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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6:06 minutes

Problem 22

Textbook Question

Disiamylborane adds only once to alkynes by virtue of its two bulky secondary isoamyl groups. Disiamylborane is prepared by the reaction of BH₃·THF with an alkene.
a. Draw the structural formulas of the reagents and the products in the preparation of disiamylborane.
b. Explain why the reaction in part (a) goes only as far as the dialkylborane. Why is Sia₃B not formed?

Verified step by step guidance

Step 1: Understand the preparation of disiamylborane. Disiamylborane is synthesized by reacting borane-tetrahydrofuran (BH₃·THF) with an alkene, specifically 2-methyl-2-butene (an isoamyl group). The reaction involves the addition of two isoamyl groups to the boron atom, forming disiamylborane (Sia₂BH). Write the structural formulas for BH₃·THF, 2-methyl-2-butene, and the resulting disiamylborane.

Step 2: Represent the reaction mechanism. The reaction proceeds via hydroboration, where the boron atom in BH₃ adds to the less substituted carbon of the alkene, and the hydrogen adds to the more substituted carbon. This process occurs twice, as BH₃ has three hydrogens, but steric hindrance prevents the addition of a third isoamyl group.

Step 3: Explain why the reaction stops at the dialkylborane stage. The bulky isoamyl groups attached to boron create significant steric hindrance, preventing a third alkene molecule from approaching the boron atom. This steric hindrance ensures that the reaction stops at the disiamylborane (Sia₂BH) stage, and a trialkylborane (Sia₃B) is not formed.

Step 4: Highlight the role of steric hindrance in the reaction. Steric hindrance refers to the physical crowding around the boron atom caused by the two bulky isoamyl groups. This crowding makes it energetically unfavorable for a third alkene to react with the boron atom, thus limiting the reaction to the formation of disiamylborane.

Step 5: Summarize the structural formulas and reasoning. The reagents are BH₃·THF and 2-methyl-2-butene, and the product is disiamylborane (Sia₂BH). The reaction stops at the dialkylborane stage due to steric hindrance, which prevents the formation of Sia₃B. This property of disiamylborane makes it a useful reagent for selective hydroboration of alkynes.

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Key Concepts

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

Disiamylborane Structure and Preparation

Disiamylborane is a dialkylborane compound formed from the reaction of borane (BH3) with an alkene, specifically using tetrahydrofuran (THF) as a solvent. The structure features two isoamyl groups attached to the boron atom, which significantly influences its reactivity. Understanding the structural formula of both the reagents (BH3·THF and the alkene) and the product (disiamylborane) is crucial for visualizing the reaction mechanism.

Steric Hindrance

Steric hindrance refers to the prevention of reactions due to the spatial arrangement of atoms within a molecule. In the case of disiamylborane, the two bulky isoamyl groups create significant steric hindrance, which limits further reactions with additional alkynes. This concept is essential for understanding why disiamylborane adds only once to alkynes and does not form the trialkylborane (Sia3B).

Reactivity of Alkynes with Boranes

The reactivity of alkynes with boranes involves the formation of a boron-alkyne complex, which can lead to the addition of boron across the triple bond. However, the specific steric and electronic properties of disiamylborane dictate that it can only add once to the alkyne, resulting in a stable dialkylborane. This understanding is key to explaining why further addition to form Sia3B does not occur in this reaction.

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Related Practice

Textbook Question

How can the following compounds be synthesized, starting with a hydrocarbon that has the same number of carbons as the desired product?

b. CH₃CH₂CH₂CH₂OH

Textbook Question

We have studied a number of pericyclic reactions previously. Draw the mechanism of the steps shown. The section number where this material was first studied is given for your review.

(c)

Textbook Question

What reagents are needed to synthesize the following alcohols?

Textbook Question

What is the major product obtained from hydroboration–oxidation of the following alkenes?

a. 2-methyl-2-butene

Textbook Question

In the hydroboration of 1-methylcyclopentene shown in Solved Problem 8-3, the reagents are achiral, and the products are chiral. The product is a racemic mixture of trans-2-methylcyclopentanol, but only one enantiomer is shown. Show how the other enantiomer is formed.

Textbook Question

Show how you would synthesize each compound using methylenecyclohexane as your starting material.

(c)

Textbook Question

When (Z)-3-methylhex-3-ene undergoes hydroboration–oxidation, two isomeric products are formed. Give their structures, and label each asymmetric carbon atom as (R) or (S). What is the relationship between these isomers?

Textbook Question

When 1,2-dimethylcyclopentene undergoes hydroboration–oxidation, one diastereomer of the product predominates. Show why this addition is stereospecific, and predict the stereochemistry of the major product.

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