Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

3. Acids and Bases

pKa

Organic Chemistry

3. Acids and Bases

pKa

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4:44 minutes

Problem 48b

Textbook Question

Given the data in Problem 47:
b. What pH would you make the water layer to cause the carboxylic acid to dissolve in the water layer and the amine to dissolve in the ether layer?

Verified step by step guidance

Identify the chemical properties of the functional groups involved: Carboxylic acids are acidic and can donate a proton (H⁺), while amines are basic and can accept a proton. This means their solubility can be manipulated by adjusting the pH of the solution.

Understand the solubility behavior: Carboxylic acids are more soluble in water when deprotonated (forming the carboxylate anion, RCOO⁻), which occurs at a pH above their pKa. Amines are more soluble in water when protonated (forming the ammonium ion, RNH₃⁺), which occurs at a pH below their pKa.

Determine the pKa values: Look up or use the provided pKa values for the carboxylic acid and the amine in the problem. These values will help you decide the pH range needed to selectively dissolve each compound in the desired layer.

Adjust the pH for selective solubility: To dissolve the carboxylic acid in the water layer, increase the pH above its pKa so it forms the carboxylate anion. To dissolve the amine in the ether layer, decrease the pH below its pKa so it remains neutral and nonpolar.

Verify the separation: After adjusting the pH, confirm that the carboxylic acid is in the water layer as the carboxylate anion and the amine is in the ether layer as the neutral compound. This ensures effective separation based on their solubility properties.

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Key Concepts

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

Carboxylic Acids and pH

Carboxylic acids are weak acids that can donate a proton (H+) in solution. The pH of a solution affects the ionization of these acids; at lower pH values, they remain mostly in their protonated form, while at higher pH values, they can deprotonate to form carboxylate ions. To dissolve a carboxylic acid in water, the pH must be adjusted to a level that favors its ionization.

Amine Solubility in Organic Solvents

Amines are basic compounds that can accept protons, and their solubility in organic solvents like ether is influenced by their protonation state. At lower pH levels, amines are protonated and more soluble in water, while at higher pH levels, they remain unprotonated and are more soluble in organic solvents. Understanding this behavior is crucial for separating amines from aqueous solutions.

Liquid-Liquid Extraction

Liquid-liquid extraction is a technique used to separate compounds based on their solubility in two different immiscible liquids, typically water and an organic solvent. The choice of pH can selectively alter the solubility of compounds, allowing for the separation of acids and bases. By adjusting the pH, one can manipulate the distribution of the carboxylic acid and amine between the water and ether layers.

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Master The 12 pKa values you want to memorize (because they are important!). with a bite sized video explanation from Johnny

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Related Practice

Textbook Question

Estimate the pKa values of the following compounds:

c. CH₃CH₂COOH

d. CH₃CH₂CH₂N⁺H₃

Textbook Question

a. Which is a stronger acid: one with a pK_a of 5.2 or one with a pK_a of 5.8?

b. Which is a stronger acid: one with an acid dissociation constant of 3.4 × 10⁻³ or one with an acid dissociation constant of 2.1 × 10⁻⁴?

Textbook Question

Like nitrogen and carbon, oxygen also shows this same hybridization effect on acidity. Both of the following compounds can lose a proton from a positively charged oxygen with three bonds to give a conjugate base containing a neutral oxygen with two bonds. One of these structures has pK_a= −2.4, while the other has pK_a= −8.0.

a. Show the reaction of each compound with water.

b. Match each structure with its pK_a, and explain your choice.

Textbook Question

Acetic acid can also react as a very weak base (pK_b = 20). Two different sites on acetic acid might become protonated to give the conjugate acid. Draw both of these possible conjugate acids, and explain (resonance) why the correct one is more stable. Calculate the pK_a of this conjugate acid.

Textbook Question

Butyric acid, the compound responsible for the unpleasant odor and taste of sour milk, has a pK_a value of 4.82. What is its Ka value? Is it a stronger acid or a weaker acid than vitamin C?

Textbook Question

For each indicated proton, suggest an approximate pK_a value from Table 4.5. Rationalize your choice.

(d)

Textbook Question

For the bases shown, draw the conjugate acid and identify a pK_a value from Table 4.5 that would help you accurately estimate its stability.

(b)

Textbook Question

For the bases shown, draw the conjugate acid and identify a pK_a value from Table 4.5 that would help you accurately estimate its stability.

(e)

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Given the data in Problem 47:b. What pH would you make the water layer to cause the carboxylic acid to dissolve in the water layer and the amine to dissolve in the ether layer?

Key Concepts

Carboxylic Acids and pH

Amine Solubility in Organic Solvents

Liquid-Liquid Extraction

Watch next

Given the data in Problem 47:
b. What pH would you make the water layer to cause the carboxylic acid to dissolve in the water layer and the amine to dissolve in the ether layer?