25.3. Organic Reactions http://www.ck12.org
FIGURE 25.29
Amide synthesis.We can see from the diagram that the –OH of the carboxyl group and a hydrogen from the amine form water as a
byproduct. The nitrogen will attach to the carboxyl carbon in the same way the oxygen atom of the alcohol did in the
ester synthesis. The reaction shown in theFigure25.29 uses a primary amine, which has two hydrogen atoms that
could potentially be removed to form the final product. The same process can also occur with a secondary amine,
which still has one hydrogen attached to the nitrogen. Tertiary amines will not form amides, because the new bond
to the carbonyl carbon must replace an existing N-H bond.
The reverse reaction (amide hydrolysis) is very useful in the study of protein structure. Proteins are long chains
of amino acids (each amino acid contains an amine group and a carboxyl group, both attached to a central carbon
atom). The amino acids are linked together by amide bonds to form the long protein chain. One of the techniques
for looking at protein structure is to break those amide linkages so we can learn the identity of the amino acids in
the chain. We do this through a hydrolysis reaction:
FIGURE 25.30
Hydrolysis of an amide linkage.Usually, an acid such as HCl is used for the hydrolysis. The amino acids can then be separated and identified. More
complex reaction conditions allow the amino acids to be broken off one by one, allowing the amino acid sequence
of an unknown protein to be determined. The three-dimensional structures and functions of large protein molecules
are ultimately determined by the sequence of amino acids from which they are constructed.
Organic Polymers
What Are Polymers?
Polymers are a pervasive part of modern life. It is very likely that at least some part of your clothing is made of
nylon, rayon, or polyester. The milk or juice that you have for breakfast often comes in a polyethylene container. If
you don’t have breakfast at home, you might get coffee from a fast food establishment, where your order is delivered
to you in a Styrofoam® container. While getting out of your car, you bang the door on another vehicle. One reason
you may not have dented the door is the fact that it could be made out of a polymeric plastic material that resists
deformation more than simple metallic structures. Elsewhere on your car, both synthetic and natural rubber are
probably present in the tires. All these materials are examples of organic polymers.
Polymersare long-chain organic molecules made up of many smaller subunits (calledmonomers) that are connected
together in a repeating pattern. Some polymers are constructed by living organisms, such as starch, cellulose,
proteins, and DNA. These natural polymers are covered in the following chapter onBiochemistry. In this section,
we will focus on synthetic polymers that are made in the laboratory.