24 Questions
What is the primary goal of molecular orbital theory?
To describe molecules in a similar way to how we describe atoms
What type of orbitals are formed between the carbon atoms in ethyne?
σ and σ* MOs, and π and π* MOs
What is the reason for ethane being less reactive than ethyne and ethene?
Because it has no / * HOMO / LUMO molecular orbitals
What is the concept that accounts for the planarity and shorter bond length of 1,3-butadiene?
Delocalized π bonds
How many π molecular orbitals are formed in 1,3-butadiene?
Four
What is the length of the C2-C3 bond in 1,3-butadiene?
148 pm
What is the reason for the barrier to rotation about the C2-C3 bond in 1,3-butadiene?
Due to the delocalization of π bonds
What is the shape of the molecule 1,3-butadiene?
Planar
According to the 4n+2 rule, what is the condition for a molecule to be aromatic?
n must be a positive integer
What is the number of π electrons in the benzene molecule?
6
What type of orbitals do π electrons lie in?
p orbitals
What is the condition for a molecule to be fully conjugated?
Every molecule in the cyclic compound is sp2 hybridized
What is the result of molecular orbital calculations for cyclooctatetraene?
The molecule is not aromatic
What is the characteristic of the pi4 and pi5 molecular orbitals in cyclooctatetraene?
They are non-bonding MOs
What type of compounds can also show aromatic stabilization?
Carbanions and carbocations
What is the characteristic of the cyclopentadienyl anion?
It is an aromatic ion
What is the effect of conjugation on the stability of a molecule?
It increases the stability of the molecule
According to Hückel's MO theory, what is a characteristic of particularly stable compounds?
All of their bonding molecular orbitals are filled with paired electrons
What is the condition for two or more pi bonds to be considered conjugated?
They must be separated by only one single bond
What is the hybridization of the carbon atoms in benzene?
sp2
How many pi electrons are in the lowest energy orbital of benzene?
2
What is the shape of the benzene molecule?
Planar hexagonal
What is the result of molecular orbital theory on the pi electrons in 1,3-butadiene?
They are partially delocalized over the whole pi system
What is the reason for the barrier to rotation in 1,3-butadiene?
The presence of pi-bonding interaction between C2 and C3
Study Notes
Molecular Orbital Theory
- The goal of molecular orbital theory is to describe molecules in a similar way to atoms, using orbitals, orbital diagrams, and electron configurations.
- Methane molecular orbitals involve forming sigma and sigma-star MOs between two carbon atoms and hydrogen atoms.
- Ethyne molecular orbitals involve forming sigma, sigma-star, pi, and pi-star MOs between carbon atoms and hydrogen atoms.
Ethane and Ethene
- Ethane has no / * HOMO / LUMO molecular orbitals, making it less reactive than ethyne and ethene.
- Ethene molecular orbitals involve forming pi and pi-star MOs between carbon atoms.
Conjugation and Aromaticity
- In 1,3-butadiene, molecular orbital theory accounts for the concept of delocalized pi bonds, where four 2p atomic orbitals combine to form four pi molecular orbitals of increasing energy.
- The 4n+2 rule is used to determine aromaticity, where the number of π electrons is set equal to 4n+2 and solved for n.
- A molecule is conjugated if it has a planar, sp2-hybridized structure, with pi electrons in p orbitals.
Cyclooctatetraene
- Molecular orbital calculations show that the lowest and highest energy MOs (pi1 and pi8*) have unique energy levels, while the other six form degenerate pairs.
- The molecule is not aromatic because it doesn't have a perfect filled shell of bonding MOs.
Charged Aromatic Compounds
- Carbanions and carbocations may show aromatic stabilization, such as the cyclopentadienyl anion.
Heterocyclic Aromatic Compounds
- Two bonding pi orbitals are lower in energy than the p atomic orbitals, while two antibonding pi* orbitals are higher in energy.
- By the aufbau principle, the four electrons from the isolated 2pz atomic orbitals are placed in the bonding pi1 and pi2 MOs.
Conjugation and Stability
- Conjugation is associated with an inherent thermodynamic stability, which can be measured experimentally by comparing the heat of hydrogenation of two different dienes.
- According to Hückel's MO theory, a compound is particularly stable if all of its bonding molecular orbitals are filled with paired electrons, making aromatic compounds quite stable.
This quiz covers the molecular orbital theory, describing molecules in terms of orbitals, orbital diagrams, and electron configurations. It includes the formation of sigma and sigma-star MOs between carbon atoms and hydrogen atoms.
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