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Chapter 7: Problem 26

Alkyne anions react with the carbonyl groups of aldehydes and ketones to form alkynyl alcohols, as illustrated by the following sequence.

Short Answer

Expert verified
Question: Explain the reaction between alkyne anions and carbonyl groups in aldehydes and ketones, and describe the product formed. Answer: The reaction between alkyne anions and carbonyl groups (present in aldehydes and ketones) is a nucleophilic addition reaction. The alkyne anion acts as a nucleophile, attacking the electrophilic carbonyl carbon. This process forms a carbocation intermediate, followed by the abstraction of a proton from the alpha carbon of the alkyne anion. Ultimately, this reaction produces alkynyl alcohols as the final product.

Step by step solution

01

Identify the reactants

Alkyne anions are negatively charged species containing a carbon-carbon triple bond, while aldehydes and ketones are carbonyl compounds in which a carbonyl group (C=O) is bonded to hydrogen or carbon atoms. For this specific reaction, let's choose a generic alkyne anion (\(RC\equiv C^-\)), an aldehyde (\(R_1CHO\)), and a ketone (\(R_2C=O\)) as reactants.
02

Understand the mechanism

The reaction between alkyne anions and carbonyl groups is a nucleophilic addition reaction. In this process, the nucleophile (the alkyne anion) attacks the electrophilic carbon of the carbonyl group. A carbocation intermediate is formed, followed by the abstraction of a proton from the alpha carbon of the alkyne anion, leading to the final product.
03

Reaction of alkyne anion with aldehyde

The alkyne anion attacks the electrophilic carbonyl carbon in the aldehyde, forming a carbocation intermediate. Next, the negatively charged alpha carbon (the carbon next to the carbonyl group) in the alkyne anion abstracts a proton from the hydrogen atom on the carbonyl group of the aldehyde, generating an alkynyl alcohol as the final product. $$ RC\equiv C^- + R_1CHO \rightarrow (R_1C)C(-OH)=CR $$
04

Reaction of alkyne anion with ketone

Similarly, the alkyne anion attacks the electrophilic carbonyl carbon in the ketone. The carbocation intermediate is formed followed by the abstraction of a proton from the alpha carbon of the alkyne anion, leading to an alkynyl alcohol product. $$ RC\equiv C^- + R_2C=O \rightarrow (R_2C)C(-OH)=CR $$
05

Summary

Alkyne anions react with carbonyl compounds (aldehydes and ketones) in a nucleophilic addition reaction to form alkynyl alcohols. The mechanism involves the attack of the alkyne anion on the electrophilic carbonyl carbon, followed by proton abstraction from the alpha carbon of the alkyne anion, resulting in the final alkynyl alcohol product.

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Most popular questions from this chapter

Rimantadine was among the first antiviral drugs to be licensed in the United States for use against the influenza \(A\) virus and in treating established illnesses. It is synthesized from adamantane by the following sequence (we discuss the chemistry of Step 1 in Chapter 8 and the chemistry of Step 5 in Section \(16.8 \mathrm{~A}\) ). Rimantidine is thought to exert its antiviral effect by blocking a late stage in the assembly of the virus. (a) Propose a mechanism for Step 2. Hint: As we shall see in Section 21.1A, reaction of a bromoalkane such as 1-bromoadamantane with aluminum bromide (a Lewis acid, Section 4.7) results in the formation of a carbocation and \(\mathrm{AlBr}_{4}{ }^{-}\). Assume that adamantyl cation is formed in Step 2 and proceed from there to describe a mechanism. (b) Account for the regioselectivity of carbon-carbon bond formation in Step 2 . (c) Describe experimental conditions to bring about Step \(S\). (d) Describe experimental conditions to bring about Step \(4 .\)

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