8.12 Reaction Stereochemistry: Addition of H2O to an Achiral Alkene
3 min read•may 7, 2024
Water addition to alkenes creates new centers. When reacts with H2O, it forms a . This allows equal attack from both sides, producing a of (R) and .
Lab reactions often yield racemic mixtures due to conditions. In contrast, enzymes provide chiral environments, favoring one enantiomer. This selectivity in biological systems leads to stereospecific products, unlike the equal distribution in typical lab reactions.
Reaction Stereochemistry: Addition of H2O to an Achiral Alkene
Addition of H2O to 1-butene
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- 1-butene is an achiral alkene has a planar geometry around the double bond due to of the carbon atoms
- Addition of H2O to 1-butene creates a new at the C2 position of the product, , as the carbon becomes hybridized
- Reaction proceeds through a planar carbocation intermediate formed by protonation of the double bond ()
- H2O can attack the carbocation from either the top or bottom face with equal probability since both faces are sterically and electronically equivalent
- Attack from the top face leads to the formation of while attack from the bottom face leads to the formation of (S)-2-butanol
- Equal amounts of (R) and (S) enantiomers are produced, resulting in a racemic mixture of 2-butanol with a 50:50 ratio of the two enantiomers
Formation of racemic mixtures
- Achiral reactants, such as 1-butene, have a are superimposable on their mirror image making them indistinguishable from each other
- When an achiral reactant undergoes a reaction that creates a new chirality center, the reaction intermediate is typically planar (carbocation or ) due to hybridization
- Planar intermediate allows equal access to both faces by the attacking reagent (H2O) since there is no steric hindrance or electronic bias favoring one face over the other
- Equal probability of attack from either face leads to the formation of equal amounts of both enantiomers (R and S) in the product
- Resulting product is a racemic mixture, containing a 50:50 ratio of the (R) and (S) enantiomers that are non-superimposable mirror images of each other
Stereochemistry in lab vs enzyme reactions
- Laboratory reactions:
- Typically use achiral reagents and conditions that do not favor the formation of one enantiomer over the other
- Proceed through planar intermediates (carbocations) that allow equal access to both faces due to lack of steric or electronic bias
- Result in the formation of racemic mixtures when new chirality centers are created with equal amounts of both enantiomers (R and S)
- Enzyme-catalyzed reactions:
- Enzymes are chiral catalysts with specific three-dimensional structures that create a in the active site
- Active sites provide a chiral environment that favors the formation of one enantiomer over the other due to steric hindrance and specific interactions
- Enzymes can shield one face of the planar intermediate, allowing attack from only one side leading to stereospecific product formation
- Lead to the formation of or products with a high proportion of one enantiomer (R or S) over the other
- catalyze the stereospecific addition of H2O to alkenes, producing optically active alcohols with high (ee)
Selectivity in Addition Reactions
- : Determines which carbon of the alkene the nucleophile (H2O) will attack, often following Markovnikov's rule
- : Refers to the preferential formation of one stereoisomer over another in a reaction
- reactions involve the addition of H2O to alkenes, often proceeding through mechanisms
- occurs when the nucleophile (H2O) attacks the electrophilic carbocation intermediate
Key Terms to Review (31)
(R)-2-butanol: (R)-2-butanol is a chiral alcohol with the chemical formula CH3CH(OH)CH2CH3. The '(R)' prefix indicates the specific stereochemical configuration of the molecule, where the hydroxyl group (-OH) is in the R orientation around the chiral carbon. This term is important in the context of understanding isomerism and the stereochemistry of reactions involving the addition of water to achiral alkenes.
(S)-2-butanol: (S)-2-butanol is a chiral alcohol with the (S) stereochemical configuration. It is one of the four possible stereoisomers of 2-butanol, which is a secondary alcohol with the molecular formula C4H10O.
$sp^2$ Hybridization: $sp^2$ hybridization is a type of orbital hybridization in which one $s$ orbital and two $p$ orbitals of an atom combine to form three equivalent $sp^2$ hybrid orbitals. This hybridization is commonly observed in planar molecules, such as those formed by the addition of water to achiral alkenes.
$sp^3$ Hybridization: $sp^3$ hybridization is a type of atomic orbital hybridization that occurs in molecules with four bonding pairs of electrons around a central atom. This results in a tetrahedral arrangement of the bonding pairs and a characteristic bond angle of approximately 109.5 degrees.
1-butene: 1-butene is a simple alkene compound with the molecular formula C$_{4}$H$_{8}$. It is a colorless gas that is the first member of the alkene homologous series and is commonly used as a building block in the production of various petrochemicals and polymers.
2-Butanol: 2-Butanol is a secondary alcohol with the chemical formula CH3CH2CHCH3. It is an isomer of 1-butanol, with the hydroxyl group (-OH) attached to the second carbon atom in the butane chain. This positioning of the hydroxyl group gives 2-butanol unique properties and reactivity compared to other butanol isomers.
