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Lab 6 Classification of Alcohols!

Objectives! • •

Use qualitative tests to study and distinguish primary, secondary, and tertiary alcohols Classify an unknown alcohol

Background Properties of Alcohols Alcohols are a class of organic compounds that have a hydroxyl, -OH, group in the structure. Because there is an O-H bond present in alcohols, they have the ability to hydrogen bond, which has a big effect on their properties compared to other organic molecules. The hydrogen bonding between molecules of the same alcohol lead to higher melting points and boiling points. Hydrogen bonding between an alcohol molecule and water molecules, as shown in Figure 1, greatly increases the solubility of alcohols compared to hydrocarbons. The solubility of each alcohol also depends on the length of the hydrocarbon chain, with shorter chains leading to more water-soluble alcohols.

Figure 1 Hydrogen bonding of methanol in water

The reactivity of an alcohol generally depends on is classification as a primary, secondary, or tertiary alcohol. Examples of each are given in Figure 2.

Figure 2 Examples of primary, secondary, and tertiary alcohols

A primary alcohol has at least two hydrogens attached to the same carbon as the hydroxyl group, so the hydroxyl group is at the end of the molecule. Primary alcohols are the easiest to oxidize as they have multiple C-H bonds available by the hydroxyl group. A primary alcohol can be oxidized twice, first to an aldehyde and then again to a carboxylic acid. In contrast, primary alcohols are the least reactive toward any reaction that involves the hydroxyl group acting as a leaving group. The

Lab 6 carbocation intermediate formed is not stable as the carbon has at most one C-C to help stabilize it. A secondary alcohol has two carbons and one hydrogen attached to the same carbon as the hydroxyl group. Therefore, the hydroxyl group is attached to a carbon in the middle of a chain. A secondary alcohol is moderately reactive toward both oxidation and the hydroxyl group acting as a leaving group. A secondary alcohol has only one C-H bond to be oxidized, so the oxidation of secondary alcohol results in a ketone. When a carbocation intermediate is formed from a secondary alcohol has two C-C bonds to help stabilize it. A tertiary alcohol has three carbons attached to the same carbon as the hydroxyl group. There are no hydrogens available on the carbon with the hydroxyl group. Therefore, a tertiary alcohol can generally not be oxidized as oxidation would involve breaking C-C bonds. However, tertiary alcohols are most reactive in reactions that involve a carbocation intermediate, as there are three C-C bonds to increase stability.

Qualitative Tests to Distinguish Alcohols Functional groups with different reactivity, like primary, secondary, and tertiary alcohols, can be distinguished by qualitative tests. In general, qualitative tests have some apparent visual change indicating when a reaction has occurred. To best understand qualitative tests, it is important to identify a positive and negative result by testing with known compounds. Control tests ensure that the reagents are working properly and that you know what the different results look like.

Ceric Ammonium Nitrate Test for Alcohols Ceric ammonium nitrate, known as CAN, reacts with all types of alcohols, so it is a good test to confirm the presence of the hydroxyl group. CAN is an ammonium salt of a cerium and nitrate complex ion that can be made from cerium(III) oxide and hot nitric acid. CAN is also available as a prepared reagent that appears yellow. When an aliphatic alcohol is present, some of the nitrate ligands in CAN are replaced with alkoxy ligands, changing the solution color to a reddish or even magenta color, as shown in Figure 3. The exact color will depend on the alcohol present. Phenolic compounds also react with CAN, but the presence of the aromatic ring changes the color of the complex. Rather than the reddish appearance of aliphatic alcohols, phenolic alcohols appear brownish black.

Figure 3 Reaction that occurs in a positive CAN test

Lab 6

Ferric Chloride Test for Phenols The ferric chloride test can be used to distinguish phenols from other alcohols based on the interaction of the iron ion with the phenol. Phenols are a class of alcohols in which the hydroxyl group is attached to a benzene ring. When a phenol is mixed with an ferric chloride solution, the phenol can add as a deprotonated ligand to the iron ion, as shown in Figure 4. The resulting complex with phenol is a purple or violet color. If a sample does not contain a phenol, even if it contains another type of alcohol, the solution will stay the yellow brown color of the starting ferric chloride.

Figure 4 Reaction that occurs in a positive ferric chloride test

Chromic Acid Structural Test for Primary and Secondary Alcohols The chromic acid test is an oxidation that works with primary and secondary alcohols. Tertiary alcohols do not react with the chromic acid test because an oxidation would require breaking a CC bond. The general form of the chromic acid test, shown in Figure 5, starts with an alcohol and uses chromium(VI) to oxidize the alcohol to a carboxylic acid, for primary alcohols, or a ketone, for secondary alcohols. The counterpart in each successful oxidation is the reduction of chromium(VI) to chromium(IV). Two chromium(IV) cations undergo a disproportionation reaction to form a chromium(V) ion and the stable chromium(III) ion. The chromium(III) ion turns the solution a shade of green dependent on the other compounds present.

