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Organic Chemistry
Table of Contents
Unit 1 Overview: Structure and Bonding
1.1 Atomic Structure: The Nucleus
1.2 Atomic Structure: Orbitals
1.3 Atomic Structure: Electron Configurations
1.4 Development of Chemical Bonding Theory
1.5 Describing Chemical Bonds: Valence Bond Theory
1.6 sp3 Hybrid Orbitals and the Structure of Methane
1.7 sp3 Hybrid Orbitals and the Structure of Ethane
1.8 sp2 Hybrid Orbitals and the Structure of Ethylene
1.9 sp Hybrid Orbitals and the Structure of Acetylene
1.10 Hybridization of Nitrogen, Oxygen, Phosphorus, and Sulfur
1.11 Describing Chemical Bonds: Molecular Orbital Theory
1.12 Drawing Chemical Structures

🥼organic chemistry review

1.11 Describing Chemical Bonds: Molecular Orbital Theory

Citation:

Covalent bonds form when atomic orbitals overlap, creating molecular orbitals. These can be bonding or antibonding, affecting the molecule's stability. Understanding this helps explain why some molecules form and others don't.

In ethylene, a carbon-carbon double bond includes a pi bond. This forms when p orbitals overlap sideways, creating bonding and antibonding pi orbitals. This structure gives ethylene its unique properties and reactivity.

Molecular Orbital Theory and Covalent Bonding

Molecular orbital theory for covalent bonds

  • Describes covalent bonding as overlap and combination of atomic orbitals forms molecular orbitals
    • Atomic orbitals with similar energy and symmetry combine into molecular orbitals
    • Electrons shared between atoms in these molecular orbitals (H2, N2)
  • Constructive interference of atomic orbitals forms bonding molecular orbitals
    • Electron density concentrated between nuclei stabilizes the molecule
    • Lower energy than original atomic orbitals
  • Destructive interference of atomic orbitals forms antibonding molecular orbitals
    • Electron density depleted between nuclei destabilizes the molecule
    • Higher energy than original atomic orbitals
  • Number of molecular orbitals formed equals number of atomic orbitals combined
  • Electrons occupy molecular orbitals in order of increasing energy follows Aufbau principle
    • Each molecular orbital holds maximum of two electrons with opposite spins follows Pauli exclusion principle (CO2, H2O)
  • Linear combination of atomic orbitals (LCAO) is used to mathematically describe the formation of molecular orbitals

Bonding vs antibonding molecular orbitals

  • Energy levels
    • Bonding molecular orbitals have lower energy than original atomic orbitals
    • Antibonding molecular orbitals have higher energy than original atomic orbitals
  • Shapes
    • Bonding molecular orbitals have constructive overlap increases electron density between nuclei
    • Antibonding molecular orbitals have destructive overlap decreases electron density between nuclei
  • Effects on atomic nuclei
    • Bonding molecular orbitals stabilize molecule by attracting nuclei towards each other
    • Antibonding molecular orbitals destabilize molecule by repelling nuclei away from each other
  • Notation
    • $\sigma$ (sigma) denotes bonding molecular orbitals from s orbital overlap
    • $\pi$ (pi) denotes bonding molecular orbitals from p orbital overlap
    • $\sigma*$ (sigma star) denotes antibonding molecular orbitals from s orbital overlap
    • $\pi*$ (pi star) denotes antibonding molecular orbitals from p orbital overlap
  • Bond order can be calculated by comparing the number of electrons in bonding and antibonding orbitals

Pi molecular orbitals in ethylene

  • Ethylene ($C_2H_4$) is planar molecule with carbon-carbon double bond
  • Double bond consists of one $\sigma$ bond and one $\pi$ bond
  • Formation of $\pi$ molecular orbitals
    1. Each carbon atom contributes one 2p orbital perpendicular to molecular plane
    2. These 2p orbitals overlap laterally to form two $\pi$ molecular orbitals: bonding ($\pi$) and antibonding ($\pi*$)
  • Energy levels of $\pi$ molecular orbitals
    • Bonding $\pi$ orbital is lower in energy than original 2p atomic orbitals
    • Antibonding $\pi*$ orbital is higher in energy than original 2p atomic orbitals
  • Shapes of $\pi$ molecular orbitals
    • Bonding $\pi$ orbital has constructive overlap above and below molecular plane
    • Antibonding $\pi*$ orbital has node (zero electron density) along molecular plane and destructive overlap above and below plane
  • Two electrons from carbon 2p orbitals occupy bonding $\pi$ orbital strengthens carbon-carbon bond

Atomic and Molecular Properties

  • Wave function describes the quantum state of an electron in an atom or molecule
  • Hybridization explains the mixing of atomic orbitals to form new hybrid orbitals
  • Electron configuration determines the arrangement of electrons in atomic or molecular orbitals

Key Terms to Review (27)

