The second type of stereoisomerism is optical isomerism.

Syn- and anti- have similar meanings but are used when the groups are not identical or closely related. For example, there are two geometric isomers of 2-butene, CH 3CH=CHCH 3: 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. Geometric isomers differ in physical properties such as melting point and boiling point. 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. 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. Ethanol and dimethyl ether (see the example, above) are functional group isomers. 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).įunctional group isomers, on the other hand, have very different chemical properties because differences in their structure give rise to different functional groups.

For example, 1-propanol, CH 3CH 2CH 2OH, and 2-propanol, CH 3CH(OH)CH 3, are position isomers, as are 1-butene, CH 2=CHCH 2CH 3, and 2-butene, CH 3CH=CHCH 3. Position isomers occur among substituted alkanes and other compounds. In isobutane, CH 3CH(CH 3) 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 (CH 3) groups. In n-butane, CH 3CH 2CH 2CH 3, the carbon atoms are joined in a so-called straight, or unbranched, chain. For example, there are two chain isomers of butane, C 4H 10. Structural isomers are subdivided as chain, position, and functional group. Tautomers are structural isomers that readily convert from one isomeric form to another and therefore exist in equilibrium. 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. Ethanol and dimethyl ether are isomers because they differ in the way the atoms are joined together in their molecules: 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℃. One is ethanol (also called ethyl alcohol), CH 3CH 2OH, a colorless liquid alcohol the other is dimethyl ether, CH 3OCH 3, a colorless gaseous ether. For example, there are two compounds with the molecular formula C 2H 6O. Isomers have the same number of atoms of each element in them and the same atomic weight but differ in other properties. Pasteur's results were given theoretical basis by J. Early work with stereoisomers was carried out by Louis Pasteur, who separated racemic acid into its two optically active tartaric acid components by crystallization (1848). Isomerism is the occurrence of such compounds. Isomer (īˈsəmər), in chemistry, one of two or more compounds having the same molecular formula but different structures (arrangements of atoms in the molecule).

Asymmetric carbon atom