Isomers Role in Chemistry

 Isomers: The Importance of Structural Formulas

 The structural theory allowed early organic chemists to begin to solve a fundamental problem that plagued them: the problem of isomerism. These chemists frequently found examples of different compounds that have the same molecular formula. Such compounds are called isomers. Let us consider an example involving two compounds that have practical uses: acetone, used in nail polish remover and as a paint solvent, and propylene oxide, used with seaweed extracts to make food-grade thickeners and foam stabilizers for beer (among other applications).

Both of these compounds have the molecular formula C3H6O and therefore the same molecular weight. Yet acetone and propylene oxide have distinctly different boiling points and chemical reactivity that, as a result, lend themselves to distinctly different practical applications. Their shared molecular formula simply gives us no basis for understanding the differences between them. We must, therefore, move to a consideration of their structural formulas.

On examining the structures of acetone and propylene oxide several key aspects are clearly different . Acetone contains a double bond between the oxygen atom and the central carbon atom. Propylene oxide does not contain a double bond, but has three atoms joined in a ring. The connectivity of the atoms is clearly different in acetone and propylene oxide. Their structures have the same molecular formula but a different constitution. We call such compounds constitutional isomers.*   Constitutional isomers are different compounds that have the same molecular formula but differ in the sequence in which their atoms are bonded, that is, in their connectivity.   Constitutional isomers usually have different physical properties (e.g., melting point, boiling point, and density) and different chemical properties (reactivity).

The Tetrahedral Shape of Methane In 1874, the structural formulas originated by Kekulé, Couper, and Butlerov were expanded into three dimensions by the independent work of J. H. van’t Hoff and J. A. Le Bel. van’t Hoff and Le Bel proposed that the four bonds of the carbon atom in methane, for example, are arranged in such a way that they would point toward the corners of a regular tetrahedron, the carbon atom being placed at its center. The necessity for knowing the arrangement of the atoms in space, taken together with an understanding of the order in which they are connected, is central to an understanding of organic chemistry.

The first explanations of the nature of chemical bonds were advanced by G. N. Lewis (of the University of California, Berkeley) and W. Kössel (of the University of Munich) in 1916. Two major types of chemical bonds were proposed:

 1. Ionic (or electrovalent) bonds are formed by the transfer of one or more electrons from one atom to another to create ions. 2. Covalent bonds result when atoms share electrons. The central idea in their work on bonding is that atoms without the electronic configuration of a noble gas generally react to produce such a configuration because these configurations are known to be highly stable. For all of the noble gases except helium, this means achieving an octet of electrons in the valence shell.

 2.  The tendency for an atom to achieve a configuration where its valence shell contains eight electrons is called the octet rule. The concepts and explanations that arise from the original propositions of Lewis and Kössel are satisfactory for explanations of many of the problems we deal with in organic chemistry today. For this reason we shall review these two types of bonds in more modern terms.

Post a Comment