5.1 Enantiomers and the Tetrahedral Carbon
2 min read•may 7, 2024
Carbon atoms bonded to four different groups create molecules with unique spatial arrangements. This 3D geometry leads to , where molecules can't be superimposed on their mirror images. Chiral molecules come in pairs called , with identical properties except for how they interact with light.
Enantiomers are crucial in nature and medicine. Many biological molecules exist as only one , like left-handed amino acids or right-handed sugars. This matters because our bodies can tell the difference, leading to varied effects from seemingly similar compounds.
Stereochemistry and Chirality
Molecular handedness origins
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- geometry arises when a carbon atom is bonded to four different substituents (chlorine, hydrogen, methyl, ethyl) oriented toward the vertices of a tetrahedron
- Chirality describes a molecule that is non-superimposable on its mirror image and lacks an internal plane of symmetry (bromochlorofluoromethane)
- refers to a carbon atom bonded to four different substituents also called a or (glyceraldehyde)
- Handedness determined by the arrangement of substituents around a leads to the existence of two mirror-image forms known as enantiomers ( and )
- These mirror-image forms are also called
Identification of enantiomers
- Enantiomers are molecules that are non-superimposable mirror images of each other have the same molecular formula and bonding arrangement but differ in the spatial orientation of their substituents ( and )
- Identifying enantiomers requires the presence of one or more stereogenic centers the absence of an internal plane of symmetry and the ability to draw mirror images that are non-superimposable (lactic acid)
- Non-enantiomeric molecules include those with an internal plane of symmetry () like 2,3-dichlorobutane molecules with multiple stereogenic centers that are superimposable on their mirror images () such as 2,3-butanediol and molecules lacking a stereogenic center (ethanol)
- Fischer projections can be used to represent the three-dimensional structure of molecules in a two-dimensional format
Properties and occurrence of enantiomers
- Physical properties of enantiomers are identical (melting point, boiling point, solubility) with the exception of their interaction with
- Enantiomers rotate plane-polarized light in opposite directions, a property known as
- (d or +) enantiomers rotate light clockwise (dextrose)
- (l or -) enantiomers rotate light counterclockwise (levorphanol)
- Chemical properties of enantiomers are identical in achiral environments but may exhibit different reactivity with other chiral molecules such as enzymes ( and )
- Occurrence in nature many naturally occurring molecules are chiral (amino acids, sugars, hormones)
- Often only one enantiomer is found in nature
- Amino acids in the L-configuration ()
- Sugars in the D-configuration ()
- Biological systems can distinguish between enantiomers leading to different physiological effects and pharmacological properties ( and )
- Often only one enantiomer is found in nature
Nomenclature and Mixtures
- The system is used to unambiguously describe the three-dimensional arrangement of substituents around a chiral center
- A contains equal amounts of both enantiomers, resulting in no net optical activity
Key Terms to Review (32)
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.
Asymmetric Carbon: An asymmetric carbon, also known as a chiral carbon, is a carbon atom that is bonded to four different substituents. This unique arrangement gives the molecule the ability to exist in two non-superimposable mirror-image forms, known as enantiomers, which have important implications in organic chemistry and biochemistry.
Chiral Center: A chiral center is a carbon atom with four different substituents attached, resulting in a non-superimposable mirror image. This structural feature is crucial in understanding the concepts of enantiomers, Pasteur's discovery of enantiomers, the sequence rules for specifying configuration, and the nucleophilic addition of HCN to form cyanohydrins.
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.
D-alanine: D-alanine is an amino acid that is the mirror image of the more common L-alanine. It is a key component in the synthesis of peptidoglycan, the structural backbone of bacterial cell walls, and plays a crucial role in the context of enantiomers and the tetrahedral carbon.
D-Cysteine: D-Cysteine is the enantiomer of the amino acid cysteine, where the sulfhydryl group (-SH) is oriented in the opposite direction compared to the more common L-cysteine. This stereochemical difference gives D-cysteine unique properties and functions within the context of enantiomers and the tetrahedral carbon.
D-dopa: D-dopa, or D-3,4-dihydroxyphenylalanine, is the enantiomer of the naturally occurring amino acid L-dopa. As a chiral molecule, D-dopa is one of the two possible stereoisomers of dopa, with distinct spatial arrangements of its functional groups around the tetrahedral carbon atom.
