Ch.9 - Alkynes

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch.9 - Alkynes

Problem 4

Chapter 9, Problem 4

The boiling points of hex-1-ene (64 °C) and hex-1-yne (71 °C) are sufficiently close that it is difficult to achieve a clean separation by distillation. Show how you might use the acidity of hex-1-yne to remove the last trace of it from a sample of hex-1-ene.

Verified step by step guidance

Identify the structural difference between hex-1-ene and hex-1-yne. Hex-1-ene is an alkene with a double bond, while hex-1-yne is an alkyne with a triple bond.

Understand the acidity of alkynes. Terminal alkynes like hex-1-yne have acidic hydrogen atoms due to the sp hybridization of the carbon atom bonded to hydrogen, making them more acidic than alkenes.

Consider using a strong base to deprotonate hex-1-yne. A strong base such as sodium amide (NaNH₂) can be used to remove the acidic hydrogen from the terminal alkyne, forming a sodium acetylide.

Recognize that the sodium acetylide formed is a salt and is soluble in polar solvents like water, whereas hex-1-ene remains nonpolar and insoluble in water.

Separate the mixture by adding water to dissolve the sodium acetylide, leaving hex-1-ene as the nonpolar residue. This exploits the difference in solubility to achieve separation.

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Key Concepts

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

Acidity of Alkynes

Alkynes, such as hex-1-yne, are more acidic than alkenes due to the sp-hybridized carbon atom, which holds the negative charge more stably when a hydrogen is removed. This acidity allows alkynes to react with strong bases, forming acetylide ions, which can be exploited for separation purposes.

Distillation

Distillation is a separation technique that relies on differences in boiling points to separate components of a mixture. When the boiling points are close, as with hex-1-ene and hex-1-yne, achieving a clean separation becomes challenging, necessitating alternative methods such as chemical reactivity differences.

Chemical Reactivity for Separation

Using chemical reactivity differences, such as the acidity of hex-1-yne, can aid in separation. By treating the mixture with a strong base, hex-1-yne can be selectively deprotonated to form a salt, which can then be removed, leaving behind hex-1-ene. This method leverages the unique chemical properties of the components for effective separation.

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Related Practice

Textbook Question

For each molecular formula, draw all the isomeric alkynes, and give their IUPAC names. Circle the acetylenic hydrogen of each terminal alkyne.

(a) C₅H₈ (three isomers)

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Textbook Question

For each molecular formula, draw all the isomeric alkynes, and give their IUPAC names. Circle the acetylenic hydrogen of each terminal alkyne.

(b) C₆H₁₀ (seven isomers)

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Textbook Question

Solved Problem 9-1 showed the synthesis of dec-3-yne by adding the hexyl group first, then the ethyl group. Show the reagents and intermediates involved in the other order of synthesis of dec-3-yne, by adding the ethyl group first and the hexyl group last.

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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.

a. hex-1-yne

b. hex-2-yne

c. hex-3-yne

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Textbook Question

Predict the products of the following acid–base reactions, or indicate if no significant reaction would take place.

a. H—C≡C—H + NaNH₂

b. H—C≡C—H + CH₃Li

c. H—C≡C—H + NaOCH₃

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