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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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8.13 Reaction Stereochemistry: Addition of H2O to a Chiral Alkene

Citation:

Adding water to a chiral alkene creates a new stereogenic center, but the original chiral center stays put. The reaction follows Markovnikov's rule, with the OH group attaching to the more substituted carbon through a carbocation intermediate.

The new stereogenic center forms randomly, resulting in a racemic mixture. This means the product is a mix of diastereomers, each with one defined stereocenter and one racemic center, reducing overall optical activity.

Reaction Stereochemistry: Addition of H2O to a Chiral Alkene

Stereochemistry of alkene hydration

  • Addition of H2O to a chiral alkene follows Markovnikov's rule
    • H atom adds to the less substituted carbon (terminal carbon)
    • OH group adds to the more substituted carbon (internal carbon)
  • Reaction proceeds through a carbocation intermediate
    • Carbocation forms on the more substituted carbon due to increased stability
      • Hyperconjugation and inductive effects stabilize the carbocation
  • Nucleophilic attack by H2O on the carbocation can occur from either the top or bottom face
    • Leads to the formation of a racemic mixture (equal amounts of both enantiomers)
  • Chirality of the starting alkene does not influence the stereochemistry of the product
    • Original chiral center remains unchanged
    • Newly formed stereogenic center is racemic

Reactant chirality and stereogenic centers

  • Chirality of the reactant alkene does not control the stereochemistry of the newly formed stereogenic center
    • Carbocation intermediate is planar and achiral, allowing attack from either face (top or bottom)
  • Original chiral center in the reactant remains unchanged during the reaction
    • This chiral center does not influence the stereochemistry of the new stereogenic center
  • Formation of the new stereogenic center is independent of the existing chiral center
    • Nucleophilic attack by H2O occurs randomly from either the top or bottom face of the carbocation

Optical activity from chiral reactions

  • Product of the addition reaction will contain two stereogenic centers
    • Original chiral center from the reactant alkene
    • Newly formed stereogenic center from the addition of H2O
  • Original chiral center retains its configuration and optical activity
    • This center is not affected by the addition reaction
  • Newly formed stereogenic center is racemic (equal amounts of both enantiomers)
    • Random attack by H2O from either face of the carbocation leads to a 50:50 mixture of enantiomers
  • Overall product is a mixture of diastereomers (stereoisomers)
    • Each diastereomer has one defined stereogenic center (from the reactant)
    • One racemic center (from the addition)
  • Optical activity of the product will be reduced compared to the starting alkene
    • Presence of the racemic stereogenic center dilutes the optical activity of the original chiral center

Reaction Considerations

  • Regioselectivity in addition reactions determines which carbon of the alkene the OH group attaches to
  • Stereospecificity refers to the control of stereochemistry in the product based on the stereochemistry of the reactant
  • In some cases, carbocation rearrangements may occur through an alkyl shift, potentially affecting the final product structure

Key Terms to Review (21)

Addition reactions: Addition reactions occur when two or more reactants combine to form a single product, typically involving unsaturated molecules (like alkenes and alkynes) gaining atoms, eliminating their double or triple bonds. These reactions are fundamental in organic chemistry for creating more complex molecules from simpler ones.
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.
Stereogenic center: A stereogenic center in a molecule is an atom, typically carbon, that is attached to four different groups or atoms, allowing the molecule to exist in two or more spatial arrangements (stereoisomers). These centers are crucial for the molecule's three-dimensional shape and properties.
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.
Hyperconjugation: Hyperconjugation is a type of conjugation in organic chemistry where the sigma bonds of alkyl groups (such as methyl or ethyl) interact with adjacent pi bonds, leading to increased stability of the molecule. This stabilizing effect is particularly important in understanding the stability of carbocations and the orientation of electrophilic additions.
Carbocation: A carbocation is a positively charged carbon atom that is part of an organic molecule. These reactive intermediates play a crucial role in various organic reactions, including electrophilic additions, nucleophilic substitutions, and elimination reactions.
Nucleophilic Attack: Nucleophilic attack is a fundamental chemical reaction in which a nucleophile, an electron-rich species, attacks an electrophilic (electron-deficient) center, forming a new covalent bond. This process is central to understanding many organic reactions, including polar reactions, addition reactions, and substitution reactions.
Racemic Mixture: A racemic mixture is a type of mixture that contains equal amounts of two enantiomers, which are molecules that are non-superimposable mirror images of each other. Racemic mixtures are important in the context of organic chemistry, as they relate to the concepts of chirality, optical activity, and the resolution of enantiomers.
Stereoisomers: Stereoisomers are molecules that have the same molecular formula and connectivity, but differ in the three-dimensional arrangement of their atoms in space. This spatial arrangement of atoms leads to different physical and chemical properties, even though the atoms are connected in the same way.
Optical Activity: Optical activity is the ability of certain molecules to rotate the plane of polarized light as it passes through a solution containing those molecules. This phenomenon is directly related to the concept of chirality, where molecules can exist in two non-superimposable mirror-image forms, known as enantiomers.
Enantiomers: Enantiomers are a pair of stereoisomers that are non-superimposable mirror images of each other. They have the same molecular formula and connectivity, but differ in the spatial arrangement of their atoms, resulting in a unique handedness or chirality.
Stereogenic Center: A stereogenic center, also known as a chiral center, is an atom within a molecule that has four different substituents attached to it, resulting in the formation of two non-superimposable mirror images, or enantiomers. This concept is central to understanding the stereochemistry of organic molecules and their behavior in various chemical reactions.
Diastereomers: Diastereomers are a type of stereoisomer that have the same molecular formula and connectivity, but differ in their three-dimensional arrangement of atoms in space. They are not mirror images of each other and do not exhibit the property of chirality.
Hydration: Hydration is the process of adding water to a chemical compound, typically involving the addition of water across a double bond or the incorporation of water into the structure of a molecule. This term is particularly relevant in the context of organic chemistry, where it plays a crucial role in various reactions and transformations.
Addition Reactions: Addition reactions are a type of organic chemical reaction where two or more reactants combine to form a single product. They are characterized by the addition of atoms or molecules to an unsaturated compound, such as an alkene or alkyne, resulting in the formation of a new saturated compound.
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.
Inductive Effects: Inductive effects refer to the ability of substituents or functional groups to influence the distribution of electron density within a molecule through space. This phenomenon can have significant implications on the stability, reactivity, and orientation of various organic reactions.
Alkyl Shift: An alkyl shift is a type of carbocation rearrangement that occurs during electrophilic addition reactions. It involves the migration of an alkyl group (such as methyl, ethyl, or isopropyl) from one carbon atom to an adjacent carbocation center, stabilizing the intermediate and altering the product formation.
Chiral Alkene: A chiral alkene is a carbon-carbon double bond where the two substituents attached to each carbon atom are different, resulting in a non-superimposable mirror image. This feature of asymmetry is crucial in the context of the addition of water to a chiral alkene.
Stereospecificity: Stereospecificity refers to the ability of a chemical reaction to produce a specific stereoisomer or spatial arrangement of atoms in the product, based on the stereochemistry of the reactants. This concept is crucial in understanding the outcomes of various organic reactions, particularly those involving chiral molecules.