2. Chromic Acid Test
Chromic Acid oxidizes 1° alcohols to carboxylic acids. The red brown Cr6+ in the chromic acid is reduced to green Cr3+ (Universiti Tunku Abdul Rahman-Department of Chemical Science, 2021, p.60). Chromic acid oxidizes 2° alcohols to ketones (Universiti Tunku Abdul Rahman-Department of Chemical Science, 2021, p.61). The reduction above is the same here. Chromic acid does not oxidize 3° alcohols (Universiti Tunku Abdul Rahman-Department of Chemical Science, 2021, p.61). This reaction can be used to distinguish between 1°, 2° and 3° alcohols (Universiti Tunku Abdul Rahman-Department of Chemical Science, 2021, p.61).
3. Reaction with Metallic Sodium
Alcohols and water are weak acids (Universiti Tunku Abdul Rahman-Department of Chemical Science, 2021, p.61). Alkoxide ion can be generated by hydroxyl group which can donate proton (Universiti Tunku Abdul Rahman-Department of Chemical Science, 2021, p.61). Dissolving alkoxide ions in alcohol, like dissolving hydroxide ions in water, can produce strong base which can use an alcohol to react with sodium metal to prepare (Universiti Tunku Abdul Rahman-Department of Chemical Science, 2021, p.61). Hydrogen gas will be generated in this reaction (Universiti Tunku Abdul Rahman-Department of Chemical Science, 2021, p.61).
2 R-OH + 2 Na → 2 R-O-Na+ + H2
an alkoxide
Source: (Universiti Tunku Abdul Rahman-Department of Chemical Science, 2021, p.61)
Apparatus and materials
- Test tubes
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Ethanol (C2H5OH)
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Isopropyl alcohol (C3H7OH)
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t-butyl alcohol (C4H9OH)
- Lucas reagent (mixture of concentrated hydrochloric acid and zinc chloride)
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Chromic acid (H2CrO4)
- Sodium metal
Procedures
A. Solubility of Alcohols
1. Approximately 1 mL ethanol, 1 mL isopropyl alcohol and 1 mL t-butyl alcohol are respectively put into 3 dry test tubes.
2. Approximately 2 mL water is placed to the tubes respectively and mixed then observation is made.
3. The results are written on the notebook.
4. The procedures above are repeated using 2 unknown liquids and the results are observed.
B. Lucas Test
1. 6 mL Lucas reagent is placed to 1 mL portions of ethanol, isopropyl alcohol and t-butyl alcohol respectively in dry test tubes at room temperature.
2. The tubes are closed by using cork then they are shaken, the lengths of time they take for the mixture to become cloudy or have two layers separated are recorded.
3. The results are written down.
4. The procedures above are repeated by using 2 unknown liquids and the results are observed.
C. Chromic Acid Test
1. 1 mL chromic acid solution is placed respectively to 2 mL portions of ethanol, isopropyl alcohol and t-butyl alcohol in dry test tubes.
2. Observation of any changes in color is made.
3. The procedures above are repeated by using 2 unknown liquids and the results are observed.
D. Reaction with Metallic Sodium
1. A small piece of sodium metal is placed to 2 mL portions of ethanol, isopropyl alcohol and t-butyl alcohol in dry test tubes respectively and any reaction that occurs is recorded.
2. What gas is formed from the reaction? The rate of gas evolution relative to each alcohol is compared. The alcohols are arranged in order of increasing acidity based on the results and the reason is explained.
3. The procedures above are repeated by using 2 unknown liquids and the results are observed. (Caution: The chemicals of the tubes should not be abandoned with un-reacted sodium down the drain as reaction between sodium and water is violent. Un-reacted sodium is cleaned by placing enough ethanol.)
Results
A. Solubility of Alcohols
B. Lucas Test
C. Chromic Acid Test
D. Reaction with Metallic Sodium
Discussion
A. Solubility of Alcohols
Based on the table above, it can be observed that there was one layer formed in each solution containing each kind of alcohol. This represents that all alcohols used in this experiment are water-soluble. This is because hydrogen bonds can be formed by hydroxyl groups in all those alcohol molecules with other alcohol molecules and with water molecules (Britannica, n.d.).
