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Ch.8 - Reactions of Alkenes
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh.8 - Reactions of AlkenesProblem 77
Chapter 8, Problem 77

An inexperienced graduate student treated dec-5-ene with borane in THF, placed the flask in a refrigerator, and left for a party. When he returned from the party, he discovered that the refrigerator was broken and that it had gotten quite warm inside. Although all the THF had evaporated from the flask, he treated the residue with basic hydrogen peroxide. To his surprise, he recovered a fair yield of decan-1-ol. Use a mechanism to show how this reaction might have occurred. (Hint: The addition of BH3 is reversible.)

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Step 1: Begin by understanding the reaction mechanism. The graduate student treated dec-5-ene with borane (BH₃) in tetrahydrofuran (THF). This is a hydroboration reaction, where BH₃ adds across the double bond of the alkene. The addition of BH₃ is reversible, meaning that under certain conditions, the borane can dissociate from the alkene.
Step 2: Consider the effect of the warm temperature due to the broken refrigerator. The elevated temperature likely caused the THF solvent to evaporate, leaving behind the residue of the alkene and borane complex. The heat may also have driven the reversible dissociation of the borane from the alkene, allowing the borane to redistribute and react further.
Step 3: Analyze the redistribution of borane. In the absence of THF and under warm conditions, borane can undergo disproportionation or redistribution reactions. This process can lead to the formation of trialkylboranes, where the boron atom becomes bonded to three alkyl groups. In this case, dec-5-ene could react further to form a trialkylborane intermediate.
Step 4: Examine the subsequent treatment with basic hydrogen peroxide (H₂O₂/NaOH). Basic hydrogen peroxide oxidizes the trialkylborane intermediate, converting the alkyl groups attached to boron into alcohols. This oxidation step follows the mechanism of hydroboration-oxidation, where the boron atom is replaced by a hydroxyl group (-OH) at the least substituted carbon of the original alkene.
Step 5: Conclude with the formation of decan-1-ol. The hydroboration-oxidation reaction typically results in anti-Markovnikov addition, meaning the hydroxyl group is added to the terminal carbon of the alkene. This explains why the student recovered decan-1-ol as the product, despite the unusual conditions during the reaction.

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Key Concepts

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

Hydroboration-Oxidation

Hydroboration-oxidation is a two-step reaction process used to convert alkenes into alcohols. In the first step, borane (BH3) adds across the double bond of the alkene, forming an organoborane intermediate. This addition is syn and occurs in a concerted manner, leading to the formation of a trialkylborane. The second step involves oxidation with hydrogen peroxide in a basic medium, converting the boron compound into an alcohol.
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Reversibility of Borane Addition

The addition of borane to alkenes is a reversible process, meaning that the organoborane intermediate can undergo hydrolysis or rearrangement under certain conditions. If the reaction mixture is heated or if the solvent evaporates, the equilibrium can shift, potentially leading to the formation of different products. In this scenario, the evaporation of THF and the subsequent warming may have allowed for the rearrangement of the organoborane, facilitating the formation of decan-1-ol upon oxidation.
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Mechanism of Alcohol Formation

The mechanism for the formation of decan-1-ol from dec-5-ene involves several key steps. Initially, borane adds to the alkene to form an organoborane. Upon warming and the absence of THF, the organoborane can rearrange or decompose, allowing for the migration of the alkyl group. When treated with basic hydrogen peroxide, the boron is replaced by a hydroxyl group, resulting in the formation of the primary alcohol, decan-1-ol, through a series of nucleophilic substitutions.
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Related Practice
Textbook Question

Ozonolysis can be applied selectively to different types of carbon–carbon double bonds. The compound shown below contains two vinyl ether double bonds, which are electron-rich because of the electron-donating alkoxy groups. Ozone reacts more quickly with electron-rich double bonds and more slowly with hindered double bonds. At −78 °C, this compound quickly adds two equivalents of ozone. Immediate reduction of the ozonide gives a good yield of a single product. Show the expected ozonolyis product, and label the functional groups produced, some of which are not typical from ozonolysis of simple alkenes.

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Textbook Question

The bulky borane 9-BBN was developed to enhance the selectivity of hydroboration. In this example, 9-BBN adds to the less hindered carbon with 99.3% regioselectivity, compared with only 57% for diborane.

a. Show the two organic products generated when the trialkylborane is oxidized with H2O2/NaOH.

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Textbook Question

The cationic polymerization of isobutylene (2-methylpropene) is shown in Section 8-16A. Isobutylene is often polymerized under free-radical conditions. Propose a mechanism for the free-radical polymerization of isobutylene.

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Textbook Question

We have seen many examples where halogens add to alkenes with anti stereochemistry via the halonium ion mechanism. However, when 1-phenylcyclohexene reacts with chlorine in carbon tetrachloride, a mixture of the cis and trans isomers of the product is recovered. Propose a mechanism, and explain this lack of stereospecificity.

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Textbook Question

Propose mechanisms to explain the opposite regiochemistry observed in the following two reactions.

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Textbook Question

The bulky borane 9-BBN was developed to enhance the selectivity of hydroboration. In this example, 9-BBN adds to the less hindered carbon with 99.3% regioselectivity, compared with only 57% for diborane.

b. 9-BBN is synthesized by adding BH3 across a symmetric, cyclic diene. What is the structure of the diene?

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