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Ch.9 - Alkynes
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh.9 - AlkynesProblem 41a,b
Chapter 9, Problem 41a,b

The following functional-group interchange is a useful synthesis of aldehydes.
Reaction diagram showing the conversion of a terminal alkyne to an aldehyde with structural formulas labeled.
(a) What reagents were used in this chapter for this transformation? Give an example to illustrate this method.
(b) This functional-group interchange can also be accomplished using the following sequence.
Chemical reaction diagram illustrating alkyne hydroboration and functional-group interchange to synthesize aldehydes.
Propose mechanisms for these steps.

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Step 1: Analyze the transformation from a terminal alkyne (R-C≡C-H) to an aldehyde (R-CH₂-CHO). This involves converting the triple bond into a single bond and introducing an oxygen atom to form the aldehyde functional group.
Step 2: For part (a), the reagents used for this transformation in the chapter are typically hydroboration-oxidation. The hydroboration step uses a reagent like disiamylborane (R₂BH) or 9-BBN to add boron to the less substituted carbon of the alkyne. The oxidation step uses hydrogen peroxide (H₂O₂) in a basic solution (NaOH) to replace the boron with an oxygen atom, forming the aldehyde.
Step 3: For part (b), the sequence provided involves two steps. First, the terminal alkyne reacts with sodium ethoxide (NaOCH₂CH₃) in ethanol (CH₃CH₂OH). This generates an enolate intermediate, where the triple bond is converted into a double bond and an ethoxy group (-OCH₂CH₃) is added to the carbon adjacent to the terminal carbon.
Step 4: The second step involves hydrolysis of the enolate intermediate using an acid (H₃O⁺). The ethoxy group (-OCH₂CH₃) is replaced by a hydroxyl group (-OH), and tautomerization occurs to form the aldehyde (R-CH₂-CHO).
Step 5: Propose mechanisms for each step. In the first step, the terminal alkyne undergoes nucleophilic attack by the ethoxide ion, forming the enolate intermediate. In the second step, acid-catalyzed hydrolysis replaces the ethoxy group with a hydroxyl group, followed by tautomerization to yield the aldehyde. Tautomerization involves proton transfer and rearrangement of the double bond to stabilize the molecule.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. Understanding functional groups is essential for predicting the behavior of organic compounds during chemical reactions, such as the transformation of one functional group into another, as mentioned in the question.
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Reagents in Organic Synthesis

Reagents are substances or compounds added to a system to cause a chemical reaction or to test if a reaction occurs. In organic synthesis, specific reagents are used to facilitate the conversion of one functional group to another, such as converting a ketone to an aldehyde. Identifying the correct reagents is crucial for successful synthesis.
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Reaction Mechanisms

A reaction mechanism is a step-by-step description of the process by which reactants are converted into products. It outlines the sequence of bond-breaking and bond-forming events that occur during a chemical reaction. Proposing mechanisms for the transformations mentioned in the question requires a solid understanding of how electrons move and how intermediates are formed.
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Related Practice
Textbook Question

Using any necessary inorganic reagents, show how you would convert acetylene and isobutyl bromide to

(b) (±)-2,7-dimethyloctane-4,5-diol.

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Textbook Question

When treated with hydrogen and a platinum catalyst, an unknown compound X absorbs 5 equivalents of hydrogen to give n-butylcyclohexane. Treatment of X with an excess of ozone, followed by dimethyl sulfide and water, gives the following products:

Propose a structure for the unknown compound X. Is there any uncertainty in your structure?

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Textbook Question

The following functional-group interchange is a useful synthesis of aldehydes.

(c) Explain why a nucleophilic reagent such as ethoxide adds to an alkyne more easily than it adds to an alkene.

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Textbook Question

Show how you would convert the following starting materials into the target compound. You may use any additional reagents you need.

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Textbook Question

When compound Z is treated with ozone, followed by dimethyl sulfide and washing with water, the products are formic acid, 3-oxobutanoic acid, and hexanal.

Propose a structure for compound Z. What uncertainty is there in the structure you have proposed?

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Textbook Question

Using any necessary inorganic reagents, show how you would convert acetylene and isobutyl bromide to

(a) meso-2,7-dimethyloctane-4,5-diol, (CH3)2CHCH2CH(OH)CH(OH)CH2CH(CH3)2.

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