5.11 Prochirality
3 min read•may 7, 2024
is a key concept in organic chemistry, bridging the gap between achiral and chiral molecules. It's all about potential - how a simple change can transform a molecule's symmetry and create new .
Understanding prochirality helps predict reaction outcomes and explains enzyme selectivity. By grasping Re/Si faces and / groups, you'll be better equipped to tackle challenges in synthesis and biological processes.
Prochirality and Stereochemistry
Concept of prochirality
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Organic chemistry 16: Stereochemistry - complex chirality, prochirality, topicity View original
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- Prochirality property of molecules or molecular fragments that can be converted from achiral to chiral in a single step
- molecules not chiral but have potential to become chiral
- Prochiral molecules contain not yet chiral due to presence of two identical called groups
- Converting prochiral molecule to chiral one involves selective replacement of one enantiotopic group with different substituent creates stereocenter resulting in chiral molecule
- Examples of prochiral molecules include:
- Ketones with two identical substituents on -carbon
- Alkenes with two identical substituents on one carbon
- atoms with two identical substituents (-CH2 groups)
Re vs Si faces
- Re and Si faces apply to planar atoms (carbonyl carbons, alkene carbons)
- Assign priorities to three substituents using (CIP) rules
- View molecule with lowest priority substituent pointing away from viewer
- Priority decreases clockwise direction ; counterclockwise
- Understanding Re and Si faces is crucial for predicting the outcome of reactions
Pro-R and pro-S groups
- Pro-R and pro-S groups apply to tetrahedral sp3-hybridized atoms with two identical substituents (-CH2 groups)
- Replace one identical substituent with higher priority group (-Cl)
- Assign priorities using
- Configuration R replaced substituent pro-R group; S pro-S group
- Pro-R and pro-S designations are important in understanding and predicting reaction outcomes
Prochirality in reaction outcomes
- Enzymes often distinguish between enantiotopic groups leading to
- Enzyme active sites chiral environments interact differently with enantiotopic groups
- Reduction of can result in formation of
- Stereochemical outcome depends on selectivity of reducing agent (enzymes, chiral catalysts)
- Addition reactions to can lead to formation of chiral products
- Stereochemistry of product depends on face selectivity (Re or Si) of addition reaction
- Biological methylation reactions often occur selectively on one enantiotopic hydrogen of prochiral -CH2 group
- (SAM) common biological methylating agent can distinguish between pro-R and pro-S hydrogens
- Understanding prochirality allows prediction and control of stereochemistry in organic synthesis and biological processes
Chirality and Stereoisomers
- refers to the geometric property of a molecule that is non-superimposable on its mirror image
- Stereoisomers are molecules with the same molecular formula and bonding sequence but different 3D arrangements of atoms
- Prochiral molecules can be converted to chiral molecules, resulting in the formation of stereoisomers
- The study of prochirality is essential for understanding the relationship between molecular structure and stereoisomeric outcomes in chemical reactions
Key Terms to Review (26)
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.
Asymmetric Synthesis: Asymmetric synthesis is a chemical reaction that produces a chiral molecule in a stereoselective manner, resulting in the formation of one enantiomer or diastereomer in excess over the other. This concept is crucial in understanding various topics in organic chemistry, including Pasteur's discovery of enantiomers, chirality at nitrogen, phosphorus, and sulfur, prochirality, chirality in nature and chiral environments, and the synthesis of amino acids.
Cahn-Ingold-Prelog: The Cahn-Ingold-Prelog (CIP) system is a set of rules used to assign a priority ranking to the substituents attached to a stereogenic center, allowing for the unambiguous specification of the stereochemistry of a molecule.
Chiral Alcohols: Chiral alcohols are organic compounds that contain a carbon atom bonded to four different substituents, making them non-superimposable on their mirror image. This asymmetry gives rise to the concept of chirality, which is central to the understanding of prochirality as discussed in topic 5.11.
Chirality: Chirality is a fundamental concept in organic chemistry that describes the three-dimensional arrangement of atoms in a molecule. It refers to the property of a molecule that is non-superimposable on its mirror image, resulting in the existence of two distinct forms known as enantiomers. Chirality is a crucial factor in understanding the behavior and properties of various organic compounds, including their interactions with living systems.
Chirality centers: A chirality center in organic chemistry is an atom, typically carbon, that has four different groups attached to it, leading to non-superimposable mirror image forms of the molecule. These centers are crucial for determining the 3D spatial orientation of molecules, affecting their chemical behavior and interactions.
