Murphy M P, Bond E. February 28th, 2016. Arizona State University.
Abstract
The aim of this experiment was to use infrared spectroscopy in order to determine the unknown identity of two chemical compounds. This was accomplished by placing two unknown samples (one liquid, one solid) on an ATR crystal of a spectrometer, and running a scan which produced two separate absorption spectrums. The bands of these spectrums were then analyzed independently in order to determine which specific bond frequencies or functional groups were present. By analyzing which bond frequencies or functional groups were present in the spectrum the observer could accurately identify the identity of each unknown compound. It was hypothesized that the liquid sample provided was 2-propanol due to the presence of three carbon bonds and an alcohol functional group. Also the solid sample provided was determined to be biphenyl primarily because of the presence of two
carbon bonds and two phenyl functional groups. This determination could not have been accomplished without IR spectroscopy and truly supports why it is vital to the field of analytical chemistry.
Introduction:
Infrared spectroscopy is a form of analytical chemistry that has been developed to properly identify the specific bond frequencies and functional groups present in a chemical compound. This particularly form of spectroscopy irradiates a sample with different frequencies of IR radiation, which are then absorbed by the sample and cause an increase in the amplitude of the vibrations of the bonds. A spectrometer is then used to determine exactly how much radiation the sample transmitted as a function of frequency. From this point an absorption spectrum is generated, which displays a plot of the transmission percentage of IR radiation through the sample as a function of frequency. If not previously known the infrared frequencies are listed as wavenumbers numbers with the units of , and range from 400-4000
.
This experiment used infrared spectroscopy in order to determine the chemical structure of two unknown compounds by observing which specific bond frequencies or functional groups that were present in the absorption spectrum. As previously stated IR spectroscopy irradiates a sample with different frequencies of IR radiation, which are then absorbed by the sample. This technique has been extremely useful in structural determination because specific bond frequencies and different functional groups display a characteristic frequency, which makes them easy to identify with the aid of a correlation table.
Upon the completion of this experiment the proper identification of both unknown compounds should be easily attained by the bands produced in the absorption spectrum. By observing the wavenumber present in the spectrum and comparing it to the values listed in the correlation table, one should be able to identify the specific bond frequencies and functional groups present in the compound. This is of great importance because certain compounds have specific bond frequencies and functional groups present in them, which allows them to be easily identified.
Experimental:
For this experiment two unknown samples (one liquid, one solid) were provided and examined through infrared spectroscopy so that they could be properly identified. The unknown liquid sample (A) was removed from its source using a pipette, and was placed into a 10 mL beaker so that it could be easily transferred to the spectrometer. Before placing the liquid sample on the ATR crystal it was cleaned using an isopropyl alcohol wipe and allowed to dry. From this point a small amount of the liquid sample was placed on the crystal and a scan was run, which produced an absorption spectrum for the unknown liquid sample. Once completed, the bands of the absorption spectrum were analyzed in order to determine which specific bond frequencies or functional groups were present. The second unknown sample (E) was a solid and was removed from its source using a clean stirring rod, which was then was placed onto a sheet of wax paper. This was performed because the solid crystals needed to be smashed into a fine powder in order to achieve an accurate reading from the spectrometer. Once this was accomplished the spectrometer was once again cleaned with an isopropyl alcohol wipe and allowed to dry. From this point a small amount of the solid sample was placed on the ATR crystal and the pressure arm was positioned on top of the sample. The arm was adjusted until the force gauge produced a reading of 80, and a scan was run to produce another absorption spectrum. This spectrum like the liquid sample was then analyzed in order to determine which specific bond frequencies or functional groups were present in the compound.
Results
Upon completion of the infrared spectroscopy of the unknown liquid sample (A) the following absorption spectrum was generated and is shown in figure 1.
Figure 1: Absorption spectrum produced concluding IR spectroscopy for unknown liquid sample (A).
Upon the completion of the infrared spectroscopy of the unknown liquid sample the absorption spectrum produced the following data which is shown in table 1. From this data the unknown compound was able to be properly identified by observing any specific bond frequencies or functional groups present.
Table 1: Peak Frequency, Peak % reflectance, Bond vibration, and functional groups present in the absorption spectrum of unknown liquid sample (A).
| Peak Frequency ( | Peak % Reflectance | Bond Vibration (if appropriate) | Functional Group Assignment (if appropriate) |
| 3330.07 | 80 | -O – H | Alcohol |
| 2969.92 | 65 | Cs | |
| 2932.76 | 90 | Cs | |
| 2888.31 | 90 | Cs | |
| 1466.86 | 57 | ||
| 1408.20 | 59 | ||
| 1378.64 | 73 | ||
| 1340.51 | 75 | ||
| 1306.96 | 69 | ||
| 1160.01 | 77 | ||
| 1128.07 | 73 | ||
| 1107.66 | 67 | ||
| 816.61 | 67 |
Upon completion of the infrared spectroscopy of the unknown solid sample (E) the following absorption spectrum was generated and is shown in figure 2.
Figure 2: Absorption spectrum produced concluding IR spectroscopy for unknown solid sample (E).
