American Heritage® Dictionary of the English Language, Fourth Edition
- n. The phenomenon of the existence of isomers.
- n. The complex of chemical and physical phenomena characteristic of or attributable to isomers.
- n. The state or condition of being an isomer.
Century Dictionary and Cyclopedia
- n. In chem., identity or close similarity of composition and molecular weight, with difference of physical or of both chemical and physical properties. There are three different cases of isomerism: first, where compound bodies have the same ultimate composition and the same molecular weight, but differ in physical properties and in their behavior toward the same reagents, being essentially distinct substances; second, where compounds have the same composition, the same molecular weight, and the same general reactions, but differ in certain physical or chemical properties; third, where compounds differ solely in certain physical properties. The facts of isomerism are generally explained by assuming a difference in the arrangement of the atoms which form the isomeric molecules.
- n. In the widest sense this term is applied to any two or more chemical compounds having the same percentage composition. Various degrees and kinds of isomerism are distinguished, the primary distinction being between chemical and physical isomerism. Chemical isomerism may be subdivided as follows: Polymerism, which is applied to compounds with the same percentage composition, but with molecular weights differing by some constant value: example, ethylene. C2H4, and butylene, C4H8, molecular weights 28 and 56 respectively. Polymerism may be either accidental or generic, according to whether the compounds belong to the same or different classes. Metamerism is applied to compounds with the same percentage composition and molecular weight, but containing homologous radicals in the molecule: examples are propylamine, C3H7NH2, ethylmethylamine, C2H5NHCH3, and trimethylamine, N(CH3)3. Chain or nucleus isomerism characterizes compounds which have the same number of similar atoms in the molecule, but in which the carbon atoms forming the nucleus are differently grouped: as, normal butane, CH3CH2CH2CH3, and isobutane, (CH3)3CH. Position or place isomerism exists in compounds which have similar nuclei but differ in the relative position of certain substituting atoms or radicals in this nucleus: as, primary or α-propyl chlorid, CH3CH2CH2Cl, and secondary or β-propyl chlorid, CH3CHClCH3. Structural isomerism applies to compounds which have the same percentage composition and molecular weight but differ in the arrangement of atoms in their molecules. Physical isomerism characterizes compounds which consist of varying aggregations of chemically similar molecules. It was first used of substances which are now termed geometrical, stereo-isomeric, or optically isomeric. Alloisomerism is a term occasionally applied to geometrically isomeric compounds one of which can pass into the other by the action of heat Dynamical isomerism is applied to compounds which have perfectly similar linkage and spatial arrangement of atoms, but which differ in energy content, that is. in atomic motion. (Tanatar, Ann. Chem. (Liebig), 273, 55.) Geometrical isomerism or stereo-isomerism marks compounds which have the same constitution, but which differ in the spatial arrangement of certain constituent radicals or atoms. Optical isomerism is applied to stereo-isomeric substances which differ in their action on a ray of polarized light: they either deflect it to the right (dextrorotatory), to the left (levorotatory), or are optically inactive (racemic or mesoform).
- n. chemistry the phenomenon of the existence of isomers - the existence of different substances with the same molecular formulae
- n. chemistry the interconversion of isomers
GNU Webster's 1913
- n. (Chem.) The state, quality, or relation, of two or more isomeric substances.
- n. the state of being an isomer; the complex of chemical and physical phenomena characteristic of isomers
“The word isomerism was coined by Berzelius to express this anomalous condition of things, which seemed to negative the most fundamental truths of chemistry.”
“This phenomenon is called isomerism, and the generally accepted explanation is that the atoms in isomeric molecules are differently arranged, or grouped, in space.”
“Sometimes, Mrs. Janner, it is important to remember that life is an isomerism in which the isomers change into one another with great ease so that they ordinarily exist together in equilibrium.”
“The practice of identifying a chemical compound only by its composing elements was the norm in chemistry, until the discovery of isomerism by Friedrich Woehler in 1827.”
“The phenomenon of isomerism is often regarded as a success for atomism.”
“It was there I got my introduction to optical isomerism and the tetrahedral carbon atom.”
“He gave to us the basic methods of analysis which made chemistry an exact science, and endowed us with the pregnant ideas of isomerism and catalysis.”
“Even in his earlier investigations into certain metal ammonias he was able to show that numerous cases of isomerism in complex cobalt and platinum compounds could only be satisfactorily explained by steric approach.”
“This is obviously important in such cases where we have to deal with chemical compounds of identical composition but of different structure, i.e. where isomerism is present.”
“There is no need for me to go into the familiar story of how isomerism was discovered; I should like to single out just one point.”
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