asymmetric carbon atom
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Isomers have the same number of atoms of each element in them and the same atomic weight but differ in other properties. For example, there are two compounds with the molecular formula C2H6O. One is ethanol (also called ethyl alcohol), CH3CH2OH, a colorless liquid alcohol; the other is dimethyl ether, CH3OCH3, a colorless gaseous ether. Among their different properties, ethanol has a boiling point of 78.5℃ and a freezing point of −117℃; dimethyl ether has a boiling point of −25℃ and a freezing point of −138℃. Ethanol and dimethyl ether are isomers because they differ in the way the atoms are joined together in their molecules:
Isomers are classified as structural isomers, which have the same number of atoms of each element and molecular weight but different bonding patterns (see chemical bond), or as stereoisomers, which have the same number of atoms of each element, molecular weight, and bonding pattern but in which the atoms have different spatial relationships. Tautomers are structural isomers that readily convert from one isomeric form to another and therefore exist in equilibrium.
Structural isomers are subdivided as chain, position, and functional group. Chain isomers occur among the alkanes. For example, there are two chain isomers of butane, C4H10. In n-butane, CH3CH2CH2CH3, the carbon atoms are joined in a so-called straight, or unbranched, chain. In isobutane, CH3CH(CH3)2, the carbon atoms are joined in a branched chain; the isobutane molecule can be visualized as a carbon atom bonded to one hydrogen atom and to three methyl (CH3) groups.
Position isomers occur among substituted alkanes and other compounds. For example, 1-propanol, CH3CH2CH2OH, and 2-propanol, CH3CH(OH)CH3, are position isomers, as are 1-butene, CH2=CHCH2CH3, and 2-butene, CH3CH=CHCH3. Position isomers have similar chemical properties since they differ only in the location of the functional group (e.g., the OH in an alcohol or the double bond in an alkene).
Functional group isomers, on the other hand, have very different chemical properties because differences in their structure give rise to different functional groups. Ethanol and dimethyl ether (see the example, above) are functional group isomers.
Stereoisomerism occurs when two or more molecules have the same basic arrangement of atoms in their molecules but differ in the way the atoms are arranged in space. There are two types of stereoisomerism. The first type, geometric isomerism, may occur when a compound contains a double bond or some other feature that gives the molecule a certain amount of structural rigidity. Geometric isomers differ in physical properties such as melting point and boiling point. For example, there are two geometric isomers of 2-butene, CH3CH=CHCH3: The prefix cis- means “same side” and trans- means “opposite side”; they are used when the groups on either side of the double bond are identical or closely related, e.g., methyl and ethyl. Syn- and anti- have similar meanings but are used when the groups are not identical or closely related.
The second type of stereoisomerism is optical isomerism. When plane-polarized light is passed through an optical isomer it is rotated into a different plane of polarization. Optical isomers, also know as chiral molecules or enantiomers, exhibit this optical activity in varying degrees. Optical isomers of a given compound are often identical in all physical properties except the direction in which they rotate light. The molecules of optical isomers are asymmetrical. The simplest optical isomers have a single “asymmetrical carbon atom” in their molecules. An asymmetrical carbon atom has four different atoms or radicals bonded to it, arranged approximately at the corners of a tetrahedron centered on the carbon atom. For example, there are two optical isomers of lactic acid: The atom and radical to either side of the carbon atom are visualized as being above the plane of the paper, the central carbon atom in the plane of the paper, and the radicals above and below the central carbon atom below the plane of the paper. Thus it is seen that the two molecules are mirror images of each other and, each being asymmetrical, cannot be superposed on each other. The d- and l- prefixes stand for dextro (right) and levo (left). Two optical isomers, such as these, whose molecules are asymmetrical and are mirror images of each other, are called enantiomorphs. When equal amounts of d- and l-enantiomorphs are mixed, the mixture has no effect on polarized light; such a mixture is called racemic.
When there is more than one asymmetrical carbon atom, there may be more than two optical isomers. For example, tartaric acid has two asymmetrical carbon atoms and three optical isomers: The d- and l-tartaric acids are enantiomorphs; each molecule is asymmetrical and is the mirror image of the other. There are two asymmetrical carbon atoms in meso-tartaric acid, but the molecule is symmetrical and does not exhibit optical activity; the optical activity is internally compensated, the effect of one asymmetrical carbon atom balancing the effect of the other. A pair of optical isomers such as d-tartaric acid and meso-tartaric acid, which are not enantiomorphs, are called diastereoisomers. Molecular disymmetry in optical isomers may come from some source other than an asymmetrical carbon atom, e.g., structural rigidity resulting from double bonds or ring structures within a molecule.
Stereoisomers are important in metabolism; in many cases only one of several isomeric forms of a compound can take part in biochemical reactions. For example, there are 16 stereoisomers of a simple sugar whose molecular formula is C6H12O4. Of these, only d-glucose is readily utilized in human metabolism.