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

Ranking Acidity

Organic Chemistry

3. Acids and Bases

Ranking Acidity

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5:25 minutes

Problem 78

Textbook Question

Imidazole has two potentially basic sites (a and b). Of the two, where would you expect a proton to add preferentially?

Verified step by step guidance

Identify the structure of imidazole, which is a five-membered aromatic heterocyclic compound containing two nitrogen atoms. Label the two nitrogen atoms as 'a' (pyrrole-like nitrogen) and 'b' (pyridine-like nitrogen).

Understand the difference between the two nitrogen atoms: The 'a' nitrogen is part of a double bond and has a lone pair of electrons that is delocalized into the aromatic ring, making it less available for protonation. The 'b' nitrogen has a lone pair of electrons that is not involved in the aromatic system, making it more available for protonation.

Recall the concept of basicity: A site is more basic if it has a lone pair of electrons that is readily available to accept a proton. Since the lone pair on the 'b' nitrogen is not delocalized, it is more basic than the 'a' nitrogen.

Consider the stability of the resulting conjugate acid: Protonation at the 'b' nitrogen leads to a positively charged species that retains aromaticity, whereas protonation at the 'a' nitrogen disrupts the aromaticity of the ring, making it less favorable.

Conclude that the proton will preferentially add to the 'b' nitrogen (pyridine-like nitrogen) because it is more basic and results in a more stable conjugate acid while preserving the aromaticity of the imidazole ring.

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Key Concepts

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

Basicity of Imidazole

Imidazole is a five-membered heterocyclic compound containing two nitrogen atoms. Its basicity is influenced by the electron density on the nitrogen atoms, which can accept protons. The basicity of each nitrogen site is determined by factors such as resonance stabilization and the surrounding electronic environment.

Protonation Sites

In imidazole, the two nitrogen atoms (labeled a and b) have different electronic properties. Protonation at site a typically leads to a more stable cation due to resonance, as the positive charge can be delocalized over the ring. Understanding the stability of the resulting protonated species is crucial for predicting where protonation will occur preferentially.

Resonance and Stability

Resonance refers to the delocalization of electrons across multiple structures, which can stabilize charged species. In the case of imidazole, protonation at one nitrogen may allow for greater resonance stabilization compared to the other. This concept is essential for determining the preferred site of protonation, as more stable cations are favored in chemical reactions.

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Related Practice

Textbook Question

Rank the carbanions shown in the margin from most basic to least basic.

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Textbook Question

b. How does the presence of an electronegative substituent such as Cl affect the acidity of a carboxylic acid?

c. How does the location of the substituent affect the acidity of the carboxylic acid?

Textbook Question

Explain the relative acidities.

CH₂═CHCH₂OH, CH₃CH₂CH₂OH, HC≡CCH₂OH

Textbook Question

Using the table of pK_a values given in [Appendix I] , answer the following:

d. Which is more electronegative: an sp³ oxygen or an sp² oxygen?

e. Which compounds demonstrate that the relative electronegativities of a hybridized nitrogen atom are sp > sp² > sp³?

Textbook Question

Rationalize the rather large difference in pK_a values for the two carboxylic acids shown.

Textbook Question

Rank the following amines in order of their basicity (strongest base = 1 ; weakest base = 6).

Textbook Question

Which acid in each pair would you expect to more readily donate a proton to a basic compound?

(b)

Textbook Question

Using qualitative reasoning for the acid–base reactions shown,

(i) which is stronger, the acid or the conjugate acid?

(ii) Which side of the reaction is favored?

(iii) Would you expect a K_eq greater than, equal to, or less than 1?

(a)

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