As hydroxyl group hydrogen bond with water and allow alcohol to be more water-soluble, it can be classified as a hydrophilic group (alcohol - Physical properties of alcohols, 2021). Molecular weight of alcohol is inversely proportional to its solubility in water since the hydrophobic hydrocarbon part in its molecule is proportional to its molecular weight (alcohol - Physical properties of alcohols, 2021). As solvent, alcohols are better than hydrocarbons for ionic substances and other polar compounds since alcohols are strong in polarity (alcohol - Physical properties of alcohols, 2021).
By testing solubility of those alcohols, it is hard to determine the identity of unknowns since one layer formed in each solution containing each kind of alcohol those alcohols and proved that all of them showed the same physical characteristic which is soluble in water.
B. Lucas Test
Based on the table above, it can be known that solution containing ethanol remained clean and not cloudy. This indicated the ethanol did not undergo any reaction in this test. The reason is a 1° alcohol usually generate unstable carbocation in this test, and occurrence of the carbocation is difficult (Identifying Alcohols | Protocol, 2021). Thus, no reaction will be observed at room temperature (Identifying Alcohols | Protocol, 2021).
According to the observation stated above, the solution containing isopropyl alcohol becomes slightly cloudy after placed in water bath. It can be known that isopropyl alcohol underwent a slight reaction in this test. This is because a carbocation was formed by 2° alcohol and its stability lower than the carbocation formed by 3° alcohols (Identifying Alcohols | Protocol, 2021). Placing the solution in a water bath will allow the obvious reaction occurs.
As observed during the experiment, it can be known that the solution containing unknown A was cloudy after placed in water bath. This shows that unknown A underwent a similar reaction as the reaction isopropyl alcohol underwent. Although the observation is not exactly same as the observation for solution containing isopropyl alcohol, it still can be proved that unknown A showed the similar chemical characteristic as isopropyl alcohol in this test.
Based on the table above, the observation for solution containing t-butyl alcohol instantly becomes cloudy. It can be indicated that t-butyl alcohol underwent an obvious reaction. This is because when adding Lucas reagent, a stable carbocation will be generated by 3° alcohol, and a swift reaction will occur (Identifying Alcohols | Protocol, 2021). Since the final halogenated product does not soluble in water, second phase will occur in the solution (Identifying Alcohols | Protocol, 2021).
According to the observation above, it can be known that the solution containing unknown B becomes slightly cloudy after placed in water bath for 5 minutes. The result is different with expected and it indicates that unknown B did not undergo same reaction as the reaction t-butyl alcohol underwent. The observation for solution containing unknown B should be same as the observation for solution containing t-butyl alcohol based on results from other tests. In this test, unknown B did not show the same chemical characteristic as t-butyl alcohol.
The possible reason is random error. Changes without knowing and cannot be predicted in the experiment will cause random errors in results obtained (Ooi, 2021, p.26). In the measuring instruments or in the environmental conditions, the changes may appear (Ooi, 2021, p.26). To reduce random errors, suitable measure of spread can be applied (Ooi, 2021, p.26).
C. Chromic Acid Test
According to the table above, solution containing ethanol turned from colorless to blue green. It can be known that ethanol reduced the Cr6+ in chromic acid to Cr3+. At the same time, chromic acid oxidized ethanol to a carboxylic acid.
Based on the observation above, solution containing isopropyl alcohol turned from colorless to blue. This indicated that isopropyl alcohol reduced the Cr6+ in chromic acid to Cr3+. Chromic acid oxidized isopropyl alcohol to a ketone at the same time.
Based on the table above, it can be known that solution containing unknown A turned from colorless to blue. This result is completely same as the result for isopropyl alcohol. This shows that unknown B underwent the same reaction as the reaction t-butyl alcohol underwent. Thus, it can be known that unknown B shows the same chemical characteristic as t-butyl alcohol during this test.
As observed during the test, solution containing t-butyl alcohol turned from colorless to orange. This shows that t-butyl alcohol did not react with chromic acid. This is because chromic acid cannot oxidize t-butyl alcohol.
