Ch. 10 - Alkynes: Electrophilic Addition and Redox Reactions

Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition

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Mullins 1st Edition

Ch. 10 - Alkynes: Electrophilic Addition and Redox Reactions

Problem 38

Chapter 9, Problem 38

Draw the ketone(s) you would expect to form by treating the following alkynes under the conditions of hydroboration–oxidation: (a) 6-methyloct-1-yne, (b) 1,10-dicyclohexyldec-5-yne, and (c) 5-phenylhex-2-yne.
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Verified step by step guidance

Step 1: Understand the reaction mechanism. Hydroboration–oxidation of alkynes involves the addition of borane (BH₃) to the triple bond, followed by oxidation with hydrogen peroxide (H₂O₂) in the presence of sodium hydroxide (NaOH). This reaction converts the alkyne into a ketone via anti-Markovnikov addition.

Step 2: Analyze the structure of the alkyne. For each alkyne provided, identify the triple bond and determine the regioselectivity of the reaction. Hydroboration–oxidation typically results in the formation of a ketone at the less substituted carbon of the triple bond.

Step 3: For (a) 6-methyloct-1-yne, locate the triple bond between carbons 1 and 2. The hydroboration–oxidation will yield a ketone at the terminal carbon (C1), as it is less substituted. Draw the resulting ketone structure.

Step 4: For (b) 1,10-dicyclohexyldec-5-yne, locate the triple bond between carbons 5 and 6. The hydroboration–oxidation will yield a ketone at carbon 5, as it is less substituted compared to carbon 6. Draw the resulting ketone structure.

Step 5: For (c) 5-phenylhex-2-yne, locate the triple bond between carbons 2 and 3. The hydroboration–oxidation will yield a ketone at carbon 2, as it is less substituted compared to carbon 3. Draw the resulting ketone 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 alkynes into carbonyl compounds, specifically ketones or aldehydes. 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) and a base (NaOH), which converts the boron atom into a hydroxyl group, yielding the corresponding ketone or aldehyde.

Alkyne Reactivity

Alkynes are hydrocarbons containing a carbon-carbon triple bond, which makes them highly reactive. Their reactivity allows them to undergo various addition reactions, including hydroboration. The position of the triple bond and the substituents on the alkyne influence the regioselectivity of the reaction, determining whether a ketone or aldehyde is formed after oxidation.

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 regioselectivity is influenced by the sterics and electronics of the substituents on the alkyne. This concept is crucial for predicting the specific ketone products formed from the given alkynes in the question.

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Related Practice

Textbook Question

For each of the following ketones/aldehydes, indicate whether it is possible to synthesize it from an alkyne as the only compound in good ( > 50%) yield. If so, how would you do it?

(a)

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Textbook Question

For each of the following ketones/aldehydes, indicate whether it is possible to synthesize it from an alkyne as the only compound in good (> 50%) yield. If so, how would you do it?

(d)

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Textbook Question

Draw the ketone(s) you would expect to form by reacting the following alkynes under the conditions of oxymercuration: (a) 6-methyloct-1-yne, (b) 1,10-dicyclohexyldec-5-yne, and (c) 5-phenylhex-2-yne.

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Textbook Question

When doing synthesis, you will often find yourself repeating the same series of steps. To see this in action, synthesize the following aldehydes beginning with an organic molecule containing three carbons or fewer.

(b)

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Textbook Question

Predict the alkyne and reactants you might use to make the following haloalkenes. [Providing the reactant and the reagent is how we start thinking about synthesis.]

(a)

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Textbook Question

Predict the alkyne and reactants you might use to make the following haloalkenes. [Providing the reactant and the reagent is how we start thinking about synthesis.]

(b)

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Draw the ketone(s) you would expect to form by treating the following alkynes under the conditions of hydroboration–oxidation: (a) 6-methyloct-1-yne, (b) 1,10-dicyclohexyldec-5-yne, and (c) 5-phenylhex-2-yne. <IMAGE>

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Key Concepts

Hydroboration-Oxidation

Alkyne Reactivity

Regioselectivity

Draw the ketone(s) you would expect to form by treating the following alkynes under the conditions of hydroboration–oxidation: (a) 6-methyloct-1-yne, (b) 1,10-dicyclohexyldec-5-yne, and (c) 5-phenylhex-2-yne.
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