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Ch. 20 - Carboxylic Acids
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh. 20 - Carboxylic AcidsProblem 7
Chapter 20, Problem 7

The IR spectrum of trans-oct-2-enoic acid is shown. Point out the spectral characteristics that allow you to tell that this is a carboxylic acid, and show which features lead you to conclude that the acid is unsaturated and conjugated.
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Examine the broad absorption band around 2500-3500 cm⁻¹. This is characteristic of the O-H stretching vibration in carboxylic acids, which is typically broad due to hydrogen bonding.
Identify the sharp peak near 1700 cm⁻¹. This corresponds to the C=O stretching vibration, which is a strong indicator of the presence of a carboxylic acid functional group.
Look for the absorption band near 1600 cm⁻¹. This is indicative of C=C stretching vibrations, suggesting the presence of an alkene group, which confirms unsaturation.
Check for additional peaks near 3000 cm⁻¹. These are associated with =C-H stretching vibrations, further supporting the presence of an alkene group.
Analyze the conjugation effects. The C=O stretching frequency near 1700 cm⁻¹ is slightly lower than typical values for non-conjugated carboxylic acids, which suggests conjugation between the alkene and the carboxylic acid group.

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Key Concepts

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

Infrared Spectroscopy (IR)

Infrared spectroscopy is a technique used to identify functional groups in organic compounds by measuring the absorption of infrared light. Different bonds absorb characteristic frequencies of IR radiation, resulting in a spectrum that displays peaks corresponding to these absorptions. Understanding how to interpret these peaks is crucial for identifying the presence of specific functional groups, such as carboxylic acids.
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General Features of IR Spect

Carboxylic Acid Functional Group

Carboxylic acids are organic compounds characterized by the presence of a carboxyl group (-COOH). In an IR spectrum, carboxylic acids typically show a broad O-H stretching absorption around 2500-3300 cm⁻¹ and a sharp C=O stretching absorption near 1700 cm⁻¹. Recognizing these peaks is essential for confirming the presence of a carboxylic acid in a compound.
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Identifying Functional Groups

Unsaturation and Conjugation

Unsaturation in organic compounds refers to the presence of double or triple bonds, which can be identified in an IR spectrum by the presence of C=C stretching vibrations typically around 1600-1680 cm⁻¹. Conjugation occurs when double bonds are separated by a single bond, leading to a stabilization of the molecule and shifts in the absorption peaks. Identifying these features helps determine the unsaturation and conjugation of the acid.
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The difference between saturated and unsaturated molecules.
Related Practice
Textbook Question

Suppose you have just synthesized heptanoic acid from heptan-1-ol. The product is contaminated by sodium dichromate, sulfuric acid, heptan-1-ol, and possibly heptanal. Explain how you would use acid–base extractions to purify the heptanoic acid. Use a chart, like that in Figure 20-3, to show where the impurities are at each stage.

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Textbook Question

Oxidation of a primary alcohol to an aldehyde usually gives some over-oxidation to the carboxylic acid. Assume you have used PCC to oxidize pentan-1-ol to pentanal.

(a) Show how you would use acid–base extraction to purify the pentanal.

(b) Which of the expected impurities cannot be removed from pentanal by acid–base extractions? How would you remove this impurity?

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Textbook Question

How many stereoisomers are possible for c. a ketotriose?

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Textbook Question

Phenols are less acidic than carboxylic acids, with values of pKa around 10. Phenols are deprotonated by (and therefore soluble in) solutions of sodium hydroxide but not by solutions of sodium bicarbonate. Explain how you would use extractions to isolate the three pure compounds from a mixture of p-cresol (p-methylphenol), cyclohexanone, and benzoic acid.

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Textbook Question

Show how you would synthesize the following carboxylic acids, using the indicated starting materials.

(a) oct-4-yne → butanoic acid

(b) trans-cyclodecene → decanedioic acid

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Textbook Question

Write the mechanism for the base-catalyzed conversion of D-fructose to D-glucose and D-mannose.

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