Chemistry - A Molecular Science

Chapter 13 Organic Chemistry

308

Refer to Figure 13.26 for the following discussion of the mechanism for esterification. Step 1

The Lewis acidic carbonyl carbon (Section 13

.4) is attacked by a lone pair on the

oxygen of the alcohol. The red lone pair in (a) becomes the red bond in (b).

Step 2

The solvent (green water) transfers a proton from the oxygen of what was the

alcohol to the O-H group of th

e original acid. The blue lone pair in (b) becomes the

blue bond in (c), while the blue bond in (b) becomes the blue lone pair in (c).

Step 3

A water molecule is lost as the carbonyl

π bond reforms to yield the ester. The red

bond in (c) is converted into the red lone pair on the leaving water molecule in (d).

FORMATION OF AMIDES FROM ESTERS AND AMINES Amides

(nitrogen atom attached to a carbonyl gr

oup) can be formed in the reaction of an

amine with a carboxylic acid or an ester (Figure 13.27). The mechanism for the reaction of an amine with an ester (Figure 13.28) is identical to that for esterification (Figure 13.26) except for the identity of the Lewis base. Step 1

The Lewis acidic carbon of the carbonyl is a

ttacked by a lone pair on the nitrogen

of the amine.

Step 2

The solvent (green water)

transfers a proton from the nitrogen atom to the non-

carbonyl oxygen atom of what was the ester.

Step 3

An alcohol molecule leaves as the carbonyl

bond reforms to yield the amide. π

There are dozens of reactions between Lewi

s basic atoms of different molecules and

Lewis acidic carbonyl carbons, but these reactions all have mechanisms that are very similar. This mechanistic similarity enables chemists to propose new chemical reactions.

13.6

POLYMERS Polymers

(from the Greek for ‘many units’) are large molecules made by forming

covalent bonds between

building blocks called

monomers

(single units). Successive

reactions of the monomer units result in the growth of a long chain - the polymer (Figure 13.29d). Polymers can be synthetic (rubber, fi

bers, plastics) or natural (rubber, proteins,

nucleic acids, carbohydrates). The special proper

ties of polymers are related to their very

high molecular weights. Synthetic polymers are classified as

condensation polymers

or

addition polymers,

depending on the mechanism by which they are formed. Polymers can

be prepared by different mechanisms, but we

will examine only those mechanisms that are

similar to the reactions we have already studied in this chapter.

(a)

(b)

(c)

(d)

Figure 13.29 Combining mers (units) One mer is a monomer (a), two mers form a dimer (b), three mers make a trimer (c), and many mers combine to form a polymer (d).

RC

O

H O

RC

O

H O H

H

O

+ H

O

R'

H

O

1

2

3

H O

R'H

RC

O

H O

R'

O

H

RC

O

R' O

(a)

(b)

(c)

(d)

Figure 13.26 Mechanism of esterification

O C

H^3

C

OC

H 2

5

H N

H

CH

3

O C

HCN^3

H

CH

3

C^2

H^5

OH

ester + amine amide + alcohol

+

+

Figure 13.27 Amide formation The water molecule that is produced is highlighted in red.

RC

O

O

RC

O

O H

R'

O

H

+

N

H

HR"

N

H

HR"

N

1

23

H O

H

H

R"

RC

O

R' O

R'

R'

RC

O

R'' N H

(a)

(b)

(c)

(d)

Figure 13.28 Mechanism for amide formation

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