Figure 5 Reaction that occurs in a positive chromic acid test with a primary alcohol

The chromium(VI) ion used in the test can be prepared in a couple ways. One way is to use chromium trioxide and sulfuric acid. The other way, used in this lab, is to slowly add concentrated acid to a sodium dichromate solution. Once the chromic acid reagent is prepared, its addition to an aldehyde, results in the formation of the greenish chromium(III) ion. Note that the reagent is aqueous so the test may result in two layers. A green color in either layer represents a positive test.

Lab 6

Iodoform Test for Oxidizable Alcohols with Methyl Substituents In the iodoform test, a methyl group next to an oxidizable alcohol group is used to form insoluble iodoform. Thus, the iodoform test generally indicates a secondary alcohol with a methyl group next to the hydroxyl group. Ethanol is the only primary alcohol that fits the criteria for the reaction. No tertiary alcohol can be oxidized with iodoform, as the alcohol has no C-H bond on the hydroxyl carbon.

Figure 6 Reaction that occurs in a positive iodoform test

To start the test with an alcohol, iodine and sodium hydroxide are mixed to form sodium hypoiodite, as shown in Figure 6. The hypoiodite ion oxidizes the alcohol to the corresponding carbonyl with a methyl group. The iodine can then react with the methyl group, replacing the hydrogens with iodines. The triiodomethyl group is now a better leaving group in the presence of the hydroxide. When the triiodomethyl group leaves, insoluble iodoform, CHI3, forms as a yellow precipitate.

Lucas’s Test to Distinguish Alcohol Type Lucas’s test reacts with all alcohols but on different time scales based on whether it is a primary, secondary, or tertiary alcohol. Lucas’s test uses zinc chloride and hydrochloric acid to create an alkyl chloride. For secondary and tertiary alcohols, the reaction proceeds via an SN1 mechanism, as shown in Figure 7. A tertiary alcohol reacts faster because it can form a more stable carbocation. Primary alcohols do not form stable carbocation intermediates, so they often do not react at all.

Figure 7 Reaction that occurs in a positive Lucas’s test

The addition of Lucas’s reagent to a tertiary alcohol such as tert-butyl alcohol results in the quick formation of the alkyl chloride, which presents as an oily, cloudy appearance. This happens due to the insolubility of alkyl halides in water. The test is also often successful with secondary alcohols, such as isopropanol. However, the oily, cloudy appearance is not immediate and might require a

Lab 6 few minutes in a water bath. If an alcohol is present but no alkyl chloride appears after a few minutes, it is likely a primary alcohol. You can prove this by placing the test tube in a hot water bath. The oily, cloudy alkyl chloride might appear for a primary alcohol after heating, but it often does not.

Lab 6

Materials • • • • • • • • •

Ethanol 2-Propanol 1-Butanol 2-Pentanol 2-Methyl-2-propanol 1-Octanol Phenol 1-Propanol Unknown alcohol

• • • • • • • •

Ceric ammonium nitrate solution 0.1M FeCl3 6M HCl 10% sodium dichromate 3M NaOH Iodine solution Lucas’s reagent Test tubes

Safety goggles are required! All work should be performed in the fume hood. Alcohols are flammable and should be handled with care and disposed of properly. Lucas’s reagent is made with concentrated hydrochloric acid, which is corrosive; wash thoroughly if skin contact is made. Chromic acid is corrosive and toxic, and chromium is a possible carcinogen.

Procedure Qualitative Tests to Distinguish Alcohols You will be assigned at least one test to perform for the known positive and the known negative test as well as performing all tests on your unknown. You will compare your results with the group or groups that performed the known positive and known negative tests.

Solubility Tests 1. Write the identification code for your unknown on the space provided on the data sheet. 2. Place 0.5 mL of each of the following alcohols in separate test tubes: ethanol, 2-propanol, 1-butanol, 2-pentanol, 2-methyl-2-propanol, 1-octanol, and your unknown. 3. Add 1 mL of deionized water to each test tube. 4. Shake the contents of each tube well to see if they mix. Record whether each alcohol is soluble, insoluble, or slightly soluble in water on the data sheet. 5. Dump the contents of the tubes in the non-halogenated waste container, and wash the test tubes with soap and water.

Ceric Ammonium Nitrate Test for Alcohols 1. Place 10 drops of the following in separate test tube: ethanol, 2-propanol, and your unknown.

