Organic Chemistry

PROBLEM 19

Which would show an absorption band at a larger wavenumber: a carbonyl group bonded

to an hybridized carbon or a carbonyl group bonded to an hybridized carbon?

PROBLEM 20

List the following compounds in order of decreasing wavenumber of the absorption

band:

a.

b.

c.

O

CH 3 CCH 3

O

CH 3 CH

O

HCH

O

OOO

OO

OO

O

O

NH

C“O

sp^3 sp^2

Section 13.10 The Position of Absorption Bands 507

C O

C O

CH 3 CH 2 CH 2 CH 2 COH

O

Wavelength (μm)

Wavenumber (cm−^1 )

2.52.6 2.7 2.8 2.9 3 3.5 4 4.5 5 5.5 6 7 8 9 10 11 12 13 14 15 16

4000380036003400320030002800260024002200 2000 1800 1600 1400 1200 1000 800 600

% Transmittance

C H

O H

Figure 13.20
The IR spectrum of pentanoic acid.

Absorption Bands
absorption bands are easy to detect. Polar bonds show intense absorp-
tion bands and the bands are quite broad (Figures 13.19 and 13.20). The position and
the breadth of an absorption band depend on the concentration of the solu-
tion. The more concentrated the solution, the more likely it is for the OH-containing
molecules to form intermolecular hydrogen bonds. It is easier to stretch an
bond if it is hydrogen bonded, because the hydrogen is attracted to the oxygen of a
neighboring molecule. Therefore, the stretch of a concentrated (hydrogen-
bonded) solution of an alcohol occurs at 3550 to whereas the
stretch of a dilute solution (with little or no hydrogen bonding) occurs at 3650 to
Hydrogen-bonded OH groups also have broader absorption bands
because the hydrogen bonds vary in strength (Section 2.9). The absorption bands of
non-hydrogen–bonded OH groups are sharper.

concentrated solution

3550 – 3200 cm−^1

dilute solution

3650 – 3590 cm−^1

H

RHO HOR R O

hydrogen bond

3590 cm-^1.

3200 cm-^1 , O¬H

O¬H

O¬H

O¬H

O¬H O¬H

O¬H