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Ch. 10 - Alkynes: Electrophilic Addition and Redox Reactions
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471Not the one you use?Change textbook
All textbooksMullins 1st EditionCh. 10 - Alkynes: Electrophilic Addition and Redox ReactionsProblem 44c(vii)
Chapter 9, Problem 44c(vii)

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) Structural representation of alkynes with products from hydroboration using BH3 and H2O2, NaOH.

Verified step by step guidance
1
Step 1: Recognize the reaction type. The given reagents (1. BH₃, 2. H₂O₂, NaOH) indicate a hydroboration-oxidation reaction. This reaction is used to convert an alkyne into a carbonyl compound (aldehyde or ketone) via anti-Markovnikov addition of water across the triple bond.
Step 2: Analyze the alkyne structure. Determine whether the alkyne is terminal (triple bond at the end of the chain) or internal (triple bond within the chain). This will influence the product formed. Terminal alkynes typically yield aldehydes, while internal alkynes yield ketones.
Step 3: Understand the mechanism. In the first step, BH₃ adds to the alkyne in an anti-Markovnikov fashion, forming a trialkylborane intermediate. In the second step, oxidation with H₂O₂ and NaOH replaces the boron atom with a hydroxyl group, leading to an enol intermediate.
Step 4: Account for tautomerization. The enol intermediate formed in the previous step is unstable and undergoes keto-enol tautomerization to form the more stable carbonyl compound. For terminal alkynes, this results in an aldehyde, while for internal alkynes, a ketone is formed.
Step 5: If the alkyne is symmetrical, only one product will form. If the alkyne is unsymmetrical, two different ketones may form due to the possibility of hydroboration occurring at either carbon of the triple bond. Draw the possible products based on the alkyne structure.

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Key Concepts

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

Hydroboration-Oxidation

Hydroboration-oxidation is a two-step reaction process used to convert alkenes and alkynes into alcohols. In the first step, borane (BH₃) adds across the triple bond of the alkyne, forming a trialkylborane intermediate. The second step involves oxidation with hydrogen peroxide (H₂O₂) in the presence of a base (NaOH), resulting in the formation of an alcohol. This reaction is notable for its anti-Markovnikov addition, where the hydroxyl group attaches to the less substituted carbon.
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Alkyne Reactivity

Alkynes are hydrocarbons containing at least one carbon-carbon triple bond, making them more reactive than alkenes due to the presence of π bonds. This reactivity allows them to undergo various addition reactions, including hydroboration. The nature of the substituents on the alkyne can influence the regioselectivity of the reaction, leading to different products based on the sterics and electronics of the substituents.
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Regioselectivity

Regioselectivity refers to the preference of a chemical reaction to yield one structural isomer over others when multiple products are possible. In the context of hydroboration-oxidation of alkynes, the reaction typically favors the formation of the less substituted alcohol due to the anti-Markovnikov addition mechanism. Understanding regioselectivity is crucial for predicting the major product(s) formed in reactions involving unsaturated hydrocarbons.
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Related Practice
Textbook Question

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