Textbook Question
For the alkynes shows here, show the product(s) expected to form when treated under the following conditions: (vii) 1. BH3 2. H2O2, NaOH. If you expect two products, show both.
(c)
10. Addition Reactions
Alkyne Hydroboration
3:59 minutesProblem 41a,b
Textbook Question
The following functional-group interchange is a useful synthesis of aldehydes.
(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.
Propose mechanisms for these steps.
Verified step by step guidance1
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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