16.2c Pi Molecular Orbitals of 1,3,5-Hexatriene | Organic Chemistry

Chad's Prep
11 Feb 202108:24
EducationalLearning
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TLDRThe video script provides an in-depth exploration of the pi molecular orbitals in 1,3,5-hexatriene, a complex conjugated pi system. The discussion builds upon previous lessons on ethylene, 1,3-butadiene, and the allyl system, focusing on the molecular orbital diagram for this more intricate system. With six pi electrons distributed across six molecular orbitals, the video explains how to identify the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). It also covers the concept of nodes in molecular orbitals and how they affect the phase of wave functions. The script emphasizes the importance of symmetry in molecular orbitals, highlighting that odd-numbered orbitals are symmetrical while even-numbered ones are anti-symmetrical. This understanding is crucial for pericyclic reactions. The video concludes with a reminder of the complexity of these representations and offers resources for further study, including a study guide and a premium course for organic chemistry students.

Takeaways
  • πŸ“š The pi molecular orbitals for 1,3,5-hexatriene are discussed in the context of a more complex molecular orbital diagram compared to previous lessons on ethylene, 1,3-butadiene, and the allyl system.
  • πŸŒ€ In a conjugated system like 1,3,5-hexatriene, all three pi bonds are conjugated with each other, involving six atoms with overlapping p orbitals, resulting in six molecular orbitals.
  • πŸ”΅ The lower half of the molecular orbitals (psi one, two, and three) are bonding, while the upper half (psi four, five, and six) are antibonding, indicated with an asterisk.
  • ⚫ The highest occupied molecular orbital (HOMO) is psi three, and the lowest unoccupied molecular orbital (LUMO) is psi four star.
  • πŸ“ˆ The number of nodes in the molecular orbitals increases with energy, starting from zero for the lowest energy orbital and increasing by one for each subsequent orbital.
  • πŸ” Nodes in molecular orbitals must be symmetrically distributed; for one node, it is in the middle, and for an odd number of nodes, one is always in the middle with the others distributed symmetrically around it.
  • πŸ“Š The pattern for nodes in antibonding orbitals with an odd number of nodes is to group them as one in the middle and then pairs on either side.
  • πŸ”¬ The molecular orbitals are not just simple combinations of p orbitals; they represent a more complex reality, which is depicted in the study guide provided by the lecturer.
  • πŸ” The lowest energy orbitals (psi one, psi three, psi five) are symmetrical, while the others are anti-symmetrical, which is important for understanding pericyclic reactions.
  • 🎨 The molecular orbital diagram can be challenging to draw, but using the node patterns and symmetry can help visualize and understand the orbitals' shapes and electron density distributions.
  • βœ… The lecturer suggests memorizing the node groupings for odd numbers of nodes, as it can be less intuitive to deduce their positions based on spacing alone.
  • πŸ“ˆ The importance of understanding these concepts is emphasized for their application in organic chemistry, particularly in pericyclic reactions.
Q & A
  • What is the significance of the number of atoms with overlapping p orbitals in 1,3,5-hexatriene?

    -The number of atoms with overlapping p orbitals in 1,3,5-hexatriene determines the number of molecular orbitals. Since there are six atoms, there are six molecular orbitals, each represented by six overlapping p orbitals.

  • How many pi electrons does 1,3,5-hexatriene have?

    -1,3,5-hexatriene has six pi electrons, which are distributed among the molecular orbitals psi one, psi two, and psi three.

  • What is the difference between bonding and antibonding molecular orbitals?

    -Bonding molecular orbitals, like psi one, two, and three, are lower in energy and have wave functions that are in phase, which results in a stabilizing effect. Antibonding orbitals, denoted with an asterisk (e.g., psi four star), are higher in energy and have wave functions that are out of phase, leading to a destabilizing effect.

  • What is the highest occupied molecular orbital (HOMO) in 1,3,5-hexatriene?

    -The highest occupied molecular orbital (HOMO) in 1,3,5-hexatriene is psi three.

  • What is the lowest unoccupied molecular orbital (LUMO) in 1,3,5-hexatriene?

    -The lowest unoccupied molecular orbital (LUMO) in 1,3,5-hexatriene is psi four star.

  • How does the number of nodes in a molecular orbital relate to its energy level?

    -The number of nodes in a molecular orbital correlates with its energy level. Each time you go up in energy, you get a new vertical node. The lowest energy molecular orbital has zero nodes, and the number of nodes increases with each increase in energy.

  • What is the pattern for the distribution of nodes in molecular orbitals with an odd number of nodes?

