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Organic Chemistry
Table of Contents
Unit 5 Overview: Stereochemistry at Tetrahedral Centers
5.1 Enantiomers and the Tetrahedral Carbon
5.2 The Reason for Handedness in Molecules: Chirality
5.3 Optical Activity
5.4 Pasteur’s Discovery of Enantiomers
5.5 Sequence Rules for Specifying Configuration
5.6 Diastereomers
5.7 Meso Compounds
5.8 Racemic Mixtures and the Resolution of Enantiomers
5.9 A Review of Isomerism
5.10 Chirality at Nitrogen, Phosphorus, and Sulfur
5.11 Prochirality
5.12 Chirality in Nature and Chiral Environments

🥼organic chemistry review

5.12 Chirality in Nature and Chiral Environments

Citation:

Chirality is a game-changer in organic chemistry, affecting how molecules interact with our bodies. It's like having two gloves - they look the same, but only one fits your right hand perfectly. This concept is crucial for understanding drug effects and biological processes.

Chiral molecules are picky about who they dance with in our bodies. Like a key fitting a lock, only the right-shaped molecule can bind to a specific receptor. This selectivity is why some drugs work wonders while their mirror images might do nothing or even cause harm.

Chirality and Biological Activity

Chirality's impact on biological properties

  • Chirality plays a crucial role in determining the biological properties of molecules
    • Enantiomers can have different pharmacological effects despite having the same chemical formula (thalidomide, fluoxetine)
      • One enantiomer may be biologically active while the other is inactive or even harmful
    • Fluoxetine, the active ingredient in Prozac, is a chiral molecule
      • The (S)-enantiomer is responsible for the antidepressant activity
      • The (R)-enantiomer has no significant antidepressant effect

Chiral molecules and receptor interactions

  • Chiral molecules interact with biological receptors through complementary shapes
    • Receptors have specific three-dimensional structures that allow them to bind selectively to certain molecules (enzymes, proteins)
    • The shape of the receptor's binding site is complementary to the shape of the ligand (neurotransmitters, hormones)
  • The "lock and key" model is an analogy for receptor-ligand interactions
    • The receptor is the "lock" with a specific shape
    • The ligand is the "key" that fits into the lock
    • Only the correct enantiomer (the right "key") can fit properly into the receptor's binding site (D-glucose, L-amino acids)
  • Enantiomers have different three-dimensional shapes
    • This difference in shape allows receptors to distinguish between enantiomers
    • One enantiomer may bind strongly to the receptor, while the other may not bind at all or bind weakly (morphine, ibuprofen)
  • Optical activity is a property of chiral molecules that can be used to distinguish between enantiomers

Chiral Environments and Selective Reactions

Prochiral substrates in chiral environments

  • Prochiral substrates have two identical functional groups that can be distinguished in a chiral environment (ketones, alkenes)
    • These substrates are not chiral but can give rise to chiral products when reacted selectively
  • Chiral environments can induce selective reactions on prochiral substrates
    • Enzymes are naturally occurring chiral catalysts that can promote selective reactions (alcohol dehydrogenase, cytochrome P450)
    • The chirality of the enzyme's active site influences the stereochemical outcome of the reaction
  • The ethanol-NAD+ reaction is an example of a selective reaction in a chiral environment
    • Ethanol is a prochiral substrate with two identical hydrogen atoms on the $\alpha$-carbon
    • In the presence of the enzyme alcohol dehydrogenase (ADH) and the cofactor nicotinamide adenine dinucleotide (NAD+), one of the hydrogen atoms is selectively removed
      1. ADH's active site is chiral and can distinguish between the two hydrogen atoms
      2. The enzyme selectively abstracts the pro-R hydrogen, resulting in the formation of acetaldehyde with a specific stereochemistry
    • The selectivity of this reaction is determined by the chirality of the enzyme's active site

Stereochemistry and Chiral Centers

  • Stereochemistry is the study of the three-dimensional arrangement of atoms in molecules
  • Chiral centers are atoms, typically carbon, bonded to four different groups
  • Stereoisomers are compounds with the same molecular formula but different spatial arrangements of atoms
  • Asymmetric synthesis involves the creation of chiral molecules from achiral starting materials in chiral environments

Key Terms to Review (34)

