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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 2a
Chapter 9, Problem 2a

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)

Verified step by step guidance
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Step 1: Understand the concept of isomerism. Isomers are compounds with the same molecular formula but different structural arrangements. In this case, we are looking for structural isomers of alkynes with the formula C5H8.
Step 2: Recall that alkynes are hydrocarbons containing a carbon-carbon triple bond. The simplest alkyne is ethyne (acetylene), and terminal alkynes have the triple bond at the end of the carbon chain, which includes an acetylenic hydrogen.
Step 3: Begin by drawing the longest carbon chain possible with a triple bond. For C5H8, start with a five-carbon chain and place the triple bond between the first and second carbon atoms to form 1-pentyne. This is a terminal alkyne, so circle the hydrogen attached to the first carbon.
Step 4: Next, consider moving the triple bond to different positions along the carbon chain. Place the triple bond between the second and third carbon atoms to form 2-pentyne. This is an internal alkyne, so it does not have an acetylenic hydrogen to circle.
Step 5: Finally, explore branching possibilities. Create a four-carbon chain with a methyl group attached, and place the triple bond at the end to form 3-methyl-1-butyne. This is another terminal alkyne, so circle the hydrogen attached to the first carbon.

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Key Concepts

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

Isomerism in Organic Chemistry

Isomerism refers to the phenomenon where compounds have the same molecular formula but different structural arrangements. In the context of alkynes, structural isomers can differ in the position of the triple bond or the branching of the carbon chain. Understanding isomerism is crucial for identifying and drawing all possible isomeric structures for a given molecular formula.

Alkynes and Terminal Alkynes

Alkynes are hydrocarbons that contain at least one carbon-carbon triple bond. Terminal alkynes have the triple bond at the end of the carbon chain, which results in the presence of an acetylenic hydrogen atom. This hydrogen is unique because it is directly bonded to a carbon atom involved in a triple bond, making it more acidic than hydrogens in alkanes or alkenes.

IUPAC Nomenclature for Alkynes

The IUPAC naming system provides a standardized way to name organic compounds. For alkynes, the longest carbon chain containing the triple bond is identified, and the chain is numbered to give the lowest possible number to the triple bond. The suffix '-yne' is used to indicate the presence of a triple bond, and any substituents are named and numbered accordingly. This systematic approach ensures clarity and consistency in chemical communication.
Related Practice
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.

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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

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

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Textbook Question

Draw structural formulas of at least two alkynes of each molecular formula.

1. C6H10

2. C8H12

3. C7H8

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Textbook Question

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

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