Chemistry, Third edition

INFRARED SPECTROSCOPY

20.23). The –OH bond in carboxylic acids, also very broad, is found between 3500

and 2400 cm–1(see ethanoic acid Fig. 20.20). The spectra of carboxylic acids is

easily distinguished from that of alcohols and phenols, since acid spectra also con-

tain the characteristic C=O peak.

●The NH 2 group gives rise to two weak absorptions (due to the N–H stretch) in the

range 3500–3100 cm–1(see propan-2-amine, Fig. 20.24).

●Nitriles (also known as cyanides), show a CN stretch near 2250 cm–1(see etha-

nenitrile, Fig. 20.25).

The spectra in Figs 20.17–20.25 show the potential of infrared spectroscopy for dis-

tinguishing between common types of organic molecule, particularly if combined

with information from other techniques such as mass spectrometry, nuclear mag-

netic resonance (NMR) and with the results of chemical tests.

385

Example 20.3

A compound P contains the following elements (percentage by

mass):

C (30.6%), H (3.80%), O (20.3%), Cl (45.2%)

Its mass spectrum contains a parent ion at m/e= 78. Its infrared

spectrum contains a very strong peak at 1800 cm–1. P does not

reduce Fehling’s solution. Suggest a molecular formula for P.

Answer

The simplest formula of the compound is first calculated from the percentage of

elements by mass:

Element Carbon Hydrogen Oxygen Chlorine

Percentage by mass 30.6% 3.80% 20.3% 45.2%

Molar ratio 30.6 3.80 20.3 45.2

——– = 2.55 ——– = 3.80 ——– = 1.27 ——– = 1.27

12.0 1.0 16.0 35.5

Simplest molar ratio 2.55 3.80 1.27 1.27

——– = 2.00 ——– = 3.00 ——– = 1.00 ——– = 1.00

1.27 1.27 1.27 1.27

Simplest whole number ratio 2311

Therefore, the simplest formula of the substance is C 2 H 3 OCl. The mass spectrum

gives the mass of the parent ion as 78 u (^35 Cl isotope), and we conclude that the

molecule has the formula C 2 H 3 OCl.

The peak at 1800 cm–1suggests the presence of a carbonyl group. Taking

‘C=O’ out of the formula C 2 H 3 OCl gives CH 3 Cl. P does not reduce Fehling’s

solution, which excludes the aldehyde CH 2 ClCHO. The structure for P is then most

likely to be CH 3 COCl:

HO

| ⁄⁄

H⎯C⎯C

| \

HCl

Using IR spectra in

organic analysis 1

Which of the following

compounds would be

expected to show (i)a peak

due to C=O, (ii)a broad

band due to –OH, (iii)a

peak due to C–H?

(a)butanol

(b)propanoic acid

(c)ethanal.

Exercise 20J

Using IR spectra in

organic analysis 2

A compound Y contains (by

mass) 19.2% nitrogen,

15.1% hydrogen and 65.8%

carbon. Its mass spectrum

contained an intense peak,

due to the parent ion, at

m/e= 73. Its infrared spec-

trum contained two peaks

of medium intensity at

3300 cm–1and 3370cm–1.

Suggest a formula for Y.

Exercise 20K