.org
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