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

-Based on the information in the passage, which compound will react most rapidly with the Lucas reagent?

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

-Based on the information in the passage, which compound will react most rapidly with the Lucas reagent?

Quizplus · Accepted Answer

11ecba2d_128f_798a_a3bf_bd54d3361d62_MD0008_00 Substitution reactions can occur in one of two ways: an S_N1 or S_N2 mechanism. An S_N1 reaction occurs in two steps whereas an S_N2 reaction occurs in a single step. The first step of an S_N1 reaction involves formation of a carbocation intermediate when the bond between the leaving group and the adjoining carbon is broken. In the second step, the nucleophile adds to the carbocation, forming the product of the reaction. The first step is the slowest step (rate-determining step). Substrates that form a more stable carbocation react faster in an S_N1reaction than those that form a less stable carbocation. The passage states that the Lucas test occurs through an S_N1 mechanism. Because Compound C is a tertiary alcohol, it can form a more stable carbocation than Compounds A and B, which are a secondary and primary alcohol, respectively. Therefore, Compound C will react the most rapidly with the Lucas reagent. (Choice A) Compound A forms a secondary carbocation, which is less stable than the tertiary carbocation formed by a tertiary alcohol. Therefore, Compound A reacts more slowly than Compound C in the Lucas test. (Choice B) Compound B is a primary alcohol and would form a primary carbocation in the S_N1 reaction if it reacted with the Lucas reagent. Because primary carbocations are not stable, it is unfavorable for Compound B to undergo an S_N1 reaction. (Choice D) Compounds A and B react with the Lucas reagent at different rates due to carbocation stability. Compound B does not react with the Lucas reagent because it would form an unstable primary carbocation. Educational objective: An S_N1 reaction occurs in two steps. First, a carbocation intermediate is formed when the bond between the leaving group and adjoining carbon atom is broken; next, the nucleophile adds to the carbocation to form the reaction product. Substrates that form a more stable carbocation react faster than substrates that form a less stable carbocation.

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