Upon the completion of the infrared spectroscopy of the unknown solid sample the absorption spectrum produced the following data shown in table 2. From this data the unknown compound was able to be properly identified by observing if any specific bond frequencies or functional groups were present.
Table 2: Peak Frequency, Peak % reflectance, Bond vibration, and functional groups present in the absorption spectrum of unknown solid sample (E).
| Peak Frequency ( | Peak % Reflectance | Bond Vibration (if appropriate) | Functional Group Assignment (if appropriate) |
| 3060.29 | 71 | Cs | |
| 3033.43 | 73 | Cs | |
| 1597.43 | 78 | C=C | phenyl |
| 1568.95 | 75 | C=C | phenyl |
| 1477.81 | 85 | ||
| 1428.62 | 87 | ||
| 1344.29 | 74 | ||
| 1181.68 | 84 | ||
| 1169.87 | 81 | ||
| 1111.22 | 67 | ||
| 1090.85 | 59 | ||
| 1041.40 | 67 | ||
| 1005.66 | 66 |
Discussion:
In this experiment infrared spectroscopy was performed on two unknown compounds (one liquid, on solid) in order to properly identify them. A small sample of each compound was taken and placed on the ATR crystal of the spectrometer. It is important to note that for the solid sample it was first crushed into a fine powder, which was then held in place by the pressure gauge that was set to 80. This step is crucial for the solid sample to ensure an accurate absorption spectrum from the IR spectrometer. From this point a scan was ran on both samples, which produced two different absorption spectrums. The bands on each of spectrums were then analyzed with the use of the correlation table provided in the lab manual in order to identify the identity of each compound. The distinct bands found in each spectrum are extremely important in the identification process, because specific bond frequencies and functional groups display distinct character frequencies.
For the liquid sample it was observed that there were four distinct peaks located inside the diagnostic region. The first peak was found to be at 2883.31 , and was identified to be an
carbon bond. The second and third peaks were located at 2932.76 and 2969.91
respectively, and were also found to be
carbon bonds. This was able to be determined from the correlation table provided in the lab manual, because
carbon bonds are found in the frequency range of 2850 to 2960
. The fourth peak was located towards the left end of the spectrum and displayed a wavenumber of 3330.07
. Once again referring to the correlation table this peak was consistent with that of an alcohol group, because it was inside the range of 3300 to 3400
and displayed a broad peak. Upon further observation it was determined that the unknown liquid compound must be 2-propanol, because of the three
carbon bonds and the alcohol group present. This hypothesis is supported because the two other unknown liquid compounds (hexane and 4-t-butylpehnol) listed in the selection bank were able to be eliminated. The compound of hexane was eliminated because five
carbon bonds were present, and the functional group of alcohol was absent. On the basis of 4-t-butylphenol, even though an alcohol group was present this compounds displays three
carbon bonds and was also eliminated.
For the solid sample it was observed that there were also four distinct bands located inside the diagnostic region. The first and second peaks were located at 1568.95 and 1597.34 respectively, and were confirmed to be phenyl groups because they displayed wavenumbers around 1600
. These hypotheses are supported by the correlation table provided in the lab manual. The third and fourth peaks were located at 3033.43 and 3060.29
respectively, and were determined to be
carbon bonds because they were located within the range of 3000-3100
.As the absorption spectrum of the unknown solid sample was furthered analyzed it was determined that the compound must be biphenyl, because of the two phenyl peaks located around 1600
and the two
carbon bond peaks located 3033.43 and 3060.29
respectively. This hypothesis is supported because the two other unknown solid compounds (benzoic acid, and 4-t-butylpehnol) listed in the selection bank were able to be eliminated. The compound of benzoic acid was eliminated because of the attachment of an alcohol group to an
carbon. The compound of 4-t-butylphenol was also eliminated from the selection process because of the presence of an alcohol functional group.
Upon the completion of this experiment it is easy for one to see why IR spectroscopy is a vital component of analytical chemistry. By understanding how to read and interpret data from an absorption spectrum an observer can easily identify specific bond frequencies and functional groups present in an unknown compound. This is due to the simple reason that specific bond frequencies and functional groups display distinct wavenumbers or ranges.
Conclusion:
The aim of this experiment was to use a form of analytical chemistry, specifically IR spectroscopy, in order to determine the identity of two unknown compounds. This was achieved by placing each unknown sample on the ATR crystal of the spectrometer and running a scan, which in turn produced two separate absorption spectrums. The bands of these spectrums were then analyzed to determine which specific bond frequencies or functional groups were present in the compound. This was accomplished by comparing the wavenumber listed on the peak in the spectrum to the correlation table provided in the lab manual. Following this step an unknown liquid and solid compound had to be selected from the unknown bank, which was also provided in the lab manual.
This experiment was conducted with relative ease, and it was determined that the unknown liquid sample provided was 2-propanol while the unknown solid sample was biphenyl. Again this observation is supported by comparing the wavenumbers produced in the spectrum to the correlation table provided in the lab manual.
References:
- Arizona State University (2016). Organic Chemistry, Laboratory Manual and Techniques. Tempe, AZ: Dr. Brian Woodrum.