16.2b Pi Molecular Orbitals of the Allyl System | Organic Chemistry

Chad's Prep
10 Feb 202110:26
EducationalLearning
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TLDRThe video script discusses the pi molecular orbitals of the allyl system, which includes the allyl cation, radical, and anion. It builds upon the concept of conjugated systems introduced through ethylene and butadiene, emphasizing the unique behavior of the allyl system with its three atoms. The lecture explains that regardless of the charge state (cation, radical, or anion), the molecular orbital diagrams remain similar, differing only in the number of electrons in the system. The allyl system's conjugated pi system consists of three overlapping p orbitals, resulting in three molecular orbitals: psi one (bonding), psi two (non-bonding), and psi three (anti-bonding). The script highlights the significance of the non-bonding orbital and how it affects the electron distribution across the atoms. The video also touches on the representation of the resonance hybrid and the concept of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), with a special mention of the singly occupied molecular orbital (SOMO) in radicals. The content is part of an organic chemistry series released weekly to aid students, and the presenter encourages viewers to subscribe for updates.

Takeaways
  • 🌟 The allyl system includes the allyl cation, allyl radical, and allyl anion, all of which share similar molecular orbital diagrams despite different numbers of electrons in the conjugated system.
  • πŸ”¬ In the allyl system, three overlapping p orbitals result in three molecular orbitals: ψ1 (bonding), ψ2 (non-bonding), and ψ3 (anti-bonding), with ψ2 being at the energy level of a non-bonding pair of electrons.
  • πŸ“Š The allyl cation has two Ο€ electrons, the allyl radical has three, and the allyl anion has four, which affects the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).
  • πŸ“ For the allyl cation, ψ1 is the HOMO and ψ2 is the LUMO, while for the allyl anion, ψ2 is the HOMO and ψ3* is the LUMO.
  • βš›οΈ In the allyl radical, the unpaired electron resides in a singly occupied molecular orbital (SOMO), which is different from the HOMO-LUMO convention.
  • πŸ”¬ The molecular orbital diagram for the allyl system accurately reflects the resonance hybrid, showing electron density spread across all three atoms in the system.
  • πŸ”΅ The allyl anion has a partial negative charge on the two end atoms, with the electron density centered there and not on the middle atom, which is consistent with the molecular orbital picture.
  • πŸ“š The lesson is part of an organic chemistry playlist released weekly throughout the school year, providing a comprehensive understanding of conjugated systems and molecular orbitals.
  • πŸ“ The script introduces the concept of nodes in molecular orbitals, where an odd number of nodes results in a node being symmetrically distributed down the middle, affecting electron density visualization.
  • πŸŽ“ The video script emphasizes the importance of understanding molecular orbitals for organic chemistry, particularly in the context of conjugated systems like the allyl system.
  • πŸ“ˆ The next lesson will cover 1,3,5-hexatriene, which is more complex with six atoms and six molecular orbitals involved in overlapping p orbitals.
  • πŸ“š For further study, the script suggests a premium course on Chatsprep.com that includes practice problems and a final exam rapid review.
Q & A

  • What is the allyl system?

    -The allyl system is a type of conjugated system that includes the allyl cation, allyl radical, and allyl anion. It is unique due to having only three atoms involved in the conjugation, which leads to specific molecular orbital patterns.

  • What is the significance of the p orbitals in the allyl system?

    -The p orbitals in the allyl system are significant because they overlap to form pi bonds. This overlap is crucial for the formation of the molecular orbitals and the distribution of electrons in the system.

  • How does the carbocation in the allyl cation affect the hybridization of the carbon atom?

    -The carbocation in the allyl cation results in the carbon atom being sp2 hybridized rather than sp3. This is because the sp2 hybridization allows for the empty p orbital to exist on the adjacent atom, which is necessary for the conjugation.

  • What is a resonance hybrid in the context of the allyl system?

    -A resonance hybrid is a concept that describes the actual structure of the allyl system, which is an average of the different possible resonance structures. It helps to explain the distribution of electrons and partial charges across the system.

  • How many pi electrons are there in the allyl cation?

    -In the allyl cation, there are two pi electrons. These electrons are distributed in the molecular orbitals according to the rules of quantum mechanics.

