The term isomer (meaning same member) refers to two molecules that have the same chemical formula, but which differ according to the spacial arrangement of their atoms. Molecules that are isomers have the same number and kind of atoms, but they differ in respect to the arrangement or configuration of their atoms. For example, dimethyl ether and ethyl alcohol have the formula C2H6O, but the two compounds have quite different properties. The term isomer was first used by the German chemist Jons Jakob Berzelius. Much of what is known about this class of compounds came as a result of work done by Friedrich Wohler and Justus Liebig in the early 1820s.
Isomerism can occur in one of several forms depending on the structure, position, and geometry of the chemical bonds. Chain isomerism exists among the alkanes, which consist of carbon atom chains. Because these chains may be straight or branched, they may yield different possible configurations. For example, normal butane (CH3CH2CH2CH3 is a straight chain, and isobutane CH-(CH3)3 is branched. Position isomerism occurs because not all hydrogen atoms in a molecule are identical. When these hydrogens are replaced by other elements, alternate isomeric structures can be created. Therefore, propanol (C 3H7OH) occurs in two forms: one has the hydroxyl group attached to a terminal carbon atom and the other has it on the middle carbon. Functional group isomerism applies to compounds that cannot be fully identified solely by reference to their chemical formula. For example, the formula C 3 H 8 O may be an ether or an alcohol. The final type of isomerism, geometric isomerism, refers to molecules in which the atoms are attached in the same order, but which have different spatial relationships. One type of geometric isomerism, called cis-trans isomerism, refers to asymmetry across the carbon-carbon double bond. For example, the four carbon molecule known as 2-butene can exist as either the cis isomer or trans isomer. In the cis form, the two methyl groups are on the same side of the double bond. In the trans form, the two methyl groups are on opposite sides. Each isomer is chemically identical but they have different physical properties, such as melting, boiling point, and density.
In 1848, the French scientist Louis Pasteur discovered that certain geometric isomers of tartaric acid behaved differently toward light. He manually separated the two isomers by handpicking differently shaped crystals from a mixture. Pasteur determined that these two forms, although identical in many respects, behaved differently toward polarized light. He found that one of these isomers would rotate a beam of polarized light counterclockwise. He called this form the levorotatory or (L) form. He found the other isomer would rotate the beam clockwise and he called this the dextrorotatory or (D) form. Interestingly, he found that only one of these forms could be used as a medium to culture molds.
Van't Hoff expanded on Pasteur's work by recognizing that the presence of an asymmetric center in a molecule would give rise to two isomeric molecules called enantiomers, one of which is the exact mirror image of the other. Enantiamers, also called optical isomers, are studied in the specialized branch of chemistry known as stereochemistry. These isomers are important in biology because on a molecular level nearly all biochemical processes involve the spatial recognition of one molecule by another. Such recognition serves as the means by which molecules interact and determines how complex biological structures are built.
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