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Organic Chemistry
Table of Contents
Unit 8 Overview: Alkenes
8.1 Preparing Alkenes: A Preview of Elimination Reactions
8.2 Halogenation of Alkenes: Addition of X2
8.3 Halohydrins from Alkenes: Addition of HO-X
8.4 Hydration of Alkenes: Addition of H2O by Oxymercuration
8.5 Hydration of Alkenes: Addition of H2O by Hydroboration
8.6 Reduction of Alkenes: Hydrogenation
8.7 Oxidation of Alkenes: Epoxidation and Hydroxylation
8.8 Oxidation of Alkenes: Cleavage to Carbonyl Compounds
8.9 Addition of Carbenes to Alkenes: Cyclopropane Synthesis
8.10 Radical Additions to Alkenes: Chain-Growth Polymers
8.11 Biological Additions of Radicals to Alkenes
8.12 Reaction Stereochemistry: Addition of H2O to an Achiral Alkene
8.13 Reaction Stereochemistry: Addition of H2O to a Chiral Alkene

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8.5 Hydration of Alkenes: Addition of H2O by Hydroboration

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Hydroboration-oxidation turns alkenes into alcohols in two steps. First, borane adds to the double bond. Then, oxidation with hydrogen peroxide creates the alcohol. This reaction is special because it goes against the usual rules for adding stuff to alkenes.

The alcohol forms on the less crowded carbon, opposite to what you might expect. Also, the reaction keeps the 3D shape of the original alkene. These unique features make hydroboration-oxidation super useful for making specific alcohols from alkenes.

Hydroboration-Oxidation Reaction

Hydroboration-oxidation reaction sequence

  • Two-step reaction sequence converts alkenes to alcohols
    • Step 1: Hydroboration
      • Borane ($BH_3$) adds across the alkene double bond forming a trialkylborane intermediate
      • Boron atom attaches to the less substituted carbon due to steric factors (less hindered)
    • Step 2: Oxidation
      • Trialkylborane intermediate treated with hydrogen peroxide ($H_2O_2$) under basic conditions (usually $NaOH$)
      • Carbon-boron bond replaced by a carbon-oxygen bond forming an alcohol
      • Oxygen atom attaches to the carbon previously bonded to boron

Regiochemistry and stereochemistry of hydroboration-oxidation

  • Regiochemistry: anti-Markovnikov addition (exhibits regioselectivity)
    • Hydroxyl group ($-OH$) attaches to the less substituted carbon of the original alkene
    • Contrasts with acid-catalyzed hydration and oxymercuration-reduction (follow Markovnikov's rule)
    • Mechanism explains boron preferentially bonding to less hindered carbon in hydroboration step
  • Stereochemistry: syn addition and stereospecific
    • Syn addition: boron and hydrogen add to the same face of the alkene in hydroboration step
      • Contrasts with bromine addition to alkenes (anti addition)
    • Stereospecific: alkene stereochemistry retained in product alcohol
      • Contrasts with acid-catalyzed hydration (not stereospecific)

Product prediction in hydroboration-oxidation

  • General rule: $-OH$ group attaches to less substituted carbon of original alkene
  • Monosubstituted alkenes (1-butene): $-OH$ group on terminal carbon
  • Disubstituted alkenes:
    1. $-OH$ group on less substituted carbon
    2. If carbons equally substituted, steric hindrance determines major product
      • $-OH$ group preferentially forms on carbon with less bulky substituents
  • Trisubstituted alkenes: $-OH$ group on only unsubstituted carbon
  • Cyclic alkenes follow same rules ($-OH$ group on less substituted carbon)
  • Stereochemistry retained:
    • $(E)$-alkenes form anti product
    • $(Z)$-alkenes form syn product

Mechanism and intermediate species

  • Hydroboration step:
    • Borane acts as an electrophile, alkene as a nucleophile
    • Forms a four-centered transition state
    • Results in a trialkylborane intermediate
  • Oxidation step:
    • Hydroxide ion attacks the boron atom
    • Hydride migrates from boron to carbon
    • Peroxide displaces hydroxide, followed by rearrangement
    • Yields the final alcohol product

Key Terms to Review (23)

