Chemistry - A Molecular Science

Chapter 13 Organic Chemistry

310

POLYESTERS The reaction of an acid and an alcohol yields an ester, and the reaction of a

dia

cid and a

di

ol (an alcohol with two OH groups) results in the formation of a

polyester

. The

monomers used to make poly(ethylene tere

phthalate) (PET) are shown in Figure 13.33.

The mechanism of the reaction is identical to

that of esterification, except that each

reaction results in a molecule that still has re

active sites at each end. Successive reactions

at these terminal sites lengthen the chain. PET goes by the trade names

Mylar

®,

Dacron

®,

and

Terylene

®. The United States produces 4.5 billion pounds of PET per year. It is used

in clothing, upholstery, tires, and many other products. NYLONS The reaction of an ester with an amine results in an amide, and the reaction of a

diester

with a

diamine results in a

polyamide

. Polyamides formed in this type of reaction are also

known as

nylons

.

Nylon 66 gets its name from the fact that it is formed from the condensation of a six-

carbon diamine with a diester containing a six-carbon chain (Figure 13.34a). The amide that is formed (Figure 13.34b) has an amine group at one end of the molecule and an ester at the other end. These two reactive groups can each react with another acid or amine to produce a longer chain that still has reactive sit

es at both ends. This process continues, and

the chain grows with each reaction. POLYPEPTIDES AND PROTEINS An amino acid is both an amine and a carboxylic acid, and amino acids can condense in a manner very similar to that discussed for nylons. The resulting molecule is called a peptide.

Peptides

are simply amides that are formed from the reaction of amino acids.

Figure 13.35 shows the condensation of three amino acids: glycine + alanine + serine. The reaction of glycine and alanine results in a

peptide; addition of the serine forms a di

tri

peptide. This process continues to form a

poly

peptide and, eventu

ally, a protein.

Polypeptides

are polyamides that form by polymerization of amino acids.

Proteins

are high molecular weight polypeptides.

Proteins often contain 100 or more

amino acids. Although each amino acid has a stereocenter, only one of the two enantiomers is biologically active. It is

estimated that the human body contains over

100,000 different kinds of protein molecules! The structure of a polypeptide can be envisioned as a

backbone

of amide linkages off which hang

the various groups (referred to

as

side chains

) that characterize the individual ami

no acids. The backbone of the tripeptide

HO

OH

O OH

O HO

O O

O O

+

poly(ethylene terephthalate)

X

Figure 13.33 Polyester The reaction of a diol and a diacid results in a polyester.

N

(HC)^2

6

NH^2

H H

NH^2 (HC) 26 N

H

H

(CH

) (^24) CO
CH^3 O CO O CH^3
CO
HN (HC)^2
6 NH
2
(CH
) 24
(HC)^2
6
HN
NH CO
2
NH
O C
(CH
O C) 24
NH
(CH
) 26

• CH
  OH 3


• CH
  OH 3
  (c)
  x
  6 carbonchain
  6 carbonchain

...

(a)

(b)

Figure 13.34 Formation of nylon 66 a) Lone pairs on N atoms of two diamine molecules attack carbonyl carbons of one diester to form C-N bonds, while a proton transfer (shown in circles) from amine to oxygen of diester produces CH

OH, 3

which leaves to produce an amide.

b) The diamide that is formed

still has two amine groups that can attack two more diester molecules. c) The chain grows with each successive reaction. The product of this reaction is the nylon 66 polymer.

O C

CH^2

O

HN^3

C O

CH

O

HN^3

CH^3

CH^2

C

HN^3

O

NH

CH^3 C

C O

O

O C

CH

O

HN^3

CH^2 OH

CH^2

C

HN^3

O

NH

CH^3 CH

C O

HN

CHCH

O C (^2) OH
O
Glycine
Serine
adipeptide
a tripeptide

• CH^2
  C
  HN^3
  O
  NH
  CH^3 C
  C O
  O
  a dipeptide


• 
  

  Alanine
  Figure 13.35 Polypeptides The condensation of amino acids results in polypeptides. The red bonds in the tripeptide highlight the peptide ‘backbone’. Note that the side chains of the amino acids appear to hang off the backbone.
  © by
  North
  Carolina
  State
  University