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.
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.
Enzymes: Enzymes are biological catalysts that speed up chemical reactions in living organisms without being consumed in the process. They play a crucial role in facilitating various biochemical reactions necessary for life, including those involved in metabolism and DNA replication.
Ketones: Ketones are organic compounds characterized by a carbonyl group (C=O) bonded to two other carbon atoms within the molecule. They are formed by the oxidation of secondary alcohols.
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.
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.
Acetaldehyde: Acetaldehyde is a colorless, flammable organic compound with the chemical formula CH3CHO. It is the simplest aliphatic aldehyde and is an important intermediate in various chemical processes and metabolic pathways.
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.
Thalidomide: Thalidomide is a synthetic drug that was initially marketed as a sedative and anti-nausea medication, but its use was later banned due to its devastating effects on fetal development, causing severe birth defects. This term is particularly relevant in the context of understanding chirality and the importance of molecular handedness in nature and chiral environments.
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.
Chiral Centers: Chiral centers are atoms within a molecule that have four different substituents attached, resulting in a non-superimposable mirror image. This asymmetry gives rise to the concept of chirality, which is essential in understanding optical activity, meso compounds, and the stereochemistry of various organic reactions and biomolecules.
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.
D-glucose: D-glucose is a monosaccharide, the most abundant sugar found in nature. It is an aldose, meaning it has an aldehyde group at one end, and is the stereoisomer with the D-configuration, indicating the position of the hydroxyl group on the chiral carbon farthest from the aldehyde group.
Receptor Interactions: Receptor interactions refer to the dynamic and specific binding of molecules, such as hormones or neurotransmitters, to their corresponding receptors on the surface or within cells. These interactions trigger signaling cascades that elicit various physiological responses in the body, and are crucial for understanding the mechanisms behind chiral environments and the role of chirality in nature.
Hormones: Hormones are chemical messengers produced by the endocrine system that regulate various physiological processes and bodily functions. They play a crucial role in maintaining homeostasis and coordinating the body's response to internal and external stimuli, including those related to chirality in nature and chiral environments.
Enzymes: Enzymes are biological catalysts that accelerate chemical reactions in living organisms. They are essential for a wide range of processes, from digestion to energy production, and play a crucial role in maintaining the delicate balance of biochemical reactions that sustain life.
Ibuprofen: Ibuprofen is a non-steroidal anti-inflammatory drug (NSAID) commonly used to relieve pain, reduce inflammation, and lower fever. It is a chiral compound with two enantiomeric forms, which is relevant in the context of chirality in nature and chiral environments.
Fluoxetine: Fluoxetine is a selective serotonin reuptake inhibitor (SSRI) medication commonly used to treat depression, anxiety, and other mental health conditions. It works by increasing the availability of the neurotransmitter serotonin in the brain, which can help regulate mood and other physiological processes.
(R)-enantiomer: The (R)-enantiomer is one of the two mirror-image forms of a chiral molecule. Chirality refers to the property of a molecule that has a non-superimposable mirror image, and the (R)-enantiomer is the specific orientation of the molecule that is designated with the 'R' configuration.
L-amino acids: L-amino acids are a class of amino acids that have the same absolute configuration as the naturally occurring amino acids found in proteins. They are the predominant form of amino acids utilized by living organisms in biochemical processes.
(S)-enantiomer: The (S)-enantiomer is one of the two possible stereoisomeric forms of a chiral molecule, where the 'S' refers to the spatial arrangement of the substituents around the chiral center. The (S)-enantiomer is a non-superimposable mirror image of the (R)-enantiomer, and the two forms often have different biological and chemical properties.
Proteins: Proteins are large, complex biomolecules composed of amino acids that play crucial roles in the structure and function of living organisms. They are essential for a wide range of biological processes, including catalyzing chemical reactions, transporting and storing other molecules, providing structural support, and participating in immune responses.
Neurotransmitters: Neurotransmitters are chemical messengers that transmit signals between neurons and target cells in the nervous system. They play a crucial role in the communication and function of the brain and body.
Cytochrome P450: Cytochrome P450 is a superfamily of enzymes that play a crucial role in the metabolism and biosynthesis of various endogenous and exogenous compounds, including steroids and other lipids. These enzymes are found in most living organisms and are particularly important in the context of chirality and the biosynthesis of steroids.
Prochiral Substrates: Prochiral substrates are molecules that possess a plane of symmetry, meaning they can be converted into chiral products through a chemical reaction. These substrates lack a stereocenter but have the potential to generate one upon transformation.
Morphine: Morphine is a naturally occurring opioid analgesic derived from the opium poppy plant. It is a powerful pain-relieving drug that has been widely used in medical and recreational contexts, with significant implications in the fields of chirality and polycyclic aromatic compounds.
Chiral Catalysts: Chiral catalysts are enantioselective catalysts that can preferentially promote the formation of one enantiomer of a product over the other in a chemical reaction. These catalysts possess a chiral environment that allows them to interact differently with the reactants, leading to the selective formation of a desired stereoisomer.
Ketones: Ketones are a class of organic compounds containing a carbonyl group (C=O) bonded to two alkyl or aryl groups. They are characterized by the presence of a carbonyl carbon flanked by two carbon atoms. Ketones are important in various organic chemistry topics, including chirality, oxidation reactions, mass spectrometry, infrared spectroscopy, and NMR spectroscopy.
Alcohol Dehydrogenase: Alcohol dehydrogenase (ADH) is an enzyme that catalyzes the oxidation of alcohols to aldehydes or ketones. It plays a crucial role in the metabolism of ethanol and other alcohols in the body, as well as in the regulation of chiral environments and the catalysis of enzymatic reactions.
Pro-R Hydrogen: The pro-R hydrogen refers to the specific hydrogen atom on a chiral carbon that is positioned on the right side of the molecule when viewed from the front. This orientation is crucial in understanding the concept of chirality and the significance of chiral environments in nature.
Nicotinamide Adenine Dinucleotide: Nicotinamide adenine dinucleotide (NAD) is a coenzyme found in all living cells that plays a crucial role in various metabolic processes, including energy production, cellular signaling, and DNA repair. It is a key component in the electron transport chain and is involved in numerous redox reactions within the body.
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.
Ethanol-NAD+ Reaction: The ethanol-NAD+ reaction is a crucial metabolic process in which ethanol is oxidized to acetaldehyde, with the concomitant reduction of the coenzyme NAD+ to NADH. This reaction is a key step in the metabolism of ethanol and has important implications in the context of chirality and chiral environments.