Achiral: Achiral refers to a molecule or object that is not chiral, meaning it is superimposable on its mirror image. Achiral molecules lack the necessary structural features, such as the presence of a stereogenic center, that would give rise to non-superimposable enantiomers.
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.
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.
Chiral: Chirality refers to the geometric property of a molecule or object that makes it non-superimposable on its mirror image. Chiral molecules are an important concept in organic chemistry, as they are central to understanding topics such as racemic mixtures, reaction stereochemistry, and the SN2 reaction.
Chiral environment: A chiral environment is a spatial arrangement in which the distribution of parts lacks an internal plane of symmetry, often leading to different interactions with other chiral molecules. This concept is crucial in organic chemistry, especially when considering how molecules recognize and react with each other based on their three-dimensional shape.
Chirality Center: A chirality center, also known as a stereogenic center, is an atom within a molecule that is attached to four different substituents, resulting in the formation of two non-superimposable mirror-image molecules, or enantiomers. This concept is crucial in understanding the stereochemistry of organic reactions, particularly the addition of water to achiral alkenes.
Electrophilic Addition: Electrophilic addition is a type of organic reaction where an electrophile, a species that is attracted to electrons, adds to the carbon-carbon double bond of an alkene. This results in the formation of a new carbon-carbon single bond and the incorporation of the electrophile into the molecule.
Electrophilic addition reaction: An electrophilic addition reaction is a chemical process in which an electrophile reacts with a nucleophile, typically an alkene or alkyne, forming a new sigma bond by adding across the double or triple bond. This reaction is key in organic synthesis, resulting in the addition of atoms or groups to the carbon atoms involved in the multiple bond.
Enantioenriched: Enantioenriched refers to a compound or mixture that has an excess of one enantiomer over the other, meaning it is not a racemic mixture. This term is particularly relevant in the context of the addition of water to achiral alkenes, where the resulting alcohol product can be enantioenriched depending on the specific reaction conditions and mechanisms involved.
Enantiomeric Excess: Enantiomeric excess is a measure of the difference in the amounts of two enantiomers (mirror-image molecules) present in a mixture. It is a key concept in understanding the resolution of racemic mixtures and the stereochemistry of addition reactions to achiral alkenes.
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.
Enantiopure: Enantiopure refers to a compound that contains only one enantiomer, or mirror-image form, of a chiral molecule. This is in contrast to a racemic mixture, which contains equal amounts of both enantiomers.
Enolate: An enolate is a negatively charged oxygen-containing species that arises from the removal of a proton from the α-carbon of a carbonyl compound. Enolates are important reactive intermediates in various organic reactions, including aldol condensations, Claisen condensations, and α-substitution reactions.
Hydratases: Hydratases are enzymes that catalyze the addition of water (H2O) to a substrate, typically an alkene or a nitrile, to form a new product. These enzymes play a crucial role in the context of the reaction stereochemistry involving the addition of water to an achiral alkene.
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.
Markovnikov Addition: Markovnikov addition is a fundamental organic chemistry concept that describes the regiochemical outcome of the addition of a polar molecule, such as hydrogen halides or water, to an unsymmetrical alkene or alkyne. It predicts the formation of the more stable carbocation intermediate, leading to the addition of the electrophilic component to the carbon atom that can best stabilize the resulting positive charge.
Nucleophilic Addition: Nucleophilic addition is a fundamental organic reaction in which a nucleophile, a species that donates electrons, adds to an electrophilic carbon center, typically a carbonyl carbon, to form a new product. This reaction is central to understanding many important topics in organic chemistry, including functional groups, polar reactions, carbocation stability, reaction stereochemistry, and the chemistry of aldehydes, ketones, alcohols, and other carbonyl-containing compounds.
Nucleophilic addition reaction: A nucleophilic addition reaction is a chemical process where a nucleophile forms a bond with an electrophilic carbon atom of a compound, typically found in aldehydes and ketones. This reaction results in the conversion of the carbonyl group into a more complex, often larger, molecule.
Planar: Planar refers to a geometric configuration where all the atoms or functional groups involved lie in the same flat or two-dimensional plane. This term is particularly relevant in the context of understanding the stereochemistry of chemical reactions, such as the addition of water to an achiral alkene.
Plane of symmetry: A plane of symmetry in a molecule is an imaginary plane that divides the molecule into two mirror-image halves. This concept is crucial in determining the chirality of molecules, as chiral molecules lack this plane of symmetry due to their non-superimposable mirror images.
Plane of Symmetry: A plane of symmetry is a hypothetical plane that divides a molecule or object into two equal and mirror-image halves. It is an important concept in understanding the symmetry and stereochemistry of organic compounds, particularly in the context of meso compounds and the stereochemistry of addition 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.
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.
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.
Stereoselectivity: Stereoselectivity refers to the preference of a chemical reaction to form one stereoisomeric product over another. It is a crucial concept in organic chemistry that describes the ability of a reaction to control the spatial arrangement of atoms in the final product.