Objective: To increase the power of observation and deduction, to give a practice at obtaining and interpreting IR spectra and to identify an unknown compound G.
Organic compound are so numerous and of such wide variety that the problem of identification is formidable unless approached in a systematic and logical manner. Both spectroscopic and chemical techniques are useful for solving these problems. The general procedures for the identification of an organic compound are:
1. Preliminary physical examination.
2. Determination of the solubility properties of the compound.
3. Qualitative element analysis- for element present in the compound other than carbon, hydrogen, and oxygen.
4. Chemical characterization tests- to determine the chemical class of the compound.
5. Spectroscopic analysis- to determine the chemical class of the compound and its structure by interpretation of the IR spectra.
6. Literature search- compares the physical and the spectroscopic properties of the unknown compound with compounds of the same chemical class.
7. Further experiment comparisons- to provide addition chemical tests data that are necessary to complete and confirm the identification.
First of all, the physical state, color, size and shape and odor of the unknown compound G were recorded. Secondly, ignition test and solubility test were carried out.
Then sodium fusion test was carried out to prepare the “fusion” solution, followed by the test for the presence of nitrogen and halogens. Afterwards, a serious of chemical characterization tests were carried out. Finally, an IR spectrum of the unknown sample G was obtained for the presence of functional group.
Data and Results:
Sample G was toxic and flammable.
The data given for the unknown sample G
Unknown sample G
1. Preliminary physical examination
Unknown sample G
burns with a luminous flame
2. Solubility test
Unknown sample G
miscible with water
3. Qualitative analysis of elements (Sodium fusion test)
For unknown sample G
Prussian blue ppt. was formed
Detection of halogens
no ppt. has formed
Absent of halogens
4. Chemical characterization tests
For unknown sample G
Bromine addition test
No color changed
Carbon-carbon double bond was absent
Ferrrous hydroxide test
No ppt. formed
Nitro compound was absent
Copper sulfate test
The solution remains light blue color.
Amine was absent
The solution change from pale yellow to dark purple.
Nitrile was present
5. Spectroscopic analysis
During the experiment, we used sodium fusion test (only for pure compound) for detecting whether nitrogen and halogen are presence in the unknown sample. In sodium fusion test, we used sodium carbonate to react with organic vapor of the unknown organic compound, which was released during heating, and it aims to prevent toxic vapour from escaping.
The IR spectra for sample G have lots of un-expected peaks present, it may due to the present of impurities left on the salt plate. In order to obtain better spectra and reduce the errors, we should clean the salt plate twice or more to ensure the chemicals left behind was cleaned.
For unknown compound G, the result of solubility test showed that it is possible to be amides amines, nitriles, nitro compounds and halogen substituted. In sodium fusion test, it was found that nitrogen was present while halogen was absent. This indicates that the sample belongs to class M and excluded halogen substituted.
As the molecular weight of sample G was 45.01 only, we made a hypothesis that it cannot be amide. Amide functional group (i.e. 2HN-C=O ) at least has a molecular weight 44.02, which implied sample G was not possible to be an amide. Therefore amide has been excluded in the class.
In bromine addition test, the color of bromine had no change, which indicated that the unknown sample does not contained carbon-carbon double bond or carbon-carbon triple bond.
As it was possible to be amines, nitriles and nitro compounds, we have to carry out a serious of test to determine which group the sample belongs. The copper sulfate test was carried out to test the presence of amine. The result shows there was absent of amine. The ferrous hydroxide test was carried out to test the presence of nitro compound. As there was no ppt. formed, it indicates that the sample was not nitro compound.
To ensure the sample G was nitrile, the hydroxamate test was carried out to test the presence of nitrile. The solution turns from pale yellow to dark purple, which indicated that the present of nitrile.
To further confirm the structure of the sample, a IR spectra was obtained. There was a sharp peak at frequency 2253.77 cm-1 (nitrile stretching) which implied the present of nitrile. Finally we inferred the structure of sample G has a molecular formula C2H3N. By calculation we found out the molecular mass is: 14.01+ 2×12.01+ 3×1.01, which match the actual molecular mass of sample G=41.05 g/mol.
By checking the reference book, sample G was found to be acetonitrile having poiling point 81-82?, which matched with the data provided. So the sample G was inferred to be acetonitrile. The structure of it is as follow:
Throughout the experiment, the result from the chemical tests were difficult to observe which is also difficult to identify a real unknown, therefore spectroscopic analysis are crucial to determine its correct structure. We can also compare the molecular weight, boiling point, melting point and other relevant data with the reference books for confirmation.
Through a series of tests, we found out that sample G is acetonitrile, CH3CN. The molecular weight of it is 45.01 g/mol and the boiling point is 81-82?.