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Organic Chemistry
Table of Contents
Unit 7 Overview: Alkenes
7.1 Industrial Preparation and Use of Alkenes
7.2 Calculating the Degree of Unsaturation
7.3 Naming Alkenes
7.4 Cis–Trans Isomerism in Alkenes
7.5 Alkene Stereochemistry and the E,Z Designation
7.6 Stability of Alkenes
7.7 Electrophilic Addition Reactions of Alkenes
7.8 Orientation of Electrophilic Additions: Markovnikov’s Rule
7.9 Carbocation Structure and Stability
7.10 The Hammond Postulate
7.11 Evidence for the Mechanism of Electrophilic Additions: Carbocation Rearrangements

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7.5 Alkene Stereochemistry and the E,Z Designation

Citation:

Alkene stereochemistry is all about the spatial arrangement of substituents around carbon-carbon double bonds. The E,Z system helps us describe these arrangements, using priority rules to determine which groups are on the same or opposite sides.

Understanding alkene stereochemistry is crucial for predicting molecular properties and reactivity. We'll explore how to name and classify alkenes based on their substituents, and learn to apply the Cahn-Ingold-Prelog rules to determine E or Z configurations.

Alkene Stereochemistry and the E,Z Designation

E vs Z configurations in alkenes

  • E,Z system describes stereochemistry of alkenes with two different substituents on each carbon of the double bond
    • E (entgegen) higher priority substituents on opposite sides of double bond (trans)
    • Z (zusammen) higher priority substituents on same side of double bond (cis)
  • Cahn-Ingold-Prelog (CIP) sequence rules assign priorities to substituents
    • Rule 1: Higher atomic number takes precedence (Br > Cl > S > P > Si > I > N > C > H)
    • Rule 2: For isotopes, higher mass number takes precedence ($^{14}C$ > $^{13}C$ > $^{12}C$)
    • Rule 3: For double or triple bonds, count them as equivalent number of single-bonded atoms (C=O treated as C(-O)(-O))
    • Rule 4: If decision can't be reached, move stepwise away from double bond until point of difference found (compare atomic numbers of first, second, third, etc. atoms until difference observed)

Naming of trisubstituted and tetrasubstituted alkenes

  • Trisubstituted alkenes have three substituents attached to double bond carbons
    • Assign priorities to three substituents using CIP rules
    • Compare two highest priority substituents to determine E or Z configuration
    • Example: (E)-1-bromo-1-chloro-2-methylprop-1-ene
  • Tetrasubstituted alkenes have four substituents attached to double bond carbons
    • Assign priorities to all four substituents using CIP rules
    • Compare two highest priority substituents on each carbon to determine E or Z configuration
    • If both carbons have same two highest priority substituents, alkene is meso compound and not optically active
    • Example: (E)-2-bromo-2-chloro-3-methylbut-2-ene

Ranking of substituents for alkene stereochemistry

  • CIP sequence rules rank substituents on double bonds
    • Atoms with higher atomic numbers have higher priority (F > O > N > C > H)
    • Isotopes with higher mass numbers have higher priority ($^{131}I$ > $^{127}I$)
    • Multiple bonds treated as equivalent number of single-bonded atoms (-C≡N treated as -C(-N)(-N))
    • If decision can't be reached, move stepwise away from double bond until point of difference found (compare atomic numbers of substituents' first, second, third, etc. atoms until difference observed)
    • Examples:
      • (E)-1-bromo-1-chloroprop-1-ene (Br > Cl)
      • (Z)-1-chloro-1-iodoprop-1-ene (I > Cl)

Stereoisomerism and Configurational Analysis

  • Stereoisomerism occurs when molecules have the same molecular formula and bonding sequence but different spatial arrangements
  • Configurational isomers are stereoisomers that cannot be interconverted without breaking and reforming chemical bonds
    • E and Z isomers of alkenes are examples of configurational isomers
  • Priority rules (CIP rules) are used to determine the configuration of alkenes
  • Substituent ranking is crucial for applying priority rules and determining E/Z configuration
  • Conformational analysis examines different spatial arrangements that can be achieved through rotation around single bonds

Key Terms to Review (39)

