The Lewis Acid-Base Concept

 
 Gilbert Newton LewisGilbert Newton Lewis 

b. Oct. 23, 1875, Weymouth, Mass., U.S.
d. March 23, 1946, Berkeley, Calif.

American chemist whose theory of the electron pair fostered understanding of the covalent bond and extended the concept of acids and bases.

Lewis took his Ph.D. at Harva rd University (1899), studied at the universities of Leipzig and Göttingen, and entered research in thermodynamics at the Massachusetts Institute of Technology, Cambridge (1905). He became professor of physical chemistry and dean of the College of Chemistry at the University of California, Berkeley in 1912.

About 1916 Lewis began to advance the idea that a chemical bond could be formed by the sharing of valence electrons as well as by the transfer of electrons.

He pub lished his views in Valence and the Structure of Atoms and Molecules (1923). He also published, with Merle Randall, Thermodynamics and the Free Energy of Chemical Substances (1923), a textbook that became a classic work.
The first to isolate deuterium, an isotope of hydrogen, he prepared a pure sample of heavy water (deuterium oxide) in 1933. His later res
earches contributed to the understanding of fluorescence, phosphorescence, and colour in organic substances.
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A Lewis acid is an electron pair acceptor. A Lewis base is an electron pair donor. This definition is more general than those we have seen to this point; any Arrhenius acid or base, or any Brønsted-Lowry acid or base can also be viewed as a Lewis acid or base. The reaction of H 1+ with OH 1-, for instance, involves donation and acceptance of a proton, so it is certainly legitimate to call it a Brønsted-Lowry acid-base reaction. But if we look at the Lewis structures for the reactants and products, we see that it is also legitimate to call this a Lewis acid-base reaction.

 

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  blue=free orbitals | red=outer electrons | yellow= closed shell/orbitals

The hydroxide ion donates a pair of electrons for covalent bond formation, thus OH 1- is a Lewis base in this reaction. The hydrogen ion accepts the pair of electrons so it is acting as a Lewis acid. Shown below is an example of a Lewis acid-base reaction that cannot be viewed as a Brønsted-Lowry acid-base reaction.
 

[Picture]

 blue=free orbitals | red=outer electrons | yellow= closed shell/orbitals

 
The BF 3 is the Lewis acid and the N(CH 3) 3 is the Lewis base. Both of the electrons in the covalent bond formed by a Lewis acid-base reaction come from the same atom (in the above example, the nitrogen donates both electrons). Such bonds are called coordinate covalent bonds. If we want to emphasize the fact that a particular bond is a coordinate covalent bond, we draw an arrow instead of a line to represent the bond in the Lewis structure. The arrow points from the donor atom to the acceptor atom.
 

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APSIDIUM  © Created:2003-04-09    lewis.pdf
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