Ch.9 - Alkynes

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

Wade 9th Edition

Ch.9 - Alkynes

Problem 3

Chapter 9, Problem 3

What reaction would acetylene likely undergo if it were kept at 1500°C for too long?

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Acetylene (C₂H₂) is a highly reactive alkyne with a triple bond between its two carbon atoms. At elevated temperatures, such as 1500 °C, it undergoes thermal decomposition or polymerization due to the instability of the molecule under such extreme conditions.

At 1500 °C, acetylene molecules can break apart into smaller fragments or recombine to form larger molecules. This process is driven by the high energy available at this temperature, which overcomes the bond dissociation energy of the triple bond.

One common reaction at this temperature is the formation of elemental carbon (C) and hydrogen gas (H₂). The reaction can be represented as:

C 2 H 2 \to C + H 2

Another possible reaction is the polymerization of acetylene to form benzene (C₆H₆) or other polycyclic aromatic hydrocarbons. This occurs when multiple acetylene molecules combine under high temperatures. For example:

3 C 2 H 2 \to C 6 H 6

In summary, if acetylene is kept at 1500 °C for too long, it is likely to undergo thermal decomposition into carbon and hydrogen gas or polymerization to form larger hydrocarbons such as benzene. These reactions are a result of the instability of acetylene at high temperatures.

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Key Concepts

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

Acetylene Structure and Properties

Acetylene (C2H2) is a simple alkyne characterized by a triple bond between two carbon atoms. This structure makes it highly reactive, particularly at elevated temperatures. Understanding its bonding and molecular geometry is crucial for predicting its behavior in chemical reactions.

Thermal Decomposition

At high temperatures, organic compounds like acetylene can undergo thermal decomposition, where the compound breaks down into simpler molecules or elements. This process can lead to the formation of carbon and hydrogen gas, as well as other hydrocarbons, depending on the conditions and duration of heating.

Combustion Reactions

Combustion reactions involve the reaction of a substance with oxygen, producing heat and light. Acetylene can combust at high temperatures, leading to the formation of carbon dioxide and water. Understanding combustion is essential for predicting the products and energy changes associated with heating acetylene.

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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

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.

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

Count the elements of unsaturation in parsalmide, ethynyl estradiol, and dynemicin A.

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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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