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Questions and Answers
What is the primary goal of molecular orbital theory?
What is the primary goal of molecular orbital theory?
What type of orbitals are formed between the carbon atoms in ethyne?
What type of orbitals are formed between the carbon atoms in ethyne?
What is the reason for ethane being less reactive than ethyne and ethene?
What is the reason for ethane being less reactive than ethyne and ethene?
What is the concept that accounts for the planarity and shorter bond length of 1,3-butadiene?
What is the concept that accounts for the planarity and shorter bond length of 1,3-butadiene?
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How many π molecular orbitals are formed in 1,3-butadiene?
How many π molecular orbitals are formed in 1,3-butadiene?
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What is the length of the C2-C3 bond in 1,3-butadiene?
What is the length of the C2-C3 bond in 1,3-butadiene?
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What is the reason for the barrier to rotation about the C2-C3 bond in 1,3-butadiene?
What is the reason for the barrier to rotation about the C2-C3 bond in 1,3-butadiene?
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What is the shape of the molecule 1,3-butadiene?
What is the shape of the molecule 1,3-butadiene?
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According to the 4n+2 rule, what is the condition for a molecule to be aromatic?
According to the 4n+2 rule, what is the condition for a molecule to be aromatic?
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What is the number of π electrons in the benzene molecule?
What is the number of π electrons in the benzene molecule?
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What type of orbitals do π electrons lie in?
What type of orbitals do π electrons lie in?
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What is the condition for a molecule to be fully conjugated?
What is the condition for a molecule to be fully conjugated?
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What is the result of molecular orbital calculations for cyclooctatetraene?
What is the result of molecular orbital calculations for cyclooctatetraene?
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What is the characteristic of the pi4 and pi5 molecular orbitals in cyclooctatetraene?
What is the characteristic of the pi4 and pi5 molecular orbitals in cyclooctatetraene?
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What type of compounds can also show aromatic stabilization?
What type of compounds can also show aromatic stabilization?
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What is the characteristic of the cyclopentadienyl anion?
What is the characteristic of the cyclopentadienyl anion?
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What is the effect of conjugation on the stability of a molecule?
What is the effect of conjugation on the stability of a molecule?
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According to Hückel's MO theory, what is a characteristic of particularly stable compounds?
According to Hückel's MO theory, what is a characteristic of particularly stable compounds?
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What is the condition for two or more pi bonds to be considered conjugated?
What is the condition for two or more pi bonds to be considered conjugated?
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What is the hybridization of the carbon atoms in benzene?
What is the hybridization of the carbon atoms in benzene?
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How many pi electrons are in the lowest energy orbital of benzene?
How many pi electrons are in the lowest energy orbital of benzene?
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What is the shape of the benzene molecule?
What is the shape of the benzene molecule?
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What is the result of molecular orbital theory on the pi electrons in 1,3-butadiene?
What is the result of molecular orbital theory on the pi electrons in 1,3-butadiene?
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What is the reason for the barrier to rotation in 1,3-butadiene?
What is the reason for the barrier to rotation in 1,3-butadiene?
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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.
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Description
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.
