Ken Yokoyama
CHE 331
February 19, 2003
Dr. Rahni
EXPERIMENT 3: Qualitative IR Analysis of Aldehydes, Ketones, Monomers and
Polymers (2-5 II)
Objectives
In this experiment, the purpose is to become familiar with the operation of FT-IR spectrometer and sample handling in IR technique for liquid samples, and learn how to interpret
IR spectra.
Theory
IR spectroscopy, one of the vibrational spectroscopic techniques, is a valuable tool for qualitative analysis. IR radiation is generally considered to be from about 10,000-100 cm
-1
, however, most instruments are limited to the range where most application occur between
4000-400 cm
-1
. In this technique, the IR radiation is absorbed and converted by a sample molecule into energy of molecular vibration. The masses of the atoms in the molecule, the force constants of the bonds, and the geometric structure of the molecule determine the frequency of absorption. IR spectra can be used to provide information on the functional groups as well as the structure of a molecule as a whole. IR spectra are commonly divided into three main regions. The high-frequency region, between 4000-1300 (2-7.7 μm), is called functional group region because the characteristics stretching frequencies for important functional groups such as
C=O, OH, and NH occur in this region. The middle-frequency region, between 1300-900 cm
-1
(7-11 μm) is known as the fingerprint region, in which the absorptions occur are complex and normally due to combinations of interacting vibrational modes, providing a unique fingerprint for every molecule. The spectrum in this region is especially valuable if examined in reference to other regions. The region between 900-650cm
-1
(11-15 μm) provides general classification of molecules from the pattern of absorptions, such as substitution patterns on a benzene ring. The absence of absorptions in the low-frequency region can provide a good evidence for the absence of an aromatic compound. Obtaining a broad, moderately intense absorption in the lowfrequency region indicates the presence of carboxylic dimers, amines, or amides. Evaluation of

the spectra is normally begun with assigning the bands of high and medium intensities, especially in the functional group region and low-frequency region.
The Fourier transform FT-IR spectrometer will be used to collect the data in this experiment. The FT-IR instruments do not require a dispersing unit; the radiation containing all wavelengths from the light source is monitored simultaneously. Most of FT-IR instruments are based on scanning Michelson interferometer that consists of a moving mirror, a fixed mirror, and a beamsplitter. Radiation from the IR source is divided at the beamsplitter in which half the beam passes to a fixed mirror and another half is reflected off the moving mirror. The two beams will recombine at the beamsplitter, and a constructive or destructive interference will occur depending on the difference of the distance between the beamsplitter to the fixed and moving mirror. The result of a variation of intensities is an oscillatory series of destructive and constructive combinations, called an interferogram. Fourier transformation converts this interferogram from the time domain into normal IR spectra, which is in the frequency domain.
FT-IR provides several advantages such as high sensitivity, resolution, and speed.
Procedure
Part I
A.
Using the WinFirst program, record a full-range (4000 cm
-1
to 600 cm -1 ) background spectrum of just the spacer between two sodium chloride plates. Clean out the sodium chloride plates with the next sample; handling them with care, by the edges only.
B.
Record a full-range (4000 cm
-1
to 600 cm -1 ) spectrum of the selected samples from the list.
Part II
A.
Record a full-range (4000 cm
-1
to 600 cm -1 the list
) spectrum of one aldehyde and a ketone from
Part III
A.
Using a single scan, record a full-range (4000 cm -1 polystyrene. to 600 cm -1 ) spectrum of a polymer,
Reagents

Ethyl Acetate

 Acetic Anhydride
 Dimethyl Formamide

Propionic Anhydride

Benzyl Alcohol

Benzaldehyde

Cyclohexanone

Polystyrene
Data and Results
1. Ethyl Acetate CH
3
  
2

CH
3
O
-The ethyl acetate has an ester carbonyl stretch ~1730 cm
-1
.

2. Acetic Anhydride CH
3
   
O O
3
-The acetic anhydride has an ester carbonyl stretch at ~ 1700 cm
-1
.

3. Dimethyl Formamide CH
3
  
CH
3
O
N-H Stretch
-The dimethyl formamide has an amide carbonyl ~1650 cm -1 and a N-H stretch ~3300 cm -1 .

4. Propionic Anhydride CH
3

CH
2
   
2

CH
3
O O
-Propionic anhydride has a strong ester carbonyl absorption ~1700 cm
-1
and a strong saturated
C-H stretch at ~2900 cm -1 .

5. Benzyl Alcohol
-Benzyl alcohol has an alcohol broad centered peak at ~3300 cm
-1
and a benzene ring peak at
~1600 cm -1 .

6. Benzaldehyde
-Bennzaldehyde has a benzene ring stretch ~1600 cm
-1
, a C=O stretch at ~ 1650 cm
-1
, and a C-
H stretch of two peaks ~ 2700 and 2800 cm
-1
.

7. Cyclohexanone
-Cyclohexanone is a saturated aliphatic ketone that has a C=O stretch ~1700 cm -1 .

8. Polystyrene
C-H stretch
Benzene ring
- Polystyrene has a benzene ring stretch ~1600 cm -1 and a highly saturated C-H stretch at ~
2900 cm
-1
.

Conclusion
Ethyl acetate has a single peak ~ 1730 cm
-1
that is indicative for an ester carbonyl stretch. The oxygen at the α-carbon of the carbonyl group will increase the frequency of absorption. The acetic anhydride and the propionic anhydride have a similar composition to ethyl acetate, except that they have an extra carbonyl group attached to the oxygen.
Propionic anhydride seems to have to have a higher absorbance at ~ 2900 cm
-1
with the
C-H stretch due to more methyl groups. The dimethyl formamide is similar to the ethyl acetate, except it has a nitrogen at the α-carbon of the carbonyl group, which is less electronegative than the oxygen, so it will have less of an increase of the frequency.
There is an N-H stretch at ~3300 cm
-1
that clearly sets it apart from ethyl acetate.
The benzyl alcohol and benzaldehyde have benzene ring stretches. There is a clear difference between them, since one is an alcohol and the other is an aromatic aldehyde. Benzyl alcohol has an O-H broad stretch at ~ 3300 cm
-1
with the benzene ring stretch at ~1600 cm
-1
. The benzaldehyde has the benzene ring stretch, but it has a C=O stretch at ~1650 cm
-1
and also the double peak C-H stretch at ~ 2700 and 2800 cm
-1
that shows it is an aldehyde. The cyclohexanone is a saturated aliphatic ketone that should have the C=O stretch higher than the benzaldehyde, because the benzene ring at the αcarbon of the carbonyl group will reduce the frequency of the carbonyl absorption. It will also have a lower carbonyl frequency than the carbonyl in ethyl acetate.
Polystyrene was the only polymer done for this section and the spectrum was taken from a saved file. It shows a benzene ring stretch at ~1600 cm -1 and a highly saturated absorbance at ~ 2900 cm
-1
.