Electron-transport chain: The electron-transport chain is a series of protein complexes and small molecules within the mitochondrial membrane that transfer electrons, derived from nutrients, to molecular oxygen, creating a proton gradient used to produce ATP. This process is essential for aerobic respiration and plays a critical role in the body's energy production.
E,Z system: The E,Z system is a notation used in organic chemistry to describe the stereochemistry of double bonds, indicating the relative positions of substituents around an alkene. It assigns the labels "E" (from the German "Entgegen," meaning opposite) for isomers with high-priority groups on opposite sides of a double bond, and "Z" (from the German "Zusammen," meaning together) when they are on the same side.
Cahn–Ingold–Prelog sequence rules: The Cahn-Ingold-Prelog (CIP) sequence rules are a set of guidelines used to systematically name the spatial arrangement of atoms around a stereocenter, helping to determine the configuration of molecules in organic chemistry. These rules prioritize atoms or groups attached to a chiral center based on atomic number, isotopic mass, and connectivity, facilitating the assignment of R (rectus) or S (sinister) configurations.
E geometry: E geometry describes a type of stereochemistry observed in alkenes where the two highest priority substituents on either carbon of the double bond are on opposite sides. This configuration is determined using the Cahn-Ingold-Prelog priority rules, which rank substituents based on atomic number and connectivity.
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.
Z configuration: In organic chemistry, the Z configuration denotes the arrangement around a double bond where the highest priority groups on each carbon are on the same side. This designation is part of the E,Z system for describing alkene stereochemistry, which is crucial for understanding molecule shapes and reactivity.
Z geometry: Z geometry is a descriptor used in organic chemistry to indicate that the highest priority substituents on each carbon of a double bond are on the same side. This nomenclature is part of the E,Z designation system for determining alkene stereochemistry based on Cahn-Ingold-Prelog priority rules.
Meso compounds: Meso compounds are molecules with multiple stereocenters that are superimposable on their mirror images due to an internal plane of symmetry, making them achiral. Despite their complex structures, these compounds do not exhibit optical activity because they have an identical mirror image.
Conformational analysis: Conformational analysis is the study of the different shapes (conformations) that molecules can adopt due to rotation around single bonds. It particularly focuses on how these shapes affect the molecule's chemical properties and reactivity in organic chemistry.
E configuration: E configuration is a type of stereochemistry that occurs in alkenes where the two highest priority substituents on each carbon of the double bond are on opposite sides. This designation is part of the E,Z system for specifying the geometry of double bonds.
Double bond: A double bond in organic chemistry is a chemical bond between two atoms involving four bonding electrons instead of the usual two. It results in stronger attraction and shorter distance between the bonded atoms compared to a single bond.
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.
Carbon-Carbon Double Bond: A carbon-carbon double bond is a covalent chemical bond in which two carbon atoms share four electrons, with two pairs of electrons forming the bond. This type of bond is a fundamental structural feature in many organic compounds and is central to understanding the properties and reactivity of alkenes, a class of unsaturated hydrocarbons.
Alkyl Groups: Alkyl groups are hydrocarbon substituents derived from alkanes by the removal of one hydrogen atom. They are non-polar, saturated, and can be straight-chain or branched. Alkyl groups play a crucial role in understanding the properties and behavior of various organic compounds, including alkanes, alkenes, and benzene derivatives.
Conformational Analysis: Conformational analysis is the study of the three-dimensional arrangements or conformations that a molecule can adopt. It involves examining the relative stability and interconversion of different conformations, which is crucial for understanding the behavior and reactivity of organic compounds.
Cahn-Ingold-Prelog rules: The Cahn-Ingold-Prelog rules are a set of guidelines used to unambiguously describe the stereochemistry of organic molecules, particularly in the context of chirality, cis-trans isomerism, and alkene stereochemistry.
Prelog: Prelog is a set of rules used to specify the configuration of stereoisomers, particularly in the context of alkene stereochemistry and the assignment of E/Z designations. These rules, developed by the chemist Vladimir Prelog, provide a systematic approach to unambiguously determine the spatial arrangement of atoms around a stereocenter.
Double Bond: A double bond is a covalent chemical bond that forms between two atoms, with the sharing of four valence electrons. This type of bond is commonly found in organic compounds, particularly in alkenes, and is a key structural feature that influences the properties and reactivity of these molecules.
