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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 8c,d
Chapter 9, Problem 8c,d

Show how you would synthesize each compound, beginning with acetylene and any necessary additional reagents
(c)
(d)

Verified step by step guidance
1
Step 1: Begin with acetylene (C≡CH) as the starting material. Deprotonate acetylene using a strong base such as sodium amide (NaNH₂) to generate the acetylide anion (C≡C⁻). This anion is highly nucleophilic and can be used for alkylation reactions.
Step 2: For compound c (2-phenylbut-3-yn-2-ol), perform an alkylation reaction by reacting the acetylide anion with benzyl bromide (PhCH₂Br) to introduce the phenyl group. This forms phenylacetylene (PhC≡CH).
Step 3: React phenylacetylene with acetone (CH₃COCH₃) in the presence of a strong base such as sodium amide (NaNH₂) or potassium tert-butoxide (KOtBu). The acetylide anion attacks the carbonyl carbon of acetone, forming the alcohol group at the 2-position and extending the chain to create 2-phenylbut-3-yn-2-ol.
Step 4: For compound d (3-methylhex-4-yn-3-ol), perform a similar alkylation reaction. React the acetylide anion with ethyl bromide (CH₃CH₂Br) to introduce the ethyl group, forming ethylacetylene (CH₃CH₂C≡CH).
Step 5: React ethylacetylene with acetone (CH₃COCH₃) in the presence of a strong base such as sodium amide (NaNH₂) or potassium tert-butoxide (KOtBu). The acetylide anion attacks the carbonyl carbon of acetone, forming the alcohol group at the 3-position and extending the chain to create 3-methylhex-4-yn-3-ol.

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Key Concepts

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

Alkyne Reactivity

Alkynes are hydrocarbons containing at least one carbon-carbon triple bond. They are more reactive than alkenes and alkanes due to the high electron density of the triple bond, which can participate in various reactions such as addition, substitution, and polymerization. Understanding the reactivity of alkynes is crucial for synthesizing compounds like 2-phenylbut-3-yn-2-ol and 3-methylhex-4-yn-3-ol.
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Alkyne Hydration

Hydroboration-Oxidation

Hydroboration-oxidation is a two-step reaction used to convert alkynes into alcohols. In the first step, the alkyne reacts with diborane (B2H6) to form an organoborane intermediate. The second step involves oxidation with hydrogen peroxide (H2O2) and sodium hydroxide (NaOH), resulting in the formation of an alcohol. This method is particularly useful for synthesizing terminal and internal alcohols from alkynes.
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General properties of hydroboration-oxidation.

Grignard Reagents

Grignard reagents are organomagnesium compounds that are highly reactive and can be used to form carbon-carbon bonds. They are created by reacting magnesium with an alkyl or aryl halide. In the synthesis of compounds like 2-phenylbut-3-yn-2-ol, Grignard reagents can be employed to introduce functional groups or to extend carbon chains, making them essential for constructing complex organic molecules.
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Related Practice
Textbook Question

When 2,2-dibromo-1-phenylpropane is heated overnight with sodium amide at 150°C, the major product (after addition of water) is a different foul-smelling compound of formula C9H8. Propose a structure for this product, and give a mechanism to account for its formation.

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Textbook Question

Show how you might synthesize the following compounds, using acetylene and any ­suitable alkyl halides as your starting materials. If the compound given cannot be ­synthesized by this method, explain why.

d. 4-methylhex-2-yne

e. 5-methylhex-2-yne

f. cyclodecyne

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Textbook Question

Show how you would synthesize each compound, beginning with acetylene and any necessary additional reagents

(a)

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Textbook Question

When 2,2-dibromo-1-phenylpropane is heated overnight in fused KOH at 200 C, the major product is a foul-smelling compound of formula C9H8. Propose a structure for this product, and give a mechanism to account for its formation.

Problem-Solving Hint:

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Textbook Question

Show how you would synthesize each compound, beginning with acetylene and any necessary additional reagents

(b)

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Textbook Question

Show how you would synthesize 2-phenylhex-3-yn-2-ol, starting with acetophenone (PhCOCH3) and any other reagents you need. ("2-ol" means there is an OH group on C2.)

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