According to the observation above, it can be known that solution containing unknown B turned from colorless to orange. The result is completely same as the result for t-butyl alcohol. This indicated unknown B underwent the same reaction as the reaction t-butyl alcohol underwent. Thus, it can be known that unknown B shows the same chemical characteristic as t-butyl alcohol during this test.
D. Reaction with Metallic Sodium
Based on the table above, it can be known that the rate of gas evolution in solution containing ethanol is the highest among all the compounds. This shows that the reaction between ethanol and sodium metal was the most obvious between all the reactions in this part.
According to the observation above, the rate of gas evolution in solution containing isopropyl alcohol is high. This indicated that isopropyl alcohol reacted with sodium metal obviously.
As observed during the test, the rate of gas evolution in solution containing unknown A is high. This result is same as the result for solution containing isopropyl alcohol. This shows that unknown A underwent the same reaction as the reaction isopropyl alcohol underwent. Based on this, it can be known that unknown A shows the same chemical characteristic as isopropyl alcohol during this test.
According to the observation above, the rate of gas evolution in solution containing t-butyl alcohol is low. This indicated that reaction between t-butyl alcohol and sodium metal was not obvious.
As observed during the test, the rate of gas evolution in solution containing unknown B is low. This result is same as the result for solution containing t-butyl alcohol. This shows that unknown B underwent the same reaction as the reaction t-butyl alcohol underwent. Based on this, it can be known that unknown B shows the same chemical characteristic as t-butyl alcohol during this test.
From 1° alcohol to 3° alcohol, acidity of alcohols is lessened (Reactions of Alcohols, 2021). Increasing electron density of oxygen atom on more highly substituted alcohol, and steric resistance (caused by alkyl groups that can restrain solvation of the final alkoxide ion) (Reactions of Alcohols, 2021). The activation energy to remove proton is increased by these situations (Reactions of Alcohols, 2021).
From 1° alcohol to 2° alcohol and 3° alcohol, the basicity of alkoxide ions enhances because a weak acid has a strong conjugate base (Reactions of Alcohols, 2021). Acidity of the acid is inversely proportional to strength of the conjugate base (Reactions of Alcohols, 2021).
Precaution steps
1. Ethanol, isopropyl alcohol and t-butyl alcohol are flammable so avoid putting them near the fire or at high temperature place.
2. Do not ingest isopropyl alcohol and t-butyl alcohol as they are irritant chemicals.
3. Treat the chromic acid properly before discarding it because it can cause environmental hazard.
4. Wearing safety googles and lab coat is necessary when conducting an experiment.
5. Wear shoes or gloves to prevent legs from any hurt caused by chemicals.
Conclusion
In this experiment, test of solubility of alcohols, Lucas Test, Chromic Acid Test and Reaction of Alcohols with Metallic Sodium were conducted. Based on the observations for different compounds from different tests, the identity of unknown A can be determined as a secondary alcohol because it shows same chemical characteristics as isopropyl alcohol in different tests conducted in this experiment. The identity of unknown B can be determined as a tertiary alcohol since it shows same chemical characteristics as t-butyl alcohol in different tests conducted in this experiment except Lucas Test. In Lucas Test, the result of solution containing unknown B is different with the result of solution containing t-butyl alcohol. The possible reason is random error.
Reference
1. Universiti Tunku Abdul Rahman-Department of Chemical Science, 2021. Chemistry Laboratory II Lab Manual May 2020. Kampar: Universiti Tunku Abdul Rahman.
2. Ooi, Z.X., 2021. Topic 5: Handling of Analytical Data. Kampar: Universiti Tunku Abdul Rahman.
3. Encyclopedia Britannica. 2021. alcohol - Physical properties of alcohols. [online] Available at: [Accessed 22 August 2021].
4. Jove.com. n.d. Identifying Alcohols | Protocol. [online] Available at: [Accessed 21 August 2021].
5. Cliffsnotes.com. n.d. Reactions of Alcohols. [online] Available at: [Accessed 21 August 2021].
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