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9C Focus on Health & Medicine: Olestra, a Synthetic Fat

9C Focus on Health & Medicine: Olestra, a Synthetic Fat

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528



CARBOXYLIC ACIDS, ESTERS, AND AMIDES



Olestra has many ester groups formed from long-chain carboxylic acids and sucrose, the sweettasting carbohydrate in table sugar. Olestra has many properties similar to the triacylglycerols in

fats and oils. In one way, however, olestra is different. Olestra has so many ester units clustered

together that they are too crowded to be hydrolyzed. As a result, olestra is not metabolized nor is it

absorbed. Instead, it passes through the body unchanged, providing no calories to the consumer.



PROBLEM 17.34



What products are formed when the following triacylglycerol is hydrolyzed with water and

H2SO4?

O



CONSUMER NOTE



17.10



CH2



O



C

O



(CH2)10CH3



CH



O



C

O



(CH2)12CH3



CH2



O



C



(CH2)14CH3



SYNTHETIC POLYMERS IN MODERN SOCIETY—

POLYAMIDES AND POLYESTERS



All natural and synthetic fibers are polymers. Natural fibers are obtained from either plant or

animal sources, and this determines the fundamental nature of their chemical structure. Fibers

like wool and silk obtained from animals are proteins, and so they are joined together by many

amide linkages.

O

CH



C



R



DuPont built the first commercial

nylon plant in 1938. Although it was

initially used by the military to make

parachutes, nylon quickly replaced

silk in many common clothing

articles after World War II.



CONSUMER NOTE



O

N

H



CH



C



R



O

N

H



CH



C



N

H



R



wool and silk—polymers with many amide bonds

(labeled in red)



An important practical application of organic chemistry has been the synthesis of synthetic

fibers, many of which have properties that are different from and sometimes superior to their

naturally occurring counterparts. Two common classes of synthetic polymers are polyamides

and polyesters.



17.10A



NYLON—A POLYAMIDE



The search for a synthetic fiber with properties similar to silk led to the discovery of nylon, a

polyamide. There are several different kinds of nylon, but the best known is nylon 6,6.

O

N

H



(CH2)6



N

H



C



O

(CH2)4



C



O

N

H



(CH2)6



N

H



C



O

(CH2)4



C



nylon 6,6

[The amide bonds are labeled in red.]



Nylon is used in a wide variety of

consumer products, including rope,

fabric, backpacks, hot air balloons,

and parachutes. Nylon sutures are

used in medicine.



smi26573_ch17.indd 528



To synthesize nylon 6,6, two monomers are needed. Each monomer has six carbons (hence

its name) and two functional groups. One monomer is a dicarboxylic acid, adipic acid

[HO2C(CH2)4CO2H], and the second monomer is a diamine, hexamethylenediamine

[H2N(CH2)6NH2]. Heating the two monomers together at high temperature forms the amide

bonds of the long polymer chain. When each amide is formed, a molecule of H2O is lost from

the starting materials. Nylon is an example of a condensation polymer.



12/16/08 9:57:43 AM



SYNTHETIC POLYMERS IN MODERN SOCIETY—POLYAMIDES AND POLYESTERS



Polymers were first discussed in

detail in Section 13.8.



529



Each NH2 and COOH react.

O

H2N



(CH2)6



NH2



+



HO



O



C



(CH2)4



O



C



+



OH



H2N



(CH2)6



NH2



+



HO



C



O

(CH2)4



C



OH



CONSUMER NOTE

loss of H2O



loss of H2O



loss of H2O





O

H

N



H

N



(CH2)6



C



O

(CH2)4



C



O

H

N



H

N



(CH2)6



C



O

(CH2)4



C



new amide bonds

nylon 6,6



• A condensation polymer is a polymer formed when monomers containing two functional

groups come together with loss of a small molecule such as water.



Kevlar is a polyamide formed from terephthalic acid and 1,4-diaminobenzene. The aromatic

rings of the polymer backbone make the chains less flexible than those of nylon 6,6, resulting in

a very strong material. Kevlar is light in weight compared to other materials that are similar in

strength, so it is used in many products, such as bulletproof vests, army helmets, and the protective clothing used by fire fighters.



Armadillo bicycle tires reinforced

with Kevlar are hard to pierce

with sharp objects, so a cyclist

rarely gets a flat tire.



H

O



OH

C



+



C



HO



NH2



O

terephthalic acid



PROBLEM 17.35



H2N



N



O







C



(−H2O)



N



C



H

O



1,4-diaminobenzene



Kevlar



new amide bond



What two monomers are needed to prepare nylon 6,10?

O

N

H



(CH2)6



N

H



C



O

(CH2)8



C



O

N

H



(CH2)6



N

H



C



O

(CH2)8



C



nylon 6,10



17.10B POLYESTERS

Polyesters constitute a second major class of condensation polymers. The most common polyester is polyethylene terephthalate (PET), which is sold under a variety of trade names depending on its use (Dacron, Terylene, and Mylar). Plastic soft drink bottles are made of PET.



