7.4 Cis–Trans Isomerism in Alkenes
3 min read•may 7, 2024
Carbon-carbon double bonds are the defining feature of . These bonds consist of a and a , creating a planar structure with . This unique geometry gives rise to , where different spatial arrangements of substituents lead to distinct molecules.
Understanding alkene structure and isomerism is crucial for predicting reactivity and properties. We'll explore how to identify, draw, and name alkene isomers using cis-trans and E/Z systems. This knowledge forms the foundation for understanding more complex organic reactions and mechanisms.
Structure and Isomerism of Alkenes
Structure of carbon-carbon double bonds
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- Carbon-carbon double bond composed of one sigma () bond and one pi () bond
- bond formed by direct overlap of along the internuclear axis
- bond formed by sideways overlap of unhybridized above and below the molecular plane
- Electron density of the bond concentrated above and below the plane of the molecule
- Restricts rotation around the double bond axis (restricted rotation)
- Rotation would require breaking the bond, an energetically unfavorable process ()
- Each carbon of the double bond has a planar, with bond angles of approximately 120°
- Remaining sp2 hybrid orbitals used to form bonds to other atoms or groups (methyl, ethyl)
- is a key feature of alkene structure
Conditions for cis-trans isomerism
- Alkene must have two different substituents attached to each carbon of the double bond
- Substituents can be atoms or groups other than hydrogen (chlorine, bromine)
- Isomers are with the same connectivity but different spatial arrangement ()
- has both higher priority substituents on the same side of the double bond plane
- has higher priority substituents on opposite sides of the double bond plane
- Cis-trans isomers cannot interconvert without breaking the bond of the double bond
- , not conformational isomers that can rotate freely (butane)
- Alkenes with two identical substituents on one or both carbons do not exhibit cis-trans isomerism
- with a plane of symmetry bisecting the double bond ()
Drawing and naming alkene isomers
- Determine the higher priority substituent on each carbon using
- Higher atomic number takes precedence (bromine > chlorine)
- For substituents with the same first atom, compare subsequent atoms until a difference is found
- Draw the isomers with higher priority substituents on the same side (cis) or opposite sides (trans)
- Represent the double bond with a solid and dashed wedge to depict the spatial arrangement
- Name the isomers using the system
- Choose the longest continuous carbon chain containing the double bond as the base name (butene)
- Number the chain to give the double bond the lowest possible number (1-butene, not 3-butene)
- Indicate the position of the double bond with the lower numbered carbon (1-butene, not 2-butene)
- Assign cis or trans as a prefix based on the arrangement of higher priority substituents
- Include any additional substituents as prefixes with their corresponding position numbers (4-chloro-2-pentene)
Stereochemistry and E/Z System
- is the study of the three-dimensional arrangement of atoms in molecules
- The is an alternative nomenclature for describing geometric isomers
- E (entgegen) corresponds to higher priority groups on opposite sides
- Z (zusammen) corresponds to higher priority groups on the same side
- The E/Z system is particularly useful for more complex alkenes where cis-trans terminology may be ambiguous
Key Terms to Review (22)
2-butene: 2-butene is an unsaturated hydrocarbon with the molecular formula C4H8. It is an alkene with a carbon-carbon double bond located at the second carbon position of the four-carbon chain. This structural feature of 2-butene is central to understanding its behavior and properties in the context of various organic chemistry topics.
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.
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.
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.
Cis Isomer: A cis isomer is a type of geometric isomer where two identical substituents are positioned on the same side of a carbon-carbon double bond. This arrangement contrasts with the trans isomer, where the identical substituents are on opposite sides of the double bond.
Cis-Trans Isomerism: Cis-trans isomerism is a type of stereoisomerism that occurs when two identical substituents are either on the same side (cis) or on opposite sides (trans) of a carbon-carbon double bond or a ring structure. This concept is crucial in understanding the properties and behaviors of cycloalkanes and alkenes.
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.
Double Bond Planarity: Double bond planarity refers to the planar arrangement of the atoms involved in a carbon-carbon double bond. This structural feature has important implications for the geometry and reactivity of alkenes, a class of organic compounds containing carbon-carbon double bonds.
E/Z System: The E/Z system is a method of designating the stereochemistry of alkenes (carbon-carbon double bonds). It is used to determine the relative positions of substituents around the double bond, indicating whether they are on the same side (cis) or opposite sides (trans) of the molecule.
Ethene: Ethene, also known as ethylene, is a simple unsaturated hydrocarbon with the chemical formula C₂H₄. It is the simplest alkene and plays a crucial role in various topics within organic chemistry, including calculating the degree of unsaturation, naming alkenes, understanding cis-trans isomerism, and evaluating the stability of alkenes.
Geometric Isomerism: Geometric isomerism, also known as cis-trans isomerism, is a type of stereoisomerism that arises when two identical substituents are located on the same side (cis) or opposite sides (trans) of a carbon-carbon double bond or a cyclic structure. This isomerism affects the spatial arrangement of atoms and can lead to significant differences in the physical and chemical properties of the molecules.
IUPAC Nomenclature: IUPAC nomenclature is a standardized system for naming organic compounds, developed by the International Union of Pure and Applied Chemistry (IUPAC). It provides a consistent and unambiguous way to identify and communicate the structure of organic molecules.
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.
P Orbitals: p Orbitals are a type of atomic orbital in which the electron is distributed in a dumbbell-shaped region around the nucleus. They are critical in understanding the formation of chemical bonds, the geometry of molecules, and the behavior of conjugated systems.
Pi Bond: A pi (π) bond is a type of covalent chemical bond formed by the side-to-side overlap of atomic orbitals, resulting in electron density concentrated above and below the internuclear axis between two atoms. Pi bonds are crucial in the structure and reactivity of many organic compounds.
Restricted Rotation: Restricted rotation refers to the limited ability of certain molecules, particularly alkenes, to freely rotate around a carbon-carbon bond due to steric hindrance or electronic effects. This concept is crucial in understanding cis-trans isomerism in alkenes.
Sigma Bond: A sigma bond is a type of covalent chemical bond formed by the overlap of atomic orbitals along the internuclear axis between two atoms. Sigma bonds are the strongest type of covalent bonds and are responsible for the structural stability and geometry of molecules.
Sp2 Hybrid Orbitals: sp2 hybrid orbitals are a type of atomic orbital that arise from the hybridization of one s orbital and two p orbitals in an atom. This hybridization occurs in molecules with trigonal planar geometry, such as the carbon atom in alkenes, and is essential for understanding the bonding and structure of these compounds.
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.
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.
Trans Isomer: A trans isomer is a type of geometric isomer in which two identical substituents are located on opposite sides of a carbon-carbon double bond. This arrangement results in a more linear, extended molecular structure compared to its cis isomer counterpart.
Trigonal Geometry: Trigonal geometry is a molecular geometry in which a central atom is bonded to three other atoms, forming a triangular planar arrangement. This geometry is commonly observed in organic chemistry, particularly in the context of cis-trans isomerism in alkenes.