Textbook Question
What is the major product of the reaction of 2-methyl-2-butene with each of the following reagents?
k. BH3/THF, followed by H2O2, HO- , H2O
10. Addition Reactions
Hydroboration
5:24 minutesProblem 77c
Textbook Question
Draw the products, including their configurations, obtained from the reaction of 1-ethylcyclohexene with the following reagents:
c. R2BH/THF, followed by HO–, H2O2, H2O
Verified step by step guidance1
Identify the reaction type: The given reagents (R2BH/THF followed by HO⁻, H2O2, H2O) indicate a hydroboration-oxidation reaction. This reaction adds water (H and OH) across the double bond of an alkene in an anti-Markovnikov fashion, meaning the OH group will attach to the less substituted carbon of the double bond.
Analyze the structure of the starting material: 1-ethylcyclohexene is a cyclohexene ring with an ethyl group attached to the first carbon of the double bond. The double bond is between carbons 1 and 2 of the cyclohexene ring.
Perform the hydroboration step: In the first step, R2BH adds across the double bond. The boron atom (B) attaches to the less substituted carbon (C2), and the hydrogen (H) attaches to the more substituted carbon (C1). This step occurs with syn addition, meaning both groups add to the same face of the double bond.
Perform the oxidation step: In the second step, the boron atom is replaced by a hydroxyl group (OH) using H2O2 and HO⁻. The stereochemistry is retained from the hydroboration step, so the OH group remains on the same face as the hydrogen added in the previous step.
Draw the product: The final product is a cyclohexanol derivative with the OH group on C2 (the less substituted carbon) and the ethyl group on C1. Ensure the stereochemistry is clearly indicated, showing the syn addition of H and OH.
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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, an alkene reacts with borane (BH3) or its derivatives in a non-polar solvent like THF, resulting in the formation of an organoborane intermediate. The second step involves oxidation, typically using hydrogen peroxide (H2O2) in the presence of a base, which converts the organoborane into an alcohol, with anti-Markovnikov selectivity.
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Regioselectivity
Regioselectivity refers to the preference of a chemical reaction to yield one structural isomer over others when multiple possibilities exist. In the context of hydroboration-oxidation, the reaction exhibits anti-Markovnikov regioselectivity, meaning that the hydroxyl group (–OH) ends up on the less substituted carbon of the alkene, while the boron atom attaches to the more substituted carbon. This is crucial for predicting the structure of the final alcohol product.
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Stereochemistry
Stereochemistry is the study of the spatial arrangement of atoms in molecules and how this affects their chemical behavior. In the hydroboration-oxidation process, the addition of boron and subsequent oxidation can lead to the formation of chiral centers, resulting in stereoisomers. Understanding the stereochemical outcomes is essential for accurately drawing the products, as the configuration (R or S) of the resulting alcohol must be determined based on the original alkene's geometry.
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