O



O

C



O



O



C



O

C



O



CH2CH2



O



CH2CH2



O



C



O

C



O



O



C

O



CH2CH2



polyethylene terephthalate

PET

[Ester bonds are drawn in red.]



smi26573_ch17.indd 529



12/16/08 9:57:47 AM



530



CARBOXYLIC ACIDS, ESTERS, AND AMIDES







FIGURE 17.5 Medical Uses of PET



a. PET is used to make the knitted material

present in artificial heart valves.



b. Some replacement blood vessels are

made with PET.



One method of synthesizing PET is by the acid-catalyzed reaction of a dicarboxylic acid

(terephthalic acid) with ethylene glycol (HOCH2CH2OH) in a Fischer esterification reaction.

Each OH and COOH react.

HO



loss of H2O

HO



O

C



+



C



O



+



HOCH2CH2OH



loss of H2O



O

C



loss of H2O



O



C



O



O

C



O



CH2CH2



HOCH2CH2OH



OH



acid catalyst



O



+



C



O



OH

terephthalic

acid



O

C



CH2CH2



C



O



O



polyethylene terephthalate

PET



Because these polymers are easily and cheaply prepared and they form strong and chemically

stable materials, they have been used in clothing, films, tires, and plastic drink bottles. Polyester

fiber is also used in artificial heart valves and replacement blood vessels (Figure 17.5).



PROBLEM 17.36



Draw the structure of Kodel, a polyester formed from 1,4-dihydroxymethylcyclohexane and

terephthalic acid. Suggest a reason why fabrics made from Kodel are stiff and crease resistant.

O

HOCH2



CH2OH



+



O

C



C



HO

1,4-dihydroxymethylcyclohexane



17.10C



OH

terephthalic acid



FOCUS ON HEALTH & MEDICINE

DISSOLVING SUTURES



Although PET is a stable material, the ester bonds in some other polyesters are quite readily

hydrolyzed to carboxylic acids and alcohols in aqueous medium. Polyesters of this sort are suited

for applications in which slow degradation is a useful feature. One example of a degradable



smi26573_ch17.indd 530



12/16/08 9:57:53 AM



SYNTHETIC POLYMERS IN MODERN SOCIETY—POLYAMIDES AND POLYESTERS



531



polymer is formed from two α-hydroxy acids—glycolic acid (HOCH2COOH) and lactic acid

[CH3CH(OH)COOH].

Join the C O of one acid with the

OH group of the second acid.

O

HOCH2



C



O



O

OH



HO



C



CH



OH



OCH2



C



O

O



CH



OCH2



C



CH3



CH3

glycolic acid



C



O



lactic acid



new C



O

O



C



CH

CH3



O bonds



dissolving suture material



When glycolic acid and lactic acid are combined, the hydroxyl group of one α-hydroxy acid

reacts with the carboxyl group of the second α-hydroxy acid. The resulting polymer is used by

surgeons in dissolving sutures. Within weeks, the polymer is hydrolyzed to the monomers from

which it was prepared, which are metabolized readily by the body. These sutures are used internally to hold tissues together while healing and scar formation occur.



17.10D



FOCUS ON THE ENVIRONMENT

POLYMER RECYCLING



The same desirable characteristics that make polymers popular materials for consumer products—

durability, strength, and lack of reactivity—also contribute to environmental problems. Many polymers do not degrade readily, and as a result, billions of pounds of polymers end up in landfills every

year. Recycling polymers is one way to help combat the waste problem they create.



CONSUMER NOTE



The most common polymers are each assigned a recycling code (1–6); the lower the number, the

easier the polymer is to recycle. PET materials are assigned recycling code “1” because they are

the easiest to recycle. Since recycled polymers are often still contaminated with small amounts of

adhesives and other materials, these recycled polymers are generally not used for storing food or

drink products. Recycled PET is used to make fibers for fleece clothing and carpeting.

PET can also be converted back to the monomers from which it was made. For example, heating

PET with water and acid breaks the ester bonds of the polymer chain to give ethylene glycol

(HOCH2CH2OH) and terephthalic acid. These monomers then serve as starting materials for

more PET.



The lower the recycling code, the

easier the polymer is to recycle.



Break each ester C



O bond.



O

O



O



O

C



The monomers are

re-formed.



C



O

C



O



CH2CH2



O



C

O



CH2CH2



H2O

H2SO4



polyethylene terephthalate

PET



HO



O

C



C

OH



O

terephthalic acid

+ HOCH2CH2OH

ethylene glycol



chemical recycling



PROBLEM 17.37



smi26573_ch17.indd 531



A polyester can be converted back to the monomers from which it is made much more easily

than a polymer like polyethylene, [ CH2CH2 ]n. Explain why this is so.



12/16/08 9:57:55 AM



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