Chapter 16 Text
Chapter 16 Text
Chapter 16 Text
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<strong>16</strong>.11 Lewis Acids and Bases 649<br />
Chemistry and Life The Amphoteric Behavior of Amino Acids<br />
Artrino acids are the building blocks of proteins. The general<br />
structure of amino acids is shown here, where different amino<br />
acids have different R groups attached to the central carbon<br />
atom.<br />
H R :O:<br />
I II ..<br />
H—N—C—C—O—H<br />
I<br />
H<br />
Amine group Carboxyl group<br />
(basic) (acidic)<br />
For example, in glycine, which is the simplest amino acid, R is<br />
a hydrogen atom, whereas in alanine R is a CHs group.<br />
H<br />
H,N—C—COOH<br />
H<br />
Glycine<br />
CHa<br />
H,N—C—COOH<br />
H<br />
Alanine<br />
Amino acids contain a carboxyl group and can therefore<br />
serve as acids. They also contain an NH2 group, characteristic<br />
of amines (Section <strong>16</strong>.7), and thus they can also act as bases.<br />
Amino acids, therefore, are amphoteric. For glycine, we might<br />
expect that the acid and base reactions with water would be as<br />
follows:<br />
Acid: H2N—CH2—COOH(ag) + H2O(1) ^=^<br />
H2N—CH2—COCT(flg) + H3O+(fl (Section 8.2) Crystalline amino acids (Figure<br />
<strong>16</strong>.14 T) have relatively high melting points, usually above<br />
200°C, which is characteristic of ionic solids. Amino acids are far<br />
more soluble in water than in nonpolar solvents. In addition, the<br />
dipole moments of amino acids are large, consistent with a large<br />
separation of charge in the molecule. Thus, the ability of amino<br />
acids to act simultaneously as acids and bases has important<br />
effects on their properties.<br />
A Figure <strong>16</strong>.14 Lysine, one of the amino acids found in<br />
proteins, is available as a dietary supplement.<br />
reactions, including those that do not involve proton transfer, as acid-base reactions.<br />
To avoid confusion, a substance like BF3 is rarely called an acid unless it is<br />
clear from the context that we are using the term in the sense of the Lewis definition.<br />
Instead, substances that function as electron-pair acceptors are referred to<br />
explicitly as "Lewis acids."<br />
Lewis acids include molecules that, like BF3/ have an incomplete octet of<br />
electrons. In addition, many simple cations can function as Lewis acids. For<br />
example, Fe3+ interacts strongly with cyanide ions to form the ferricyanide ion,<br />
Fe3+ + 6:C=N<br />
3'<br />
[Fe(C=N:)6]