The Lewis Acid-Base Concept
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
Bronsted-Lowry acid or base
can also be viewed as a Lewis acid or base. The reaction of H1+ with OH1-,
for instance, involves donation and acceptance of a proton, so it is certainly legitimate
to call it a Bronsted-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
The hydroxide ion donates a pair of electrons for covalent bond formation, thus OH1- 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 Bronsted-Lowry acid-base reaction.
The BF3 is the Lewis acid and the N(CH3)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.