Lab 6 2. Add 10 drops of ceric ammonium nitrate solution to each test tube, and record your observations. 3. Discard the solutions in the non-halogenated waste container, and wash the test tubes with soap and water.

Ferric Chloride Test for Phenols 1. Place 10 drops of the following in separate test tubes: ethanol, phenol, and your unknown. 2. Add 10 drops of the 0.1 M FeCl3 solution to each test tube, and record your observations. 3. Dump the contents of the tubes in the halogenated waste container, and wash the test tubes with soap and water.

Chromic Acid Structural Test for Primary and Secondary Alcohols 1. Place 1 mL of the following in separate test tubes: ethanol, 2-propanol, 2-methyl-2propanol, and your unknown. 2. Add 10 drops of 6M HCl to each test tube, and shake well. 3. Add 1 to 2 drops of 10 % Na2Cr2O7, and shake the mixture well again. Record your observations for each. 4. Dump the contents of the tubes in the halogenated waste container, and wash the test tubes with soap and water.

Iodoform Test for Oxidizable Alcohols with Methyl Substituents 1. Place 3 to 5 drops of each the following to separate test tubes: ethanol, 1-propanol, 2propanol, 2-methyl-2-propanol, and your unknown. 2. Dilute the contents of each test tube with 2 mL of water, and mix well. 3. Add 2 mL of 3 M NaOH to each test tube, and mix well again. 4. Add 3 mL of the iodine solution to each test tube, and mix well once more. Record your observations for each. 5. Dispose of the contents of the tubes in the halogenated waste container, and wash the test tubes with soap and water.

Lucas’s Test to Distinguish Alcohol Type 1. Add 1 mL of the following to separate test tubes: ethanol, 2-propanol, 2-methyl-2propanol, and your unknown. 2. Add 10 mL of Lucas’s reagent to each tube and shake to mix well. 3. Measure the amount of time that is needed to form a cloudy layer in each test tube. If there is no reaction in 15 minutes, assume the sample is a primary alcohol. Record your observations and times on the data sheet. 4. Dispose of the content of the tube in the halogenated waste container, and wash the test tube with soap and water.

Lab 6

Pre-Lab Questions Prepare for lab by completing and understanding the answers to these questions. Refer to the Background or another resource, such as your textbook, if necessary. 1. What is the functional group that determines if a compound is an alcohol?

2. What observation is expected for a positive iodoform test?

3. Name and draw an example of a primary, secondary, and tertiary alcohol.

4. What factor is associated with the increase in boiling point of alcohols?

5. What factor is associated with the increase in solubility of alcohols?

Lab 6 Report Sheet

Lab 6: Classification of Alcohols Report Sheet Name _______________________ Date _______________________

Section Instructor

_______________________ _______________________

Qualitative Tests to Distinguish Alcohols UNKNOWN CODE: __________________________

Solubility Tests Alcohol

Water solubility

ethanol

_____________________________________

2-propanol

_____________________________________

1-butanol

_____________________________________

2-pentanol

_____________________________________

2-methyl-2-propanol

_____________________________________

1-octanol

_____________________________________

Unknown

_____________________________________

Ceric Ammonium Nitrate Test for Alcohols Alcohol

Observations

ethanol

_____________________________________

2-propanol

_____________________________________

Unknown

_____________________________________

Ferric Chloride Test for Phenols Alcohol

Observations

ethanol

_____________________________________

phenol

_____________________________________

Unknown

_____________________________________

Lab 6 Report Sheet

Chromic Acid Structural Test for Primary and Secondary Alcohols Alcohol

1o, 2o, or 3o?

Observations

ethanol

_______

_____________________________

2-propanol

_______

_____________________________

2-methyl-2-propanol

_______

_____________________________

Unknown

_______

_____________________________

Iodoform Test for Oxidizable Alcohols with Methyl Substituents Alcohol

Observations

1-propanol

_____________________________________

2-propanol

_____________________________________

ethanol

_____________________________________

2-methyl-2-propanol

_____________________________________

Unknown

_____________________________________

Lucas’s Test to Distinguish Alcohol Type Alcohol

1o, 2o, or 3o?

Observations

ethanol

_______

_____________________________

2-propanol

_______

_____________________________

2-methyl-2-propanol

_______

_____________________________

Unknown

_______

_____________________________

Describe the information you have gathered from the tests about your unknown.

Lab 6 Report Sheet

Post-lab Questions 1. If an alcohol is oxidized by chromic acid and does not react with Lucas’s reagent, what class of alcohol is it? Explain.

2. Draw the structure of 2-methyl-2-pentanol.

3. Predict the results of each following test with 2-methyl-2-propanol. a. Chromic acid test

b. Lucas’s test

c. Iodoform test

4. Is your unknown a primary, secondary, or tertiary alcohol?...