Wave function: A wave function is a mathematical description of the quantum state of an isolated quantum system, representing the probability amplitude of a particle's position and other variables. In organic chemistry, it particularly helps in understanding the behavior of electrons within atomic orbitals.
Pauli exclusion principle: The Pauli Exclusion Principle states that no two electrons in an atom can have the same set of four quantum numbers. This principle helps determine electron configurations within atoms, crucial for understanding chemical bonding and molecular structure in organic chemistry.
Electron configuration: Electron configuration describes the distribution of electrons in an atom's atomic orbitals. It follows a set of rules, including the Pauli exclusion principle and Hund's rule, to show how these electrons are arranged around the nucleus.
Aufbau principle: The Aufbau principle is a guideline used in organic chemistry to determine the electron configuration of an atom, molecule, or ion. It states that electrons occupy the lowest energy orbitals first before filling higher energy levels.
Wave Function: The wave function is a mathematical function that describes the quantum state of an object, such as an electron or a particle. It is a fundamental concept in quantum mechanics that provides a complete description of the behavior and properties of a particle or system.
Aufbau Principle: The Aufbau principle is a fundamental concept in quantum mechanics that describes the order in which electrons occupy atomic orbitals. It is a crucial principle that governs the electron configurations of atoms and is closely tied to the understanding of atomic structure and chemical bonding.
Atomic Orbitals: Atomic orbitals are the wave-like functions that describe the behavior and spatial distribution of an electron in an atom. They are the fundamental building blocks of atomic structure and play a crucial role in understanding chemical bonding and reactivity.
Covalent Bonds: Covalent bonds are chemical bonds formed by the sharing of electrons between two atoms. They are a fundamental type of chemical bonding that helps stabilize molecules and compounds by creating strong, directional interactions between atoms.
Hybridization: Hybridization is a fundamental concept in chemistry that describes the process of mixing atomic orbitals to form new hybrid orbitals, which are used to explain the geometry and bonding patterns of molecules. This term is closely related to the development of chemical bonding theory, valence bond theory, and molecular orbital theory, as well as the structure and properties of various organic compounds.
Node: A node is a fundamental concept in various fields, including atomic structure, molecular orbital theory, and the stability of conjugated dienes. It represents a specific point or location within a system, where important properties or interactions occur.
Pauli Exclusion Principle: The Pauli exclusion principle is a fundamental principle in quantum mechanics that states that no two identical fermions (particles with half-integer spin) can occupy the same quantum state simultaneously. This principle has far-reaching implications for the structure of atoms, the organization of the periodic table, and the behavior of chemical bonds.
Electron Configuration: Electron configuration refers to the arrangement of electrons in an atom's orbitals, which determines the atom's chemical properties and behavior. This concept is central to understanding the structure and behavior of atoms, as well as the formation of chemical bonds and the properties of molecules.
Constructive Interference: Constructive interference is a phenomenon that occurs when two or more waves, such as light or sound waves, interact in a way that reinforces or amplifies the resulting wave. This happens when the crests of the waves align, causing the amplitudes to add together, resulting in a larger overall wave.
Destructive Interference: Destructive interference is a phenomenon that occurs when two waves, such as sound or light waves, interact in a way that results in the cancellation or reduction of the wave amplitude at certain points. This happens when the crests of one wave align with the troughs of another wave, causing them to cancel each other out.
Bond Order: Bond order is a fundamental concept in chemical bonding theory that describes the strength and stability of a chemical bond between atoms. It is a measure of the number of shared electron pairs between two atoms and is directly related to the bond's overall strength and length.
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.
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.
Molecular Orbital Theory: Molecular Orbital Theory is a model that describes the behavior of electrons in a molecule by considering the formation of molecular orbitals from the combination of atomic orbitals. This theory provides a more comprehensive understanding of chemical bonding compared to the earlier Valence Bond Theory.
Ethylene: Ethylene is a colorless, flammable gas with the chemical formula C₂H₄. It is the simplest alkene and is widely used in the chemical industry for the production of various organic compounds and polymers. Ethylene is a key term that connects to several important topics in organic chemistry, including the structure of alkenes, chemical bonding, and industrial applications.
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.
Molecular Orbitals: Molecular orbitals are the wave functions that describe the behavior of electrons in a molecule. They are formed by the combination of atomic orbitals and play a crucial role in understanding the structure, bonding, and reactivity of chemical compounds.
Antibonding Molecular Orbitals: Antibonding molecular orbitals are higher-energy molecular orbitals that are formed when atomic orbitals combine in a way that results in destructive interference, leading to a decrease in electron density between the bonded atoms. These orbitals are essential in understanding the stability and reactivity of molecules.
Bonding Molecular Orbitals: Bonding molecular orbitals are the regions of high electron density that form between atoms when they share electrons to create a chemical bond. These orbitals are crucial in understanding the stability and properties of molecules, as they determine the strength and nature of the bonds that hold atoms together.
Linear Combination of Atomic Orbitals (LCAO): The linear combination of atomic orbitals (LCAO) is a fundamental concept in molecular orbital theory, which describes the formation of molecular orbitals by combining the wave functions of individual atomic orbitals. This approach is used to understand the electronic structure and bonding in molecules.
Sigma Star: Sigma star (σ*) is a type of molecular orbital that is formed by the constructive interference of atomic orbitals in a molecule. It represents the antibonding interaction between atoms, which weakens the bond between them and increases the energy of the overall molecular system.
2p Orbital: The 2p orbital is one of the possible electron configurations that can exist in an atom. It is a higher energy orbital within the second principal energy level (n=2) of an atom, characterized by a specific shape and angular momentum.
Pi Star (π*): Pi star (π*) is an antibonding molecular orbital that arises from the constructive interference of atomic p-orbitals in a molecule. It is a key concept in molecular orbital theory, which describes the formation and properties of chemical bonds.