  • What is unique about the molecular orbitals when there is an odd number of atoms in a conjugated system?

    -When there is an odd number of atoms in a conjugated system, such as in the allyl system, there is an odd number of molecular orbitals. This leads to one of the orbitals, psi two, being non-bonding, which is a unique feature not found in systems with an even number of atoms.

  • How does the number of electrons in the pi system change between the allyl cation, radical, and anion?

    -The allyl cation has two pi electrons, the allyl radical has three pi electrons (including the unpaired electron), and the allyl anion has four pi electrons. This change affects the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for each species.

  • What is the term used for the molecular orbital with an unpaired electron in a radical species?

    -The molecular orbital with an unpaired electron in a radical species is often referred to as the SOMO, which stands for singly occupied molecular orbital.

  • How does the distribution of electron density in the allyl system reflect in the molecular orbital diagrams?

    -The distribution of electron density in the allyl system is reflected in the molecular orbital diagrams through the representation of the pi electrons smeared across the entire region of the conjugated system, as well as the depiction of partial positive and negative charges on the end atoms.

  • What is the significance of the nodes in molecular orbital diagrams?

    -Nodes in molecular orbital diagrams are points where the wave function goes to zero, indicating no electron density. They are important for understanding the phase and energy of the orbitals, with an odd number of nodes leading to unique distributions of electron density.

  • What is the next system that will be discussed in the series of lessons?

    -The next system to be discussed in the series of lessons is 1,3,5-hexatriene, which involves six atoms and six molecular orbitals in the conjugated system.

Outlines
00:00
🌟 Orbitals of the Allyl System: Cation, Radical, and Anion

This paragraph delves into the pi molecular orbitals of the allyl system, which includes the allyl cation, radical, and anion. It explains how these orbitals are similar regardless of the charge state of the system. The discussion includes the unique characteristics of a conjugated system with an odd number of atoms, leading to an odd number of molecular orbitals. The paragraph also covers the concept of non-bonding orbitals and how they differ from bonding and antibonding orbitals. The summary of the molecular orbital diagrams for the allyl cation is provided, including the identification of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).

05:01
πŸ”¬ Allyl Radical and Anion: Molecular Orbital Analysis

The second paragraph continues the exploration of the allyl system, focusing on the allyl radical and anion. It emphasizes that the molecular orbital diagrams remain identical across different charge states, with variations in electron filling affecting the HOMO and LUMO. The paragraph explains how the radical has an unpaired electron in the singly occupied molecular orbital (SOMO), which is significant for one-electron transfer reactions. The summary also touches on the resonance hybrid representation of the allyl anion, highlighting the distribution of electron density across the atoms and the partial charges at the ends of the system.

10:02
πŸ“š Resources for Further Learning and Support

The final paragraph serves as a call to action for viewers to support the educational content by liking and sharing the video. It also provides information on additional resources available for those seeking further study materials, practice problems, and exam reviews. The paragraph directs interested students to the premium course on Chatsprep.com, where a free trial is offered to enhance their learning experience.

