ISOMERISM

the existence of sets of two or more substances that have identical molecular formulas but different molecular structures or configurations, and hence different properties. The substances that make up these sets differ only in the arrangement of their component atoms and are called isomers. It is now generally accepted that the differences in the properties of isomers result from differences in the arrangements of atoms within the molecules, but at the time (1830) that isomerism was recognized and named (by the Swedish chemist Jns Jakob Berzelius), even the existence of molecules was not well established. The idea that atoms might unite in a given ratio but different patterns was quite novel; indeed, the investigation of isomerism contributed to the development of the concept that many substances are made up of particles whose composition is defined by not only the numbers and proportions of different atoms but also their arrangement. For the rest of the 19th century, the study of isomerism was practically a branch of organic chemistry. In 1893, however, the Swiss chemist Alfred Werner published his findings on the structures of the inorganic coordination compounds. These structures correctly account for the complex isomerism of covalent inorganic compounds. The two principal classes of isomers are constitutional (or structural) isomers and stereoisomers. Substances are said to be constitutional isomers if they have the same elemental composition and molecular weight but differ in the order in which the atoms are bonded. On the other hand, they are called stereoisomers if they have the same composition, molecular weight, and bonding pattern but differ in the spatial arrangements that their atoms can assume without breaking any covalent bonds. (At ordinary temperatures all polyatomic molecules constantly undergo internal vibration, torsion, and flection, but the differing shapes that result are not regarded as isomers.) A typical example of constitutional isomerism is provided by the two compounds ethyl alcohol and methyl ether, which both have the molecular formula C2H6O. At atmospheric pressure ethyl alcohol is a liquid that boils at 78 C (172 F) and is converted to ethyl iodide upon reaction with hydriodic acid. Methyl ether is a gas that liquefies at -24 C (-11 F) and is converted to methyl iodide by reaction with hydriodic acid. The molecular structure of ethyl alcohol is CH3-CH2-OH; that of methyl ether is CH3-O-CH3. Constitutional isomers that can be readily changed into each other are called tautomers. Typical is the ester ethyl acetoacetate, which under ordinary circumstances is a mixture of the so-called keto form (about 92 percent) and the enol form (about 8 percent). Two classes of stereoisomeric compounds are generally recognized: optical isomers and geometric isomers. Optical isomers occur in pairs that are nonsuperimposable mirror images of each other, like right- and left-handed gloves. They take their name from their effect of rotating the plane of polarization of a beam of polarized light that is directed through them: one member of each pair causes rotation in one direction, the other in the opposite direction. This phenomenon, called optical activity, arises from any of several kinds of imbalance in the three-dimensional structure of the molecule that cause it to have no point, line, or plane of symmetry. The optical activity of these compounds is the only physical property that differentiates them. The chemical properties of optical isomers also are identical, except in their interactions with other dissymmetric compounds. Geometric isomerism results from rigidity in the molecular structure; in organic compounds this rigidity most often is associated with a double bond or a ring of atoms. In most compounds in which two carbon atoms are linked by a double bond, each of these atoms is also linked by single bonds to two other atoms (or groups of atoms), which may differ from one anothersay, A and B. The two carbon atoms and the four atoms directly bonded to them are held in a plane by the double bond. There are therefore two different arrangements: one, called cis, in which the two A atoms are on the same side of the double bond, and one, called trans, in which the two A atoms are on opposite sides. In any such pair, the cis isomer differs from the trans both physically and chemically. The molecules of the simplest cyclic hydrocarbons consist of three or more methylene (CH2) groups joined in a ring. In cyclopropane, for example, there are three such groups; the three carbon atoms define a plane, and the two hydrogen atoms bonded to each of them lie on different sides of this plane. If the hydrogen atoms attached to two of the carbon atoms are replaced by other atomssay, Atwo different structures are possible. In one (cis), the two A atoms lie on the same side of the plane; in the other (trans), the A atoms lie on opposite sides. Cis-trans isomerism also occurs in two groups of inorganic compounds. In coordination compounds such as dichlorodiammineplatinum(II), PtCl2(NH3)2, in which a metal atom is surrounded by two pairs of covalently bonded atoms or groups of atoms (ligands) that are situated at the vertices of a square, the ligands of one pairsay, the two chlorine atomsoccupy adjacent vertices in the cis isomer and diagonally opposite vertices in the trans isomer. If the metal atom is linked to six ligands, two of one kind and four of another, located at the vertices of an octahedron, the ligands of the pair occupy adjacent vertices in the cis isomer and opposite vertices in the trans isomer.