Ingold: Ingold is a concept in organic chemistry that is primarily used in the context of specifying the configuration of stereoisomers and understanding the stereochemistry of alkenes. It provides a systematic set of rules for assigning priorities to substituents around a stereocenter or double bond, which is crucial for determining the relative orientation of these groups.
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.
Meso Compound: A meso compound is a special type of organic compound that contains two or more stereogenic centers, but is achiral and has a plane of symmetry. This means that despite having multiple chiral centers, the molecule as a whole is superimposable on its mirror image, and therefore does not exhibit optical activity.
Priority Rules: Priority rules are a set of guidelines used to determine the relative importance or precedence of different substituents or functional groups when specifying the stereochemical configuration of organic compounds. These rules are particularly relevant in the context of alkene stereochemistry and the assignment of E/Z designations.
Cahn: Cahn is a term that refers to the Cahn-Ingold-Prelog (CIP) sequence rules, which are a set of guidelines used to specify the stereochemical configuration of molecules, particularly in the context of alkene stereochemistry and the E/Z designation.
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.
Z: The Z designation is a method used to describe the stereochemistry of alkenes, indicating the relative orientation of the substituents attached to the double-bonded carbons. It is derived from the German word 'zusammen' meaning 'together'.
Entgegen: Entgegen is a German prefix that denotes a sense of opposition or against. In the context of organic chemistry, it is particularly relevant to the discussion of alkene stereochemistry and the E,Z designation.
E,Z System: The E,Z system is a method used to designate the stereochemistry of alkenes, specifically the relative orientation of substituents around the carbon-carbon double bond. The terms 'E' and 'Z' are derived from the German words 'entgegen' (opposite) and 'zusammen' (together), respectively, and describe the spatial arrangement of the substituents.
Halogens: Halogens are a group of five highly reactive nonmetal elements in the periodic table, including fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). They are known for their strong oxidizing properties and ability to form a wide range of compounds with other elements.
Tetrasubstituted Alkenes: Tetrasubstituted alkenes are organic compounds containing a carbon-carbon double bond where all four substituents attached to the double-bonded carbons are different. These unique alkenes are of particular interest in the context of alkene stereochemistry, the stability of alkenes, and the Wittig reaction.
Trans: The term 'trans' refers to the spatial arrangement of atoms or functional groups in a molecule, particularly in the context of alkenes, Diels-Alder reactions, and steroids. It describes a configuration where the substituents are on opposite sides of a carbon-carbon double bond or a cyclic structure.
E: E is a designation used in organic chemistry to describe the stereochemistry of alkenes, specifically the arrangement of substituents around a carbon-carbon double bond. The 'E' stands for the German word 'entgegen', meaning 'opposite'.
Stereoisomerism: Stereoisomerism refers to the phenomenon where molecules have the same molecular formula and connectivity, but differ in the spatial arrangement of their atoms. This concept is particularly important in the context of alkene stereochemistry and the E,Z designation.
Isotopes: Isotopes are atoms of the same element that have the same number of protons in the nucleus but a different number of neutrons, resulting in different atomic masses. Isotopes are an important concept in understanding the stereochemistry of alkenes and the E,Z designation.
Trisubstituted Alkenes: Trisubstituted alkenes are organic compounds containing a carbon-carbon double bond with three substituents attached to one of the carbon atoms. These types of alkenes are important in the context of understanding alkene stereochemistry, the stability of alkenes, and the hydration of alkenes through hydroboration reactions.
Optically Active: Optically active refers to a molecule's ability to rotate the plane of polarized light. This property arises from the presence of one or more chiral centers within the molecular structure, which give the molecule a non-superimposable mirror image.
Substituent Ranking: Substituent ranking is a systematic method used to determine the priority or precedence of different substituents attached to a carbon-carbon double bond (alkene) in order to assign the correct E/Z configuration. This term is crucial in the context of understanding alkene stereochemistry and the E,Z designation.
Cis: The term 'cis' refers to a spatial arrangement of atoms or groups in a molecule where two identical substituents are on the same side of a carbon-carbon double bond or a ring structure. This orientation is in contrast to the 'trans' configuration, where the identical substituents are on opposite sides.
Zusammen: Zusammen is a German word that means 'together' or 'jointly'. In the context of organic chemistry, specifically 7.5 Alkene Stereochemistry and the E,Z Designation, this term is used to describe the relative orientation of substituents around a carbon-carbon double bond.