Henry SuMay 8, 2003
Lab Experiment #3CHE 331
8-1,2,3Dr. Rahni
Qualitative IR Analysis of Aldehydes, Ketones, Monomers, and Polymers
Purpose
The purpose of this laboratory experiment is to demonstrate the various methods of sample preparation for analysis in the infrared region, illustrate how to identify functional groups from infrared spectra, and determine the principle structure of an unknown compound from these groups. It will also serve to compare the carbonyl absorption frequencies for selected aldehydes and ketones to illustrate the effect of substituents and conjugation
Theory
Infrared wavelengths are those that are longer than wavelengths associated with the visible region. The region that begins at about 0.8 um is called the overtone region. The fundamental region (2.5 um to 1.5 um) is the area generally used for infrared measurements, and is divided into the group family frequency region (2.5 um to 8 um) and the fingerprint region (8 um to 15 um). The position of the absorption peaks is more or less dependent on the complete molecular structure. Thus, they are more difficult to identify and correlate. The far infrared region (15 um to 400 um) extends from the fundamental region and goes to the microwave region.
Atoms in a molecule move together in a straight line translation and vibrate periodically about their centers of mass. Repulsive forces between nuclei with their completed shells and the bonding forces due to valence electrons are the main causes of the vibrations. Three coordinates must be specified in order to describe this motion. Thus, for n number of atoms in a given molecule, 3n coordinates are used to describe its motions. The fundamental vibrations of a molecule are stretching, bending, wagging, rocking, and twisting.
The correlation between the expected number and observed number of bands may be increased by bands that do not fall in the classes of combination tones, overtones, and difference tones. The number of observed bands may also be decreased in certain cases. One of these situations is when the molecule is highly symmetrical. Another is when frequencies are degenerate, or not separated by the spectrophotometer. Some fundamental bands may be too weak for detection, or occur too far below the range of usual spectrophotometers.
Apparatus
The following materials will be needed in order to perform this experiment:
Material / QuantityDouble beam Infrared Spectrophotometer / 1
Volumetric Flasks / 10
KBr pellet equipment / 1
Cell Filling Equipment / 1
Reagents
Solution / QuantityEthyl Acetate / 5 ml
Acetic Anhydride / 5 ml
Dimethyl formamide / 5 ml
Propionic Anhydride / 5 ml
Benzyl Alcohol / 5 ml
Benzaldehyde / 5 ml
Cyclohexanone / 5 ml
Procedure
1-Assemble Cell Filling Equipment
-Place spacer between plates
-Tightly screw plates into place
-Inject liquid into intake vessel to fill cell, cap when full
-Place cell into spectrophotometer
2-Run Scan for Sample
Data and Calculations
See accompanying IR spectra sheets.
Conclusion:
Upon completion of the outlined procedure, IR spectra were generated for ethyl acetate, acetic anhydride, dimethyl formamide, propionic anhydride, benzyl alcohol, benzaldehyde, and cyclohexanone.