Passage
Students used three classification tests to study the reactivity of alcohol, aldehyde, and ketone functional groups in organic compounds. The organic compounds provided to the students are given in Table 1.Table 1 Organic Compounds Tested
First, the students performed the Lucas test by mixing a few drops of each compound with 2 mL of the Lucas reagent (ZnCl₂ and HCl) . This reagent is used to test for secondary and tertiary alcohols. A positive test result is indicated by a cloudy reaction mixture upon addition of the Lucas reagent due to the formation of an insoluble compound by an S_N1 reaction with a secondary or tertiary alcohol (Reaction 1) .
Reaction 1Next, a test for alcohols and aldehydes was performed by the addition of a drop of chromic acid reagent (CrO₃ dissolved in aqueous H₂SO₄) into samples of the organic compounds. The disappearance of the orange color from chromic acid and formation of a blue-green suspension is indicative of a positive result for this test. Compound A was found to be an example of a positive result, as shown in Reaction 2.
Reaction 2Finally, samples of the compounds were also dissolved in water, followed by the addition of NaOH and iodine. This test is known as the iodoform test, which is used to identify methyl ketones and secondary alcohols. A positive result is indicated by the formation of iodoform (CHI₃) , a yellow precipitate (Reaction 3) .
3Table 2 Selected Observations from the Classification Tests

-An unknown compound containing both a secondary alcohol and an aldehyde in its structure was given to a student. Suppose the Lucas test confirmed there was an alcohol present. If a protecting group were added to the alcohol, would a positive result from the chromic acid test be indicative of only the presence of the aldehyde?

Question

Passage
Students used three classification tests to study the reactivity of alcohol, aldehyde, and ketone functional groups in organic compounds. The organic compounds provided to the students are given in Table 1.Table 1 Organic Compounds Tested

First, the students performed the Lucas test by mixing a few drops of each compound with 2 mL of the Lucas reagent (ZnCl₂ and HCl) . This reagent is used to test for secondary and tertiary alcohols. A positive test result is indicated by a cloudy reaction mixture upon addition of the Lucas reagent due to the formation of an insoluble compound by an S_N1 reaction with a secondary or tertiary alcohol (Reaction 1) .

Reaction 1Next, a test for alcohols and aldehydes was performed by the addition of a drop of chromic acid reagent (CrO₃ dissolved in aqueous H₂SO₄) into samples of the organic compounds. The disappearance of the orange color from chromic acid and formation of a blue-green suspension is indicative of a positive result for this test. Compound A was found to be an example of a positive result, as shown in Reaction 2.

Reaction 2Finally, samples of the compounds were also dissolved in water, followed by the addition of NaOH and iodine. This test is known as the iodoform test, which is used to identify methyl ketones and secondary alcohols. A positive result is indicated by the formation of iodoform (CHI₃) , a yellow precipitate (Reaction 3) .

3Table 2 Selected Observations from the Classification Tests

-An unknown compound containing both a secondary alcohol and an aldehyde in its structure was given to a student. Suppose the Lucas test confirmed there was an alcohol present. If a protecting group were added to the alcohol, would a positive result from the chromic acid test be indicative of only the presence of the aldehyde?

Quizplus · Accepted Answer

11ecba2d_1291_c382_a3bf_c3470f4acf79_MD0008_00 Organic compounds often contain multiple functional groups that can both react under the same conditions. In cases where a reaction is desired in one functional group but not another functional group, protecting groups are utilized. Protecting groups are organic molecules added to a functional group to prevent it from reacting and forming an undesired product. Protecting groups tend to be stable under oxidizing and reducing conditions. After the desired reaction is complete, the protecting group can be removed to provide the original functional group. The chromic acid test is an oxidation reaction, and the alcohol and aldehyde would both react with chromic acid to give a positive result (blue-green suspension). Primary alcohols and aldehydes will be oxidized to carboxylic acids, and secondary alcohols will be oxidized to ketones. To confirm there is an aldehyde present in the unknown compound, the alcohol can be protected. Because protecting groups are unreactive under oxidizing conditions, only the aldehyde will be oxidized by chromic acid to form a carboxylic acid. (Choices A and B) An aldehyde is reactive under oxidizing conditions, and because the chromic acid test is an oxidation reaction, an aldehyde would give a positive result. Because the protected alcohol cannot react and there are no other reactive functional groups present, a positive result is solely indicative of an aldehyde in this compound. (Choice D) A protected alcohol is unreactive under oxidizing conditions, and therefore would not react in the chromic acid test. Educational objective: Chromic acid is an oxidizing agent that can oxidize primary alcohols and aldehydes to carboxylic acids and secondary alcohols to ketones. Protecting groups are organic molecules added to a functional group to prevent them from reacting under a given set of reaction conditions. Protecting groups tend to be stable against oxidation and reduction.

Passage Students Used Three Classification Tests to Study the Reactivity of of Alcohol