A carboxylic acid ( p K a = 5) is 10 11 times more acidic than an alcohol (p K a = 16). Why?

All right. Hi, everyone. So this question says that acetic acid with a PK value of 4.74 is approximately 15 times more acidic compared with acetone A PK value of 20. Why? Now here on the screen, we have four different answer choices labeled A through D. But notably, all of them discuss in part how the stability of a conjugate base impacts the PK values that we observe. So let's talk about that. So recall first and foremost, that PK values give us a direct measurement of the strength of an acid. And they followed the same logic as the PH scale. In other words, the lower the PK value, the stronger that acid happens to be. So in part, the strength of an acid can be determined by the stability of its conjugate base. When an acid loses a proton, the resulting species is known as the conjugate base. And many factors can affect the stability of the conjugate base. We have electron negativity. For example, we have the effect of resonance on conjugate based stabilization. We have hybridization to consider as well. If I can spell that right, we can also consider STAIC effects and many other factors. This is not going to be an exhaustive list. But the main point here is that many factors can affect the stability of one conjugate base over another. So in the case of acetic acid and acetone recall that acidity in general is about the ability of a compound to donate a certain proton. In the case of acetic acid, it's going to be the proton connecting to the oxygen atom on the right side of the structure. Whereas in the case of acetone, it's going to be the proton connecting to the carbon atom next to the carbo. Now, when the specific proton indicated gets removed from either of these compounds, the atom that was previously attached to that proton is going to have a negative charge. So acetic acid would have a negative charge on oxygen. Whereas Acetone or rather their conjugate base would have a negative charge on carbon. And that's important because recall that oxygen is more electron then carbon. And if oxygen, well, since oxygen is more electron than carbon, it's a lot easier for carbon. Excuse me, for oxygen to accommodate the resulting negative charge that the conjugate base is going to have. So because oxygen can better accommodate that negative charge, the conjugate base of acetic acid is more stable. And because the conjugate base of acetic acid is more stable than that of acetone, it's also weaker, which means that the acid is stronger. So there you have it, our answer here matches with option a in the multiple choice because acetic acid conjugate base is more stabilized by the electron oxygen than acetone's conjugate base. So with that being said, thank you so very much for watching and I hope you found this helpful.

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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

Ranking Acidity

Organic Chemistry

3. Acids and Bases

Ranking Acidity

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Problem 1

Textbook Question

A carboxylic acid ( pK_a = 5) is 10¹¹ times more acidic than an alcohol (pK_a = 16). Why?

Verified step by step guidance

Step 1: Understand the concept of pKa. The pKa value is a measure of the acidity of a compound. Lower pKa values indicate stronger acids, while higher pKa values indicate weaker acids. Here, the carboxylic acid has a pKa of 5, and the alcohol has a pKa of 16, meaning the carboxylic acid is significantly more acidic.

Step 2: Compare the stability of the conjugate bases. When an acid donates a proton, it forms its conjugate base. The stability of the conjugate base is a key factor in determining the acidity of the acid. The carboxylic acid forms a carboxylate ion (R-COO⁻), which is stabilized by resonance. The alcohol forms an alkoxide ion (R-O⁻), which lacks resonance stabilization.

Step 3: Analyze resonance stabilization in the carboxylate ion. The carboxylate ion has two oxygen atoms that share the negative charge through resonance. This delocalization of charge makes the conjugate base more stable, which in turn makes the carboxylic acid more acidic.

Step 4: Consider the electronegativity of oxygen. Both carboxylic acids and alcohols have oxygen atoms, but in carboxylic acids, the carbonyl group (C=O) further increases the electron-withdrawing effect. This effect stabilizes the negative charge on the conjugate base, enhancing the acidity of the carboxylic acid.

Step 5: Relate the pKa difference to the acidity difference. The difference in pKa values (16 - 5 = 11) corresponds to a factor of 10¹¹ in acidity. This exponential relationship arises because pKa is logarithmic, meaning small changes in pKa result in large changes in acidity.

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Key Concepts

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

Acidity and pKₐ

Acidity in organic chemistry is often measured using the pKₐ value, which indicates the strength of an acid. A lower pKₐ value corresponds to a stronger acid, meaning it dissociates more readily in solution to release protons (H⁺). In this case, the carboxylic acid with a pKₐ of 5 is significantly stronger than the alcohol with a pKₐ of 16, making it more likely to donate a proton.

Carboxylic Acids vs. Alcohols

Carboxylic acids contain a carboxyl group (-COOH) that can easily lose a proton, while alcohols have a hydroxyl group (-OH) that is less likely to do so. The resonance stabilization of the carboxylate ion formed after deprotonation enhances the acidity of carboxylic acids, making them much stronger acids compared to alcohols, which do not benefit from such stabilization.

Comparative Acidity

The comparative acidity of different functional groups can be understood through their ability to stabilize the negative charge after losing a proton. In this scenario, the carboxylic acid is 10¹¹ times more acidic than the alcohol, indicating that the carboxylate ion formed is much more stable than the alkoxide ion from the alcohol, thus favoring the dissociation of the carboxylic acid.

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