Glossary

are molecules with the same chemical formula but different 3D arrangements of atoms. They come in two main types: (mirror images) and (non-mirror images). Understanding these is key to grasping molecular structure and reactivity.

Knowing how to calculate and represent stereoisomers is crucial. The number of possible stereoisomers depends on , while Fischer and Newman projections help visualize their 3D structures. This knowledge is essential for predicting molecular behavior and understanding biological processes.

Stereoisomers

Enantiomers vs diastereomers

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  • Enantiomers
    • Non-superimposable mirror images of each other (like left and right hands)
    • Opposite configurations at all chirality centers (R and S)
    • Identical physical properties except for the direction of rotation (clockwise or counterclockwise)
    • Exhibit due to their chiral nature
  • Diastereomers
    • Not mirror images of each other (like cis and )
    • Different configurations at one or more but not all chirality centers
    • Different physical properties such as melting points, boiling points, and solubilities (can be separated by physical means)

Stereoisomer quantity calculation

  • Number of possible stereoisomers for a molecule determined by the formula: 2n2^n, where nn is the number of chirality centers (also called stereocenters)
    • Molecule with 2 chirality centers will have 22=42^2 = 4 possible stereoisomers (2 enantiomeric pairs)
    • Molecule with 3 chirality centers will have 23=82^3 = 8 possible stereoisomers (4 enantiomeric pairs)
  • Stereoisomers consist of:
    • Enantiomeric pairs: 2n2=2n−1\frac{2^n}{2} = 2^{n-1} (1 pair for 2 chirality centers, 2 pairs for 3 chirality centers)
    • Diastereomeric pairs: 2n−2n−12=2n−1−1\frac{2^n - 2^{n-1}}{2} = 2^{n-1} - 1 (1 pair for 2 chirality centers, 3 pairs for 3 chirality centers)
    • Molecule has multiple chirality centers but is due to an internal plane of symmetry (can be superimposed on its mirror image)
    • Meso compounds reduce the total number of stereoisomers by 1 ( has 3 stereoisomers instead of 4)

Epimers and stereoisomer relationships

    • Subset of diastereomers that differ in configuration at only one chirality center (like and )
    • Closest stereoisomeric relationship among diastereomers (most similar in structure)
  • Relationship to other stereoisomers
    • Enantiomers differ at all chirality centers, while epimers differ at only one (enantiomers are farther apart structurally)
    • Diastereomers that are not epimers differ at more than one but not all chirality centers (in between enantiomers and epimers)
  • Importance of epimers
    • Similar physical properties due to their close structural relationship (harder to separate than other diastereomers)
    • Different biochemical activities or functions in biological systems (glucose and galactose metabolism)

Stereoisomer representations

  • Fischer projections
    • 2D representation of 3D molecular structures, particularly useful for depicting stereoisomers
    • Horizontal lines represent bonds coming out of the page, vertical lines represent bonds going into the page
  • Newman projections
    • Used to visualize different conformations of molecules, especially along carbon-carbon single bonds
    • Helpful in understanding configurational and

Key Terms to Review (21)

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.
Chirality Centers: Chirality centers, also known as stereogenic centers, are atoms within a molecule that have four different substituents attached to them. This arrangement creates a non-superimposable mirror image, resulting in the formation of two distinct stereoisomers called enantiomers.
Cis Isomers: Cis isomers are a type of stereoisomer where two identical substituents are located on the same side of a carbon-carbon double bond or a cyclic structure. This term is particularly relevant in the context of understanding isomerism in cycloalkanes, cyclohexane conformations, and diastereomers.
Cis–trans isomers: Cis–trans isomers are types of stereoisomers where the same atoms or groups of atoms are positioned differently around a rigid structure, such as a double bond or a ring system, in cycloalkanes. In cis isomers, these groups are on the same side; in trans isomers, they are on opposite sides.
Configurational Isomers: Configurational isomers are a type of stereoisomers that differ in the spatial arrangement of atoms or groups around a carbon-carbon double bond or a tetrahedral carbon center, without any difference in the connectivity of atoms. These isomers cannot be interconverted without breaking and reforming covalent bonds.
Conformational Isomers: Conformational isomers are different spatial arrangements of atoms within the same molecule that can be interconverted by the rotation of single bonds without breaking any covalent bonds. These isomers are particularly relevant in the context of understanding diastereomers, as conformational differences can contribute to the distinct properties of diastereomeric compounds.
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.
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.
Epimers: Epimers are a type of stereoisomers that differ in the configuration of only one stereocenter, or chiral carbon, within a molecule. This subtle difference in the spatial arrangement of atoms can have significant implications in the context of carbohydrate chemistry and stereochemistry.
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.
Galactose: Galactose is a monosaccharide, or simple sugar, that is a C-4 epimer of glucose. It is an important component of lactose, the primary sugar found in mammalian milk, and is also produced in the body during the metabolism of lactose.
Glucose: Glucose is a simple sugar, or monosaccharide, that serves as the primary source of energy for the body's cells. It is a key component in various metabolic processes and plays a central role in carbohydrate chemistry and biochemistry.
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.
Newman projection: A Newman projection is a method used in organic chemistry to visualize the spatial arrangement of bonds and atoms in a molecule from a specific viewpoint, which is looking down the bond axis connecting two carbon atoms. This visual representation helps in understanding the different conformations (spatial arrangements) that molecules can adopt due to rotation around single bonds.
Newman Projection: The Newman projection is a way of representing the three-dimensional structure of organic molecules, particularly alkanes, on a two-dimensional plane. It provides a simplified view of the spatial arrangement of atoms and their relative positions, allowing for the analysis of conformational preferences and steric interactions.
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.
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.
Stereocenter: A stereocenter is a carbon atom in a molecule that is bonded to four different substituents, resulting in a chiral center that can exist in two non-superimposable mirror-image forms called enantiomers. Stereocenters are central to understanding the handedness and configuration of molecules, as well as their interactions in biological systems.
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.
Tartaric Acid: Tartaric acid is a naturally occurring organic acid found in many fruits, especially grapes. It is an important compound in the context of understanding the reason for handedness in molecules, Pasteur's discovery of enantiomers, and the concept of diastereomers in organic chemistry.
Trans Isomers: Trans isomers are a type of stereoisomers that occur when two identical substituents are positioned on opposite sides of a carbon-carbon double bond or a ring structure. This arrangement contrasts with cis isomers, where the identical substituents are on the same side.
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