Classifying an Unknown Compound by Functional Groups Lab Report
The purpose of this lab was to identify the functional groups in three unknown compounds. In order to accomplish this, a variety of different tests were done. First, solubility tests were run on each compound. The results of the solubility tests can indicate the size and polarity of unknown compound. The solubility tests helped to narrow down the possible functional groups in the unknown. Once the options were narrowed, further classification tests were run. These tests were used to identify specific functional groups in each unknown sample.
- Water Test
The first solubility test was done in water. Each compound was place in the water to determine whether it was soluble in water.
- 5% Sodium Hydroxide Test
If unknown compounds were insoluble in water the next test was to determine whether they were soluble in 5% sodium hydroxide (NaOH). The sodium hydroxide test demonstrated whether the unknown was a carboxylic acid or a phenol because the compound would be soluble. The reactions were as follows:
- 5% Sodium Hydrogen Carbonate (NaHCO3) Test
Afterwards, the unknowns that were soluble in NaOH, were tested in 5% sodium hydrogen carbonate (NaHCO3). This solubility test can distinguish between a carboxylic acid and a phenol. The strong acidity in the carboxylic acid will cause the unknown to be soluble. However, the phenol is not as acidic; therefore, it is not soluble.
- 5% Hydrochloric Acid (HCl) Test
Next, 5% hydrochloric acid was used to determine if an unknown, insoluble in 5% NaOH, was an amine. An amine would be soluble in HCl. If the unknown was insoluble in HCl, a wide variety of functional groups were possible such as an alkene, an alcohol, an aldehyde, or a ketone.
- Sulfuric Acid (H2SO4) Test
The solubility of the unknown compounds was finally assessed using sulfuric acid (H2SO4). Sulfuric acid dissolves molecules containing alkenes, oxygen, or nitrogen. Therefore, if the unknown was insoluble in H2SO4, the functional group was indicated to be an alkane, alkyl halide, or an aromatic compound.
- Potassium Permanganate Test
The potassium permanganate (KMnO4) test was used to determine whether the unknown was an alkene. Potassium permanganate reacts with the C=C bond, oxidizing it into a diol. If an alkene was present, it would change from the original purple color of potassium permanganate to a dark brown precipitate. This result would indicate the C=C bond was broken and the two alcohol groups were formed.
- Beilstein Test
Next, the Beilstein Test was used to determine whether an unknown was an alkyl halide. The unknown compound is placed on a copper wire and then directly into a flame. The flame turns bright green if the unknown is an alkyl halide.
- TCICA Test
The TCICA test was used to determine whether an unknown was an alcohol. The TCICA is an oxidizing agent, therefore, reacts, reducing the compound. Primary and secondary alcohols will reduce in the presence of TCICA. The duration of time it takes for a precipitate to form indicates whether the compound is a primary or secondary alcohol. Primary alcohols take longer to react, whereas, secondary alcohols react almost instantly.
- Iron (III) Chloride Test
Iron (III) chloride was used to identify unknowns containing a phenol group. Compounds that contain a phenol will react with the iron (III) chloride to produce a bright color. However, the color change can happen very quickly and some phenols may not react.
- Bromine in Water Test
The next test used bromine in water to classify a phenol. If the unknown was a phenol, the dark bromine color would disappear.
- 2,4-Dinitrophenylhydrazine (DNP) Test
2,4-Dinitrophenylhydrazine (DNP) can be used to classify aldehydes or ketone functional groups. When the 2,4-Dinitrophenylhydrazine is added to the unknown, in the presence of an aldehyde or ketone, the DNP would react forming a bright yellow, orange, or red precipitate.
