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

10. Addition Reactions

Hydroboration

Organic Chemistry

10. Addition Reactions

Hydroboration

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5:07 minutes

Problem 7a

Textbook Question

Hydroboration, an electrophilic addition reaction like those studied in Section 21.4, only gives 1,2-addition to buta-1,3-diene, regardless of temperature. Why?

Verified step by step guidance

Understand the concept of hydroboration: Hydroboration is an electrophilic addition reaction where borane (BH₃) or its derivatives react with alkenes to form organoboranes. This reaction typically proceeds via syn-addition, meaning both the boron atom and the hydrogen atom add to the same face of the double bond.

Analyze the structure of buta-1,3-diene: Buta-1,3-diene is a conjugated diene, meaning it has alternating double and single bonds (CH₂=CH-CH=CH₂). The conjugation allows for delocalization of π-electrons, which affects the reactivity of the molecule.

Consider the regioselectivity of hydroboration: Hydroboration typically adds to the less substituted carbon of the double bond due to steric and electronic factors. In the case of buta-1,3-diene, the reaction will target the terminal double bond (CH₂=CH-) because it is less sterically hindered compared to the internal double bond (-CH=CH₂).

Explain why only 1,2-addition occurs: The reaction proceeds through a concerted mechanism where the boron atom and hydrogen atom add simultaneously to the double bond. The conjugation in buta-1,3-diene stabilizes the intermediate formed during the addition, favoring the 1,2-addition product. The 1,4-addition product is not formed because the reaction does not involve a carbocation intermediate that would allow for rearrangement or resonance stabilization.

Discuss the role of temperature: Unlike other electrophilic addition reactions, hydroboration does not exhibit temperature-dependent selectivity in this case. The concerted mechanism and the inherent regioselectivity of hydroboration ensure that only the 1,2-addition product is formed, regardless of temperature.

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Key Concepts

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

Electrophilic Addition Reactions

Electrophilic addition reactions involve the attack of an electrophile on a nucleophile, leading to the formation of a more saturated product. In the case of alkenes, the double bond acts as a nucleophile, reacting with electrophiles to form carbocations. This mechanism is crucial for understanding how reagents like borane add across double bonds, influencing the regioselectivity of the reaction.

Regioselectivity

Regioselectivity refers to the preference of a chemical reaction to yield one structural isomer over others when multiple possibilities exist. In hydroboration, the reaction favors the formation of the more stable product through a specific pathway, leading to 1,2-addition in buta-1,3-diene. This concept is essential for predicting the outcome of reactions involving unsymmetrical alkenes.

Stability of Intermediates

The stability of intermediates, such as carbocations formed during electrophilic addition, significantly influences the reaction pathway. In hydroboration, the formation of a less stable, but more reactive, intermediate leads to a consistent 1,2-addition product. Understanding how the stability of these intermediates affects the reaction mechanism is key to predicting the behavior of the reaction under various conditions.

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Related Practice

Textbook Question

Predict the product(s) that would result when the following molecules are allowed to react under the following conditions: (i) 1. BH₃ 2. NaOH, H₂O₂ (ii) 1. Hg(OAc)₂ 2. NaBH₄ (iii) H₂SO₄ , H₂O (iv) 1. OsO₄ 2. NaHSO₃ (v) H₂O (vi) NaOH, H₂O . If there is no reaction, write 'no reaction.'

(d)

Textbook Question

Predict the product(s) that would result when the following molecules are allowed to react under the following conditions: (i) 1. BH₃ 2. NaOH, H₂O₂. If there is no reaction, write 'no reaction.'

(a)

Textbook Question

a. Show the reagents required to form the primary alcohol in each of the following reactions.

Textbook Question

Identify A through O:

Textbook Question

Suggest an alkene to undergo hydroboration–oxidation (1. BH₃ 2. NaOH, H₂O₂) to give exclusively the alcohols shown. Pay close attention to the relative (but not absolute) stereochemical outcome.

(c)

Textbook Question

Would you expect the following to produce an equal or unequal mixture of stereoisomers?

(c)

Textbook Question

(b)

Textbook Question

Suggest a reagent and a reactant that could be combined to make each of the following alcohols.

(a)

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