Mindmap
Keywords
πŸ’‘Pi molecular orbitals
Pi molecular orbitals are a type of molecular orbital that results from the lateral overlap of p orbitals on adjacent atoms. In the context of the video, they are crucial for understanding the electronic structure of the allyl system, which includes the allyl cation, radical, and anion. The video discusses how these orbitals are involved in conjugated systems and how their energy levels and electron occupancy differ depending on the charge state of the allyl system.
πŸ’‘Conjugated system
A conjugated system is a type of molecular structure where alternating single and double bonds are present, allowing for the delocalization of electrons across the system. The video focuses on the allyl system, which is a conjugated system with three atoms and overlapping p orbitals, and how this affects the molecular orbitals and electron distribution.
πŸ’‘Allyl cation
The allyl cation is a positively charged ion derived from the allyl system. It is characterized by having two pi electrons. The video explains how the molecular orbital diagram for the allyl cation is constructed and how it identifies psi one as the highest occupied molecular orbital (HOMO) and psi two as the lowest unoccupied molecular orbital (LUMO).
πŸ’‘Allyl radical
The allyl radical is a neutral species with an unpaired electron in the allyl system. The video discusses how the molecular orbital with the unpaired electron is referred to as the singly occupied molecular orbital (SOMO) and how this radical's behavior differs from other species in organic chemistry due to its involvement in one-electron transfer reactions.
πŸ’‘Allyl anion
The allyl anion is a negatively charged ion with an additional electron in the allyl system. The video describes how this anion has four pi electrons that are distributed among the molecular orbitals, with psi two being the HOMO and psi three being the LUMO. The anion's electron density is depicted as being centered on the two end atoms, which is consistent with the resonance picture.
πŸ’‘Resonance structures
Resonance structures are alternative ways of representing the distribution of electrons in a molecule, which do not exist independently but contribute to the overall structure through resonance. The video uses resonance structures to explain the average electron distribution in the allyl system, leading to the concept of the resonance hybrid, which is a blend of all possible resonance structures.
πŸ’‘Resonance hybrid
A resonance hybrid is a concept that describes the actual structure of a molecule as an average of all possible resonance structures. The video explains how the resonance hybrid of the allyl system reflects the electron density across the entire region of the conjugated system, which is consistent with the molecular orbital picture.
πŸ’‘Molecular orbital diagram
A molecular orbital diagram is a graphical representation that shows the arrangement of molecular orbitals in a molecule. The video uses these diagrams to illustrate the distribution of electrons in the allyl cation, radical, and anion, and how the diagrams are identical for these species despite differences in electron count.
πŸ’‘Highest Occupied Molecular Orbital (HOMO)
The HOMO is the highest energy molecular orbital that is occupied by electrons in a molecule. In the context of the video, the HOMO for the allyl cation is psi one, for the allyl radical it is the orbital with the unpaired electron (SOMO), and for the allyl anion, it is psi two.
πŸ’‘Lowest Unoccupied Molecular Orbital (LUMO)
The LUMO is the lowest energy molecular orbital that is not occupied by electrons. The video identifies the LUMO for the allyl cation as psi two and for the allyl anion as psi three. In the case of the allyl radical, the concept of LUMO is less relevant due to the presence of the unpaired electron in the SOMO.
πŸ’‘Singly Occupied Molecular Orbital (SOMO)
The SOMO is a molecular orbital that contains a single unpaired electron, typically found in radicals. The video discusses how the allyl radical has its unpaired electron in the SOMO, which is a specific molecular orbital that is neither the HOMO nor the LUMO but is significant for understanding the reactivity of the radical.
Highlights

The pi molecular orbitals of the allyl system are unique and include the allyl cation, radical, and anion.

The allyl system has three atoms and a conjugated system that is independent of charge state.

The allyl cation has sp2 hybridization, with an empty p orbital on the adjacent carbon.

Resonance structures and the concept of a resonance hybrid are introduced to explain the allyl system.

Molecular orbital diagrams for the allyl cation, radical, and anion are similar, differing only in electron count.

The allyl system's conjugated system includes three atoms with overlapping p orbitals, resulting in three molecular orbitals.

An odd number of atoms in a conjugated system leads to an odd number of molecular orbitals and unique bonding characteristics.

Psi two in the allyl system is non-bonding, similar to a lone pair of electrons on an atom.

The molecular orbital with an odd number of nodes exhibits unique behavior, with nodes symmetrically distributed.

The allyl cation has two pi electrons, which are used to identify the highest occupied molecular orbital (HOMO).

The molecular orbital diagrams are constructed by considering the overlap of three adjacent p orbitals.

The allyl radical involves a single unpaired electron, referred to as the singly occupied molecular orbital (SOMO).

The allyl anion has four pi electrons, with the additional electron contributing to the pi system.

The molecular orbitals for the allyl radical and anion are identical in structure but differ in electron filling.

The allyl system's molecular orbitals confirm the resonance picture, showing electron density across the entire region.

The allyl system's molecular orbitals provide insight into the distribution of partial positive and negative charges.

The next lesson will cover 1,3,5-hexatriene, a more complex system with six atoms and six molecular orbitals.

The study guide, practice problems, and final exam rapid review for this lesson are available on chatsprep.com.

Transcripts

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Organic ChemistryMolecular OrbitalsAllyl SystemCationRadicalAnionElectron DensityResonance HybridP OrbitalsConjugated SystemChemical BondingEducational Content