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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

Isomers are molecules with the same formula but different structures. They come in two main types: constitutional isomers have different bonding arrangements, while stereoisomers have different spatial arrangements. Understanding isomers is crucial for predicting molecular properties and reactivity.

Stereoisomers include enantiomers (mirror images), diastereomers (non-mirror images), and cis-trans isomers (geometric isomers). Chirality, the property of non-superimposable mirror images, is key in stereochemistry. Fischer and Newman projections help visualize 3D structures in 2D representations.

Types of Isomers

Constitutional vs stereoisomers

  • Constitutional isomers have the same molecular formula but different bonding arrangements or atomic connectivity (butane and 2-methylpropane, ethanol and dimethyl ether)
  • Stereoisomers have the same molecular formula and bonding arrangements but different spatial arrangements of atoms ((R)-2-butanol and (S)-2-butanol, cis-2-butene and trans-2-butene)

Types of constitutional isomers

  • Skeletal isomers have the same molecular formula but different carbon chain arrangements (pentane and 2-methylbutane)
  • Functional isomers have the same molecular formula but different functional groups (propanal and propanone (acetone))
  • Positional isomers have the same molecular formula and functional group but different positions of the functional group on the carbon chain (1-propanol and 2-propanol)

Stereoisomers

Categories of stereoisomers

  • Enantiomers are non-superimposable mirror images of each other with opposite configurations at all stereogenic centers ((R)-2-butanol and (S)-2-butanol)
    • Enantiomers exhibit optical activity, rotating plane-polarized light in opposite directions
  • Diastereomers are not mirror images of each other and have the same configuration at one or more stereogenic centers, but opposite configurations at other stereogenic centers ((2R,3S)-2,3-butanediol and (2R,3R)-2,3-butanediol)
  • Cis-trans isomers (geometric isomers) differ in the spatial arrangement of substituents across a double bond or ring
    • Cis isomers have substituents on the same side of the double bond or ring
    • Trans isomers have substituents on opposite sides (cis-2-butene and trans-2-butene)
  • Relationship between stereoisomers
    1. Enantiomers are a type of stereoisomer with a specific mirror-image relationship
    2. Diastereomers are all other stereoisomers that are not enantiomers
    3. Cis-trans isomers are a specific type of diastereomer

Chirality and Representation

  • Chirality refers to the property of molecules that are non-superimposable on their mirror images
  • Meso compounds are optically inactive despite having stereogenic centers due to an internal plane of symmetry
  • Fischer projections are two-dimensional representations of three-dimensional molecules, commonly used for depicting stereochemistry of sugars and amino acids
  • Newman projections are useful for visualizing different conformations of molecules, particularly along single bonds

Key Terms to Review (25)

Fischer projections: Fischer projections are a two-dimensional drawing method used in organic chemistry to represent the three-dimensional spatial arrangement of atoms around chiral centers, primarily in carbohydrates. This method uses horizontal lines to represent bonds projecting forward (out of the plane) and vertical lines for bonds projecting backward (into the plane).
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.
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.
Constitutional isomers: Constitutional isomers are compounds that have the same molecular formula but differ in the sequence in which their atoms are connected. These variations lead to molecules with distinct physical and chemical properties, despite having the same numbers of each type of atom.
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.
Constitutional Isomers: Constitutional isomers are a type of structural isomerism where molecules have the same molecular formula but differ in the connectivity or arrangement of their atoms. This concept is essential in understanding the properties and behavior of organic compounds across various topics in chemistry.
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.
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.
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.
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.
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.
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.
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.
Skeletal Isomers: Skeletal isomers are a type of structural isomerism where organic compounds have the same molecular formula but differ in the arrangement of their carbon skeleton. This means the compounds have the same number and type of atoms, but the atoms are connected in a different order, resulting in distinct three-dimensional structures.
Functional Isomers: Functional isomers are a type of structural isomerism where molecules have the same molecular formula but differ in the arrangement or type of functional groups present. This results in different chemical and physical properties despite the same overall composition.
Positional Isomers: Positional isomers are a type of structural isomerism where the atoms or functional groups are arranged differently within the molecule, resulting in distinct compounds with the same molecular formula. These isomers differ in the position of substituents or functional groups along the carbon chain or ring structure.
(2R,3S)-2,3-butanediol: (2R,3S)-2,3-butanediol is a chiral organic compound with two stereogenic centers, resulting in four possible stereoisomers. It is a diol, meaning it contains two hydroxyl (-OH) groups, and is an important intermediate in various chemical processes and applications.
(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.
Cis-trans Isomers: Cis-trans isomers are a type of stereoisomerism that occurs in alkenes, where the substituents on the double-bonded carbons are either on the same side (cis) or on opposite sides (trans) of the double bond. This concept is crucial in understanding the stereochemistry of alkenes and the E/Z designation.
Geometric Isomers: Geometric isomers, also known as cis-trans isomers, are a type of stereoisomerism that occurs when two identical substituents are arranged on the same side (cis) or opposite sides (trans) of a carbon-carbon double bond. This structural feature has important implications for the physical and chemical properties of organic compounds.
Fischer Projections: Fischer projections are a way of representing the three-dimensional stereochemistry of organic molecules, particularly carbohydrates, on a two-dimensional plane. They are named after the German chemist Emil Fischer, who developed this method of depicting the spatial arrangement of atoms in a molecule.
R/S System: The R/S system, also known as the Cahn-Ingold-Prelog system, is a method used to assign a configuration (R or S) to a stereocenter or chiral carbon atom in organic chemistry. This system provides a standardized way to determine the absolute configuration of a molecule, which is crucial for understanding and predicting the stereochemical properties and reactions of organic compounds.
(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.
(2R,3R)-2,3-butanediol: (2R,3R)-2,3-butanediol is a chiral organic compound with two hydroxyl groups attached to adjacent carbon atoms. It is a stereoisomer of 2,3-butanediol, specifically the enantiomer with the (2R,3R) configuration.
Newman Projections: Newman projections are a way of representing the three-dimensional structure of organic molecules on a two-dimensional plane. They provide a simplified view of the spatial arrangement of atoms in a molecule, particularly the orientation of substituents around a carbon-carbon bond.