    -For molecular orbitals with an odd number of nodes, one node is always placed right down the middle. The remaining nodes are then symmetrically distributed on either side of the central node.

  • How do you determine the phase of the wave functions when crossing nodes in molecular orbitals?

    -When you do not cross a node, the wave functions are in phase. When you cross a node, the wave functions go out of phase and alternate.

  • What is the significance of the symmetry in the molecular orbitals of 1,3,5-hexatriene?

    -The symmetry in the molecular orbitals of 1,3,5-hexatriene indicates whether the orbitals are bonding or antibonding. Orbitals with even numbers of nodes are anti-symmetric (out of phase), while those with odd numbers of nodes are symmetric (in phase).

  • Why is it important to understand the molecular orbital diagram for 1,3,5-hexatriene?

    -Understanding the molecular orbital diagram for 1,3,5-hexatriene is crucial for predicting the stability of the molecule, its reactivity in chemical reactions, and for comprehending concepts like pericyclic reactions in organic chemistry.

  • What is the role of the study guide in enhancing the understanding of the molecular orbitals for 1,3,5-hexatriene?

    -The study guide provides visual representations of the molecular orbitals alongside the actual realities they represent, which helps in understanding the complex nature of these orbitals and their significance in the overall structure and properties of 1,3,5-hexatriene.

  • How can students benefit from the study guide and practice materials for organic chemistry?

    -Students can benefit from the study guide and practice materials by gaining a deeper understanding of complex topics like molecular orbitals, practicing problems, and preparing for exams, which can ultimately lead to better performance in their organic chemistry courses.

Outlines
00:00
🌟 Introduction to Pi Molecular Orbitals in Hexatriene

This paragraph introduces the topic of pi molecular orbitals in 1,3,5-hexatriene, following on from previous lessons on ethylene, 1,3-butadiene, and the allyl system. It sets the stage for a complex molecular orbital diagram, emphasizing the conjugated pi system with six overlapping p orbitals across six atoms. The paragraph outlines the process of filling in the pi electrons (24 in total) and identifies the highest occupied molecular orbital (HOMO) as psi 3 and the lowest unoccupied molecular orbital (LUMO) as psi 4*. It also explains the concept of nodes in molecular orbitals and how they relate to energy levels, with the lowest energy orbital having zero nodes and each subsequent orbital gaining a new node as energy increases.

05:01
πŸ“ˆ Drawing Molecular Orbitals and Understanding Node Distribution

The second paragraph delves into the intricacies of drawing the molecular orbitals for 1,3,5-hexatriene, focusing on the distribution of nodes. It explains the process of placing nodes symmetrically across the diagram, with a special focus on orbitals with an odd number of nodes, which must have one node in the middle. The paragraph provides a mnemonic for the node groupings (one two, two and one for three nodes, and one one two one for four nodes) to assist in memorizing their placement. It also discusses the importance of symmetry in the orbitals, with odd-numbered orbitals being symmetrical and even-numbered orbitals being anti-symmetrical, a concept crucial for understanding pericyclic reactions. The paragraph concludes by reminding viewers of the complexity of these representations and encourages them to refer to study guides for a more detailed understanding. It ends with a call to action for likes and shares to help other students find the lesson and mentions a premium course for further study materials.

