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Gilbert Newton Lewis was one of the great chemists in history. His greatest discovery may well be the theory of the covalent bond in 1916, but he made many other contributions. One was his theory of acids and bases. In 1923, he wrote:
"We are so habituated to the use of water as a solvent, and our data are so frequently limited to those obtained in aqueous solutions, that we frequently define an acid or a base as a substance whose aqueous solution gives, respectively, a higher concentration of hydrogen ion or of hydroxide ion than that furnished by pure water. This is a very one sided definition . . . ."
What Lewis wanted was a general definition of an acid and a base, one that was universal no matter what the chemical environment. He know that the current ideas (the Arrhenius theory - dating from the late 1800's and the Bronsted-Lowry theory - announced in January 1923) were not sufficient. In the next 2 pages of discussion, after the above quote, Lewis wrote:
"When we discuss aqueous solutions of substances which do not contain hydroxyl [ion], it is simplest to define a base as a substance which adds hydrogen ion. . . . Since hydrogen is a constituent of most of our electrolytic solvents, the definition of an acid or base as a substance which gives up or takes up hydrogen ion would be more general than the one we used before, but it would not be universal."
Lewis then gives his definition of an acid and a base:
"We are inclined to think of substances as possessing acid or basic properties, without having a particular solvent in mind. It seems to me that with complete generality we may say that a basic substance is one which has a lone pair of electrons which may be used to complete the stable group of another atom, and that an acid is one which can employ a lone pair from another molecule in completing the stable group of one of its own atoms."
By the way, the italicized words are Lewis.' He finished the above paragraph with one more sentence, a restatement of what he just said:
"In other words, the basic substance furnishes a pair of electrons for a chemical bond, the acid substance accepts such a pair."
It is important to make two points here:
1) NO hydrogen ion need be involved.
2) NO solvent need be involved.
The Lewis theory of acids and bases is more general than the "one sided" nature of the Bronsted-Lowry theory. Keep in mind that Bronsted-Lowry, which defines an acid as a proton donor and a base as a proton acceptor, REQUIRES the presence of a solvent, specifically a protic solvent, of which water is the usual example. Since almost all chemistry is done in water, the fact that this limits the Bronsted-Lowry definition is of little practical consequence.
The Lewis definitions of acid and base do not have the constraints that the Bronsted-Lowry theory does and, as we shall see, many more reactions were seen to be acid base in nature using the Lewis definition than when using the Bronsted-Lowry definitions.
II. The Acid Base Theory
The modern way to define a Lewis acid and base is a bit more concise than above:
A "Lewis acid" is any atom, ion, or molecule which can accept electrons and a "Lewis base" is any atom, ion, or molecule capable of donating electrons. However, a warning: many textbooks will say "electron pair" where I have only written "electron." The truth is that it sometimes is an electron pair and sometimes it is not.
It turns out that it may be more accurate to say that "Lewis acids" are substances which are electron-deficient (or low electron density) and "Lewis bases" are substances which are electron-rich (or high electron density).
Several categories of substances can be considered Lewis acids:
1) positive ions
2) having less than a full octet in the valence shell
3) polar double bonds (one end)
4) expandable valence shells
Several categories of substances can be considered Lewis bases:
1) negative ions
2) one of more unshared pairs in the valence shell
3) polar double bonds (the other end)
4) the presence of a double bond
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