CIP Rules: The CIP (Cahn-Ingold-Prelog) rules are a set of guidelines used to systematically assign priorities to substituents around a stereogenic center, allowing for the unambiguous specification of the stereochemistry of a molecule. These rules are particularly important in the context of understanding prochirality.
Enantiotopic: Enantiotopic refers to the relationship between two identical substituents or groups on a prochiral molecule that, if replaced, would result in the formation of enantiomeric products. This concept is crucial in understanding the stereochemical outcomes of chemical reactions involving prochiral substrates.
Molecular Symmetry: Molecular symmetry refers to the arrangement and orientation of atoms within a molecule that allows for the identification of symmetry elements such as planes, axes, and centers of symmetry. This concept is crucial in understanding the conformations of molecules, their handedness, and the characteristics of nuclear magnetic resonance (NMR) spectroscopy.
Pro-R: The pro-R position refers to the spatial orientation of a substituent group or atom in a chiral molecule. It describes the position of a substituent that is on the right-hand side of a molecule when the molecule is viewed with the priority groups arranged in descending order from left to right, according to the Cahn-Ingold-Prelog (CIP) priority rules.
Pro-S: The pro-S configuration refers to the spatial arrangement of atoms or substituents around a stereogenic center, where the substituent designated as the priority group (S) is positioned on the same side as the observer. This term is particularly relevant in the context of prochirality, which describes molecules that have the potential to become chiral upon a specific chemical transformation.
Prochiral: Prochirality refers to the property of a molecule that has two enantiotopic groups or faces that are not related by any symmetry operation. In other words, a prochiral molecule has the potential to become chiral upon the introduction of a new substituent or functional group.
Prochiral Alkenes: Prochiral alkenes are alkenes that have two enantiotopic hydrogen atoms or substituents attached to a carbon-carbon double bond. This means that the two hydrogen atoms or substituents can be replaced with different groups to generate two distinct stereoisomers.
Prochiral Ketones: Prochiral ketones are a class of organic compounds where the two hydrogen atoms attached to the carbonyl carbon are enantiotopic, meaning they are non-identical and can be differentiated. This property allows for the potential to introduce stereochemistry through selective functionalization of one of the prochiral hydrogen atoms.
Prochirality: Prochirality refers to the property of a molecule or a functional group that has the potential to become chiral, or asymmetric, upon a specific chemical transformation or reaction. In other words, a prochiral molecule can be converted into a chiral molecule through a well-defined chemical process.
Prochirality center: A prochirality center is a carbon atom in a molecule that can be converted from achiral to chiral with a single substitution. It essentially has the potential to become a stereocenter upon modification of its environment or substituents.
Re face: The re face, or re-face, refers to the orientation of a substituent or functional group on a prochiral center. It describes the spatial arrangement of the substituent in relation to the other groups attached to the prochiral carbon, which is crucial in determining the stereochemical outcome of a reaction.
S-Adenosyl methionine: S-Adenosyl methionine (SAM) is a versatile metabolite that serves as a methyl donor in numerous biological reactions, playing a crucial role in cellular processes such as epigenetics and neurotransmitter synthesis. It is a key intermediate in the one-carbon metabolism pathway and is involved in both prochirality and biological substitution reactions.
Si face: The Si face refers to the side of a prochiral center where silicon is attached. This term is important in the context of prochirality and the synthesis of amino acids, as the Si face can influence the stereochemical outcome of reactions involving prochiral centers.
Sp2-Hybridized: sp2-hybridized refers to the type of hybridization that occurs in carbon atoms where three sp2 hybrid orbitals and one unhybridized p orbital are formed. This hybridization is commonly observed in molecules with trigonal planar geometry, such as those found in the context of prochirality.
Sp3-Hybridized: sp3-hybridized refers to the type of hybridization that occurs in carbon atoms with four single bonds, resulting in a tetrahedral molecular geometry. This hybridization is a key concept in understanding the structure and reactivity of organic compounds, particularly in the context of prochirality and nucleophilic addition reactions involving water.
Stereocenters: A stereocenter is a carbon atom in a molecule that is bonded to four different substituents, resulting in a chiral center that can give rise to two possible stereoisomers. These stereoisomers are non-superimposable mirror images of each other, known as enantiomers.
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.
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.
Stereospecific Reactions: Stereospecific reactions are chemical transformations where the stereochemistry (spatial arrangement) of the reactants is directly transferred to the products. The configuration and orientation of the atoms in the starting materials are preserved in the final compounds, resulting in a specific stereoisomer as the outcome.
Substituents: Substituents are atoms or functional groups that replace hydrogen atoms in a molecule's structure. They are an essential concept in organic chemistry, as they play a crucial role in determining the properties, reactivity, and naming of various organic compounds.