Mindmap
Keywords
πŸ’‘pi molecular orbitals
pi molecular orbitals are regions of space where electrons are likely to be found in a molecule that has a pi bond. They are crucial for understanding the electronic structure of conjugated systems. In the video, the focus is on the pi molecular orbitals of 1,3,5-hexatriene, which is a more complex system compared to ethylene and 1,3-butadiene discussed in previous lessons.
πŸ’‘conjugated system
A conjugated system in chemistry refers to a chain of alternating single and double bonds in a molecule. This arrangement allows for the delocalization of electrons across the entire system, which can influence the molecule's stability and reactivity. The video discusses the conjugated pi system of 1,3,5-hexatriene, which involves six atoms with overlapping p orbitals.
πŸ’‘homo and lumo
HOMO stands for the highest occupied molecular orbital, and LUMO is the lowest unoccupied molecular orbital. These terms are significant in chemistry as they determine the energy levels at which electrons reside in a molecule. In the context of the video, the HOMO for 1,3,5-hexatriene is psi 3, and the LUMO is psi 4 star, indicating the energy levels where electrons are most likely to be found or where they can be excited to.
πŸ’‘nodes
In molecular orbital theory, a node is a point or region in space where the wave function of the orbital has a value of zero. The number of nodes is indicative of the energy level of the orbital; the more nodes, the higher the energy. The video explains how to identify the number of nodes for each orbital in the pi system of 1,3,5-hexatriene, which is essential for constructing the molecular orbital diagram.
πŸ’‘in-phase and out-of-phase
These terms describe the phase relationship of wave functions in molecular orbitals. When wave functions are in phase, they add together constructively, and when they are out of phase, they subtract or interfere with each other. The video discusses how crossing a node results in a change from in-phase to out-of-phase, which is important for drawing the molecular orbital diagram and understanding the symmetry of the orbitals.
πŸ’‘molecular orbital diagram
A molecular orbital diagram is a visual representation that shows the arrangement of molecular orbitals in a molecule. It includes both bonding and antibonding orbitals and is used to fill in the electrons to determine the molecule's electronic configuration. The video presents a complex molecular orbital diagram for 1,3,5-hexatriene, illustrating the process of filling electrons and identifying the HOMO and LUMO.
πŸ’‘delocalization
Delocalization is the concept where electrons are not associated with a single atom or a specific location but are spread out over a larger region, such as across a conjugated system. This phenomenon contributes to the stability and unique properties of conjugated molecules. The video touches on delocalization in the context of the pi electrons in the conjugated system of 1,3,5-hexatriene.
πŸ’‘pericyclic reactions
Pericyclic reactions are a class of organic reactions that involve the concerted rearrangement of electrons within a cyclic transition state. These reactions are significant in organic chemistry and often involve conjugated systems. The video hints at the importance of understanding molecular orbitals for pericyclic reactions, suggesting that the concepts discussed will be helpful in that context.
πŸ’‘sigma and pi bonds
Sigma (Οƒ) and pi (Ο€) bonds are two types of covalent bonds. Sigma bonds are formed by head-on overlap of atomic orbitals, while pi bonds result from the side-by-side overlap. In the video, it is mentioned that there is a single sigma bond between the pi bonds in the conjugated system of 1,3,5-hexatriene, which is important for understanding the overall structure and bonding in the molecule.
πŸ’‘electron density
Electron density refers to the probability of an electron being present in a particular region of space. In the context of molecular orbitals, areas of high electron density are indicative of bonding regions. The video describes the electron density in the lowest energy molecular orbital (psi 1) of 1,3,5-hexatriene, noting that there is a 'big smear of electron density' with zero vertical nodes.
πŸ’‘wave function
A wave function in quantum mechanics is a mathematical description of the quantum state of a particle or system of particles. The wave function provides information about the probability of finding a particle in a particular location or state. The video discusses how the wave functions of molecular orbitals are in phase or out of phase depending on whether a node is crossed, which affects the overall shape and properties of the orbital.
Highlights

The pi molecular orbitals for 1,3,5-hexatriene are discussed, building upon previous lessons on ethylene, 1,3-butadiene, and the allyl system.

1,3,5-hexatriene has a conjugated system with three pi bonds, all interconnected.

There are six atoms with overlapping p orbitals, resulting in six molecular orbitals.

Each molecular orbital is represented by six overlapping p orbitals, indicating a more complex reality.

The molecular orbitals are filled with a total of 24 pi electrons.

The lower half of the molecular orbitals (psi one, two, and three) are bonding, while the upper half (psi four, five, and six) are antibonding.

The highest occupied molecular orbital (HOMO) is psi three, and the lowest unoccupied molecular orbital (LUMO) is psi four star.

The number of nodes in each molecular orbital corresponds to its energy level, with zero nodes being the lowest energy.

A useful tool for drawing molecular orbital diagrams is identifying the number of nodes, which correlate with energy levels.

Psi one has zero vertical nodes, indicating the lowest energy level.

Psi four star and psi five star have four and five nodes, respectively, indicating higher energy levels.

The molecular orbital diagram for psi two requires one node to be symmetrically in the middle.

For orbitals with two nodes, like psi three, a trick involves covering half the diagram and dividing the remaining half.

Odd numbers of nodes always have one node in the center, with the remaining nodes distributed symmetrically.

The grouping for nodes in orbitals with an odd number is memorized as one-two, two-one for psi four star and one-one-two-one for psi five star.

The dashed lines representing nodes are a helpful tool for drawing but are not technically part of the diagram.

The lowest energy orbitals (psi one, psi three, psi five) are symmetrical, while the others are anti-symmetrical, which is important for pericyclic reactions.

The representation of molecular orbitals is a simplified version of a more complex reality, which is depicted in study guides.

The study guide provides a comparison of the simplified representations with the actual realities of the molecular orbitals.

The video encourages viewers to like and share for the benefit of other students, and to check out additional resources on chatsprep.com.

Transcripts
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