Chapter 16 Text

16.11 Lewis Acids and Bases 649
Chemistry and Life The Amphoteric Behavior of Amino Acids
Artrino acids are the building blocks of proteins. The general
structure of amino acids is shown here, where different amino
acids have different R groups attached to the central carbon
atom.
H R :O:
I II ..
H—N—C—C—O—H
I
H
Amine group Carboxyl group
(basic) (acidic)
For example, in glycine, which is the simplest amino acid, R is
a hydrogen atom, whereas in alanine R is a CHs group.
H
H,N—C—COOH
H
Glycine
CHa
H,N—C—COOH
H
Alanine
Amino acids contain a carboxyl group and can therefore
serve as acids. They also contain an NH2 group, characteristic
of amines (Section 16.7), and thus they can also act as bases.
Amino acids, therefore, are amphoteric. For glycine, we might
expect that the acid and base reactions with water would be as
follows:
Acid: H2N—CH2—COOH(ag) + H2O(1) ^=^
H2N—CH2—COCT(flg) + H3O+(fl (Section 8.2) Crystalline amino acids (Figure
16.14 T) have relatively high melting points, usually above
200°C, which is characteristic of ionic solids. Amino acids are far
more soluble in water than in nonpolar solvents. In addition, the
dipole moments of amino acids are large, consistent with a large
separation of charge in the molecule. Thus, the ability of amino
acids to act simultaneously as acids and bases has important
effects on their properties.
A Figure 16.14 Lysine, one of the amino acids found in
proteins, is available as a dietary supplement.
reactions, including those that do not involve proton transfer, as acid-base reactions.
To avoid confusion, a substance like BF3 is rarely called an acid unless it is
clear from the context that we are using the term in the sense of the Lewis definition.
Instead, substances that function as electron-pair acceptors are referred to
explicitly as "Lewis acids."
Lewis acids include molecules that, like BF3/ have an incomplete octet of
electrons. In addition, many simple cations can function as Lewis acids. For
example, Fe3+ interacts strongly with cyanide ions to form the ferricyanide ion,
Fe3+ + 6:C=N
3'
[Fe(C=N:)6]