Unknown 7 Functional Group - PhenolUnknown 7
Water / Insoluble
5% NaOH / Soluble
5%NaHCO3 / Insoluble
FeCl3 Test / Positive (Eugenol) / Negative (Benzyl alcohol) / Unknown 7
Dark Blue / Yellowish Brown / Yellow
Bromine (Br2) Test / Positive (2-isopropylphenol) / Negative (2-propanol) / Unknown 7
Yellow / Dark Gold/Orange / Light Orange
Table 1 – Unknown 7 -Solubility Tests
Table 2 – Unknown 7 – Classification Tests
The results of the solubility tests, as seen in Table 1, indicate that Unknown 7 was a phenol. Two classification tests were used to support this conclusion. First, the FeCl3 test was performed. The results displayed in Table 2 indicate the unknown was not clearly a positive or negative for a phenol. Therefore, this test was inconclusive. Also, the bromine test was performed to determine if the unknown was a phenol. The results, also in Table 2, of this test were also inconclusive. The color change of the unknown did not match either the positive or the negative. The classification tests failed to indicate that Unknown 7 was a phenol. However, by using the solubility tests, it was suggested Unknown 7 was a phenol. One possible reason for the error in the classification tests is presence of the unknown phenol has decreased over time. Also, the dark color of the unknown made it difficult to detect the color changes in the classification tests.
Unknown 17 Functional Group – Secondary AlcoholUnknown 17
Water / Insoluble
5% NaOH / Insoluble
5%HCl / Insoluble
H2SO4 / Soluble
2,4-DNP Test / Positive (4-methyoxy-acetophenone) / Negative (benzoic acid) / Unknown 17
Orange Precipitate / Yellowish-Orange; No precipitate / Yellowish-Orange; No precipitate
KMnO4 Test / Positive / Negative / Unknown 17
None / None / Purple
TCICA Test / Positive (2,2dimethyl 1,3 propanediol) / Negative (benzoic acid) / Unknown 17
Precipitate in 30 minutes / No precipitate / Precipitate in 60 seconds
Table 3 – Unknown 17 Solubility
Table 4 – Unknown 17 Classification
According to Table 3, the results of the solubility tests indicated Unknown 17 could possibly be an alkene, alcohol, aldehyde, or a ketone. Further classification tests were used in order to narrow down the possibilities in order to reach a conclusion. First, the 2,4-DNP Test was used in order to determine whether Unknown 17 was an aldehyde or a ketone. As Table 4 shows, the lack of precipitate formed suggests that Unknown 17 was not an aldehyde or ketone. Therefore, Unknown 17 could be an alkene or an alcohol. To determine which functional group was present in Unknown 17, the potassium permanganate test was performed. The results, in Table 4, indicate Unknown 17 was not an alkene because the unknown remained purple and did not form a brown precipitate. Therefore, the results thus far suggest Unknown 17 was an alcohol. To confirm, the TCICA Test was performed. According to Table 4, Unknown 17 precipitated almost immediately during the TCICA Test. These results indicate Unknown 17 was a secondary alcohol.
Unknown D Functional Group – Alkyl HalideUnknown D
Water / Insoluble
5% NaOH / Insoluble
5%HCl / Insoluble
H2SO4 / Insoluble
Flame Test / Positive / Negative / Unknown D
Green Flame / Nothing / Green Flame
Table 5 – Unknown D – Solubility Table 6 – Unknown D - Classification
Using the results from Table 5, the solubility tests suggested that Unknown D could be an alkane or an alkyl halide. In order to determine which it was, the Beilstein Test was used. According to Table 6, Unknown D produced a bright green flame suggesting it was an alkyl halide. The result from the Beilstein Test was very clear; therefore, no further testing was done.
The purpose of the experiment was to identify the functional groups of three unknown compounds using solubility tests and chemical classification tests. The solubility tests narrowed down the potential possibilities for each unknown and then further testing was done to make a conclusion about each. Based on the solubility test results, Unknown 7 was a phenol, despite the inconclusive results from the classification tests. One possible reason for this is the low amount of the phenol present in the solution. Unknown 17 was concluded to be a secondary alcohol based on a variety of tests. The TCICA test conformed this. Lastly, based on the results of the solubility tests and the Beilstein test, Unknown D was an alkyl halide. In conclusion, identifying functional groups of an unknown compound can be done using solubility tests and classification tests.