D-methamphetamine: D-methamphetamine, also known as dextroamphetamine, is the more potent and psychoactive enantiomer of the drug methamphetamine. It is a central nervous system stimulant that has been used for the treatment of attention-deficit/hyperactivity disorder (ADHD) and narcolepsy, but its high potential for abuse and addiction has led to strict regulations on its medical use.
D-ribose: D-ribose is a monosaccharide, a type of simple sugar, that is an essential component of ribonucleic acid (RNA). It is a key player in the topics of enantiomers, D-L sugars, and the configurations of aldoses.
Dextrorotatory: Dextrorotatory, also known as dextrorotation or (+)-rotation, refers to the ability of certain chiral molecules to rotate the plane of polarized light in a clockwise direction when viewed from the direction of the light source. This property is closely linked to the concept of optical activity and enantiomers, and has important implications in various fields, including organic chemistry, biochemistry, and pharmaceutical sciences.
Enantiomer: Enantiomers are pairs of molecules that are mirror images of each other but cannot be superimposed onto one another, similar to left and right hands. They often have identical physical properties but can exhibit different behaviors in chiral environments, such as interactions with polarized light or biological molecules.
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.
Fischer Projection: A Fischer projection is a way of representing the three-dimensional structure of a molecule, particularly organic compounds with tetrahedral carbon centers, on a two-dimensional plane. It is used to depict the relative orientation of substituents around a carbon atom and is crucial for understanding concepts such as enantiomers, diastereomers, and the configuration of sugars.
L-alanine: L-alanine is a non-essential amino acid that is a fundamental building block of proteins. It is the simplest chiral amino acid, with a methyl group as its side chain, and plays a crucial role in various metabolic processes within the body.
L-cysteine: L-cysteine is a sulfur-containing amino acid that is one of the 20 standard amino acids found in proteins. It is an important building block for many proteins and plays a crucial role in various biological processes.
L-dopa: L-dopa, or levodopa, is an amino acid that is a precursor to the neurotransmitters dopamine, norepinephrine, and epinephrine. It is a key compound in the context of enantiomers and the tetrahedral carbon structure, as it exhibits chirality and the ability to exist in different stereoisomeric forms.
L-methamphetamine: L-methamphetamine is the levorotatory enantiomer of the stimulant drug methamphetamine. As a chiral molecule, methamphetamine can exist in two mirror-image forms, the L- and D-isomers, which have different biological effects.
L-threonine: L-threonine is an essential amino acid that is a fundamental building block of proteins in the human body. It is involved in various metabolic processes and plays a crucial role in the context of enantiomers and the tetrahedral carbon structure.
Levorotatory: Levorotatory refers to the ability of a chiral molecule to rotate the plane of polarized light in a counterclockwise direction when viewed from the direction of the light source. This property is closely related to the concepts of optical activity, enantiomers, and the tetrahedral carbon structure.
Meso Compounds: Meso compounds are a type of stereoisomer that possess a plane of symmetry, making them achiral despite containing chiral centers. These unique molecules exhibit properties of both enantiomers and diastereomers, bridging the gap between different types of isomerism.
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.
Phosphine: Phosphine is a colorless, flammable, and toxic gas with the chemical formula PH3, similar to ammonia but with phosphorus replacing nitrogen. It is often encountered in organic chemistry as a ligand in coordination chemistry or as a reagent in the synthesis of organophosphorus compounds.
Plane-Polarized Light: Plane-polarized light is a type of electromagnetic radiation where the electric field oscillates in a single, well-defined plane. This property of light is closely related to the concepts of optical activity, enantiomers, diastereomers, and meso compounds in organic chemistry.
R/S Configuration: R/S configuration is a system used to unambiguously describe the three-dimensional arrangement of atoms around a chiral carbon center. It allows for the classification of stereoisomers as either R (rectus) or S (sinister) based on the priority of substituents attached to the chiral carbon.
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.
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.
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.
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.
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.
Tetrahedral: Tetrahedral refers to a three-dimensional molecular geometry in which a central atom is bonded to four other atoms, forming a shape resembling a pyramid with a triangular base. This arrangement is a fundamental concept in chemistry, particularly in the context of chemical bonding theory, organic chemistry, and stereochemistry.