Organic Chemistry

Section 17.3 Physical Properties of Carbonyl Compounds 677

Esters, carboxylic acids, and amides each have two major resonance contributors.
The second resonance contributor for acyl chlorides and acid anyhdrides is much
less important.

The second resonance contributor (the one on the right) is more important for the
amide than it is for the ester or for the carboxylic acid because nitrogen is better than
oxygen at sharing its electrons. Nitrogen is less electronegative than oxygen and,
therefore, is better able to accommodate a positive charge.

PROBLEM 4

Which is longer, the carbon–oxygen single bond in a carboxylic acid or the carbon–oxygen

bond in an alcohol? Why?

PROBLEM 5

There are three carbon–oxygen bonds in methyl acetate.

a. What are their relative lengths?

b. What are the relative infrared (IR) stretching frequencies of these bonds?

PROBLEM 6

Match the compound with the appropriate carbonyl IR absorption band:

acyl chloride and

acid anhydride

ester

amide

17.3 Physical Properties of Carbonyl Compounds

The acid properties of carboxylic acids have been discussed previously (Sections 1.18
and 7.10). Recall that carboxylic acids have values of approximately 3– 5
(Appendix II; see also “Special Topics I”in the Study Guide and Solutions Manual).
The acid properties of dicarboxylic acids will be discussed in Section 17.21. The boil-
ing points and other physical properties of carbonyl compounds are listed in
Appendix I. Carbonyl compounds have the following relative boiling points:

relative boiling points

amide 7 carboxylic acid 7 nitrileWester'acyl chloride'aldehyde'ketone

pKa

' 1800 cm-^1

' 1730 cm-^1

' 1640 cm-^1

' 1800 1750 cm-^1

O

C

R OCH 3

O

C

ROH

O

C

RNH 2

C

R OCH 3

C

ROH

C

RNH 2

+

+

+

O−

O−

O−