Hydride shift: A hydride shift is a rearrangement process where a hydrogen atom with its pair of electrons moves from one carbon to an adjacent carbocation center, stabilizing the molecule during electrophilic additions. This mechanism is key in understanding how carbocations can rearrange to form more stable intermediates in reactions involving alkenes.
Anti stereochemistry: Anti stereochemistry describes the spatial arrangement in a chemical reaction where two substituents are positioned on opposite sides of a double bond or ring structure after the reaction. It is particularly relevant in the halogenation of alkenes, resulting in products where the added atoms are located across from each other.
Syn stereochemistry: Syn stereochemistry in the context of hydration of alkenes involves the addition of two substituents to the same side of a double bond during a chemical reaction. It is a crucial concept in understanding how molecules are transformed in organic synthesis, especially in hydroboration reactions where the outcome significantly affects molecular shape and function.
Stereochemistry: Stereochemistry is the study of the three-dimensional arrangement of atoms in molecules and how this arrangement affects the chemical and physical properties of the substance. It examines the spatial orientation of atoms and their relationship to one another, which is crucial in understanding many organic chemistry concepts.
Alcohols: Alcohols are organic compounds containing a hydroxyl (-OH) functional group attached to a saturated carbon atom. They are widely used in various chemical reactions and have diverse applications in industry, medicine, and everyday life.
Alkenes: Alkenes are a class of unsaturated organic compounds characterized by the presence of a carbon-carbon double bond. They are an important functional group in organic chemistry, with a wide range of applications and reactivity. Alkenes are closely related to the topics of chirality, isomerism, electrophilic addition reactions, halogenation, hydration, the E2 reaction, infrared spectroscopy, 13C NMR spectroscopy, alcohol preparation, and the Wittig reaction.
Regioselectivity: Regioselectivity refers to the preference of a chemical reaction to occur at a specific site or region of a molecule, leading to the formation of one regioisomeric product over another. This concept is particularly important in the context of electrophilic addition reactions of alkenes, electrophilic aromatic substitution, and other organic transformations.
Markovnikov's Rule: Markovnikov's rule is a principle in organic chemistry that describes the orientation of addition reactions involving unsaturated compounds, such as alkenes. It states that in the addition of a hydrogen halide (HX) to an alkene, the hydrogen atom of the HX bond attaches to the carbon atom of the alkene that can best stabilize the resulting carbocation intermediate.
Syn Addition: Syn addition is a type of organic reaction where two substituents are added to the same side of a carbon-carbon double bond, resulting in the formation of a new stereocenter with a specific stereochemical configuration. This term is particularly relevant in the context of various organic chemistry topics, including electrophilic addition reactions of alkenes, hydration of alkenes, reduction of alkenes, and oxidation of alkenes.
Trisubstituted Alkenes: Trisubstituted alkenes are organic compounds containing a carbon-carbon double bond with three substituents attached to one of the carbon atoms. These types of alkenes are important in the context of understanding alkene stereochemistry, the stability of alkenes, and the hydration of alkenes through hydroboration reactions.
Monosubstituted Alkenes: Monosubstituted alkenes are organic compounds that contain a carbon-carbon double bond with a single substituent attached to one of the carbon atoms. These molecules are important in the context of understanding the stability of alkenes and the hydration of alkenes through the process of hydroboration.
Disubstituted Alkenes: Disubstituted alkenes are organic compounds that contain a carbon-carbon double bond with two substituents attached to each carbon atom of the double bond. These types of alkenes are particularly relevant in the context of understanding the stability of alkenes and the hydration of alkenes through the process of hydroboration.
Trialkylborane: Trialkyboranes are a class of organoborane compounds containing three alkyl groups bonded to a central boron atom. These compounds play a crucial role in the hydration of alkenes through the process of hydroboration-oxidation, which is a key reaction in organic chemistry.
Anti-Markovnikov Addition: Anti-Markovnikov addition is a type of electrophilic addition reaction that occurs when a hydrogen-containing molecule, such as water or hydrogen halide, adds to an alkene or alkyne in a way that places the hydrogen on the less substituted carbon. This is in contrast to the Markovnikov addition, which places the hydrogen on the more substituted carbon.
Hydroxyl Group: The hydroxyl group (OH-) is a functional group consisting of an oxygen atom covalently bonded to a hydrogen atom. It is a key structural feature in many organic compounds, particularly alcohols and phenols, and plays a crucial role in their chemical properties and reactivity.
Hydride: A hydride is a compound in which hydrogen is bonded to a more electropositive element, such as a metal or a metalloid. Hydrides play a crucial role in various organic chemistry reactions, including the hydration of alkenes, the oxidation of alcohols, the nucleophilic addition of hydrides and Grignard reagents, and the chemistry of esters.
Hydroboration-Oxidation: Hydroboration-oxidation is a two-step reaction sequence that allows for the anti-Markovnikov addition of water to alkenes and alkynes, resulting in the formation of alcohols. This process involves the initial hydroboration of the carbon-carbon double or triple bond, followed by an oxidation step to yield the final alcohol product.
Oxidation: Oxidation is a fundamental chemical process in which a substance loses electrons, resulting in an increase in its oxidation state. This term is central to understanding various reactions and transformations in organic chemistry, from the hydration of alkenes to the oxidation of alcohols and aldehydes.
Borane: Borane is a chemical compound with the formula BH3, consisting of a boron atom bonded to three hydrogen atoms. It is a highly reactive and flammable gas that serves as a key intermediate in organic chemistry, particularly in reactions involving alkenes, alkynes, and carboxylic acids.
Oxymercuration-Reduction: Oxymercuration-reduction is a two-step organic reaction that is used to convert alkenes into alcohols. It involves the initial addition of a mercury-containing compound, followed by the reduction of the resulting mercurinium ion intermediate to form the final alcohol product.
Hydrogen Peroxide: Hydrogen peroxide (H2O2) is a colorless, slightly viscous liquid that is a common oxidizing agent used in a variety of chemical reactions. It is an important compound that plays a role in several organic chemistry topics, including the hydration of alkenes, oxidation of alkenes, hydration of alkynes, preparation of alcohols, and reactions of carboxylic acids.
Cyclic Alkenes: Cyclic alkenes are unsaturated cyclic organic compounds that contain at least one carbon-carbon double bond within a ring structure. These compounds are an important class of molecules in organic chemistry, particularly in the context of understanding the addition of water (hydration) to alkenes through the process of hydroboration-oxidation.
Stereospecific: Stereospecificity refers to the ability of a chemical reaction to produce a specific stereoisomer as the sole or predominant product. This term is particularly relevant in the context of organic chemistry reactions involving alkenes, where the stereochemistry of the reactants and products is of great importance.