FMO UPDATED ACTUAL FINAL EXAM
QUESTIONS AND CORRECT ANSWERS
What is the principle behind the formation of molecular orbitals (MOs) from atomic orbitals
(AOs)? - CORRECT ANSWERS Molecular orbitals are formed by the linear combination of
atomic orbitals. When two AOs combine, they form two MOs: one bonding MO (lower in
energy) and one antibonding MO (higher in energy) than the original AOs. This principle
explains how electrons are shared or transferred in chemical bonding.
What are the major factors influencing orbital overlap in bond formation? - CORRECT
ANSWERS Three major factors influence the effectiveness of orbital overlap: 1) Orbital
energy - orbitals close in energy interact strongly; 2) Orbital size - effective overlap requires
similar sizes; 3) Orbital symmetry - orbitals must have compatible symmetries to overlap
efficiently.
How does hybridization facilitate bonding in larger organic molecules? - CORRECT
ANSWERS Hybridization involves mixing an atom's s and p orbitals to form hybrid atomic
orbitals (HAOs) that are oriented directly towards bonding partners, optimizing orbital overlap.
This includes sp3, sp2, and sp hybridizations corresponding to tetrahedral, trigonal planar, and
linear geometries, respectively.
What defines an electrocyclic reaction in organic chemistry? - CORRECT
ANSWERS Electrocyclic reactions are a type of pericyclic reaction where a conjugated pi-
electron system forms a ring by the formation of a sigma bond between the ends of the system.
These reactions are characterized by their stereospecificity and can be either ring-opening or
ring-closing, influenced by the electronic nature of the system (thermal or photochemical
conditions).
How does Frontier Molecular Orbital Theory apply to organic reactions? - CORRECT
ANSWERS FMO Theory focuses on the interactions between the highest occupied
molecular orbital (HOMO) of one reactant and the lowest unoccupied molecular orbital (LUMO)
of another. In reactions, the HOMO of a nucleophile typically interacts with the LUMO of an
,electrophile, guiding the direction and type of chemical reaction based on orbital energies and
symmetries.
Why are the coefficients of molecular orbitals important in determining chemical reactivity? -
CORRECT ANSWERS The coefficients of molecular orbitals indicate the contribution of
each atomic orbital to the molecular orbital. Larger coefficients at a particular atom in a
molecule's highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital
(LUMO) signify higher reactivity at that atom, guiding selective chemical reactions.
What factors influence orbital overlap in organic molecules? - CORRECT ANSWERS Orbital
overlap is influenced by three main factors: 1. Orbital Energy Alignment - Orbitals must be close
in energy for effective interaction. 2. Orbital Orientation and Symmetry - Orbitals must be
properly aligned and must have compatible symmetry to overlap efficiently. 3. Overlap Extent -
Greater overlap occurs when orbitals are larger and extend towards each other, facilitating
stronger bonding.
How does hybridization describe bonding in organic molecules? - CORRECT
ANSWERS Hybridization theory explains the bonding in organic molecules by mixing
atomic orbitals (s, p, and sometimes d) to form new hybrid orbitals (sp3, sp2, sp) that are
oriented optimally to form covalent bonds with other atoms. This provides a clear structure
predicting molecular geometry (e.g., tetrahedral for sp3, trigonal planar for sp2) and bonding
capabilities.
How is bonding described in extended conjugated systems? - CORRECT ANSWERS In
extended conjugated systems, p-orbitals overlap laterally to form a delocalized π-electron
system across multiple atoms. This delocalization lowers the energy, increases stability, and
contributes to unique chemical properties like absorption of visible light and enhanced
reactivity in aromatic compounds and polyenes.
What is the Diels-Alder reaction mechanism? - CORRECT ANSWERS The Diels-Alder
reaction is a [4+2] cycloaddition between a conjugated diene and a dienophile. The reaction
typically occurs through orbital interactions where the diene's highest occupied molecular
, orbital (HOMO) overlaps with the dienophile's lowest unoccupied molecular orbital (LUMO),
forming a new six-membered ring in a concerted process.
How does Lewis acid catalysis affect the Diels-Alder reaction? - CORRECT ANSWERS Lewis
acids can enhance the Diels-Alder reaction by coordinating to the electron pairs of the
dienophile, increasing its electrophilic character. This coordination stabilizes the LUMO,
increasing reactivity with the diene's HOMO and often improving the reaction rate and
selectivity.
Why is the conformation of the diene important in the Diels-Alder reaction? - CORRECT
ANSWERS The conformation of the diene is crucial as it affects the orbital alignment and
reactivity. S-cis diene conformation (where the double bonds are conrotatory) is necessary for
effective orbital overlap with the dienophile, enabling the cycloaddition process.
What determines the stereochemical outcome of the Diels-Alder reaction? - CORRECT
ANSWERS The stereochemistry of the Diels-Alder product is determined by the
substituent orientation on the diene and dienophile, the approach geometry, and
stereochemical demands of substituents. Endo preference—where substituents orient towards
the electron-withdrawing group on the dienophile—is common due to secondary orbital
interactions.
How do thermodynamic and kinetic control differ in chemical reactions? - CORRECT
ANSWERS Thermodynamic control conditions favour the formation of the most stable
product at equilibrium, typically at higher temperatures or longer reaction times. Kinetic control
conditions favour the formation of products that are formed fastest, which are not necessarily
the most stable and occur under lower temperatures or shorter reaction times.
How can one predict the product of a Diels-Alder reaction? - CORRECT ANSWERS Identify
the Dienophile and Diene: Recognize which compound serves as the diene (must be in s-cis
conformation for effective reaction) and which as the dienophile.
QUESTIONS AND CORRECT ANSWERS
What is the principle behind the formation of molecular orbitals (MOs) from atomic orbitals
(AOs)? - CORRECT ANSWERS Molecular orbitals are formed by the linear combination of
atomic orbitals. When two AOs combine, they form two MOs: one bonding MO (lower in
energy) and one antibonding MO (higher in energy) than the original AOs. This principle
explains how electrons are shared or transferred in chemical bonding.
What are the major factors influencing orbital overlap in bond formation? - CORRECT
ANSWERS Three major factors influence the effectiveness of orbital overlap: 1) Orbital
energy - orbitals close in energy interact strongly; 2) Orbital size - effective overlap requires
similar sizes; 3) Orbital symmetry - orbitals must have compatible symmetries to overlap
efficiently.
How does hybridization facilitate bonding in larger organic molecules? - CORRECT
ANSWERS Hybridization involves mixing an atom's s and p orbitals to form hybrid atomic
orbitals (HAOs) that are oriented directly towards bonding partners, optimizing orbital overlap.
This includes sp3, sp2, and sp hybridizations corresponding to tetrahedral, trigonal planar, and
linear geometries, respectively.
What defines an electrocyclic reaction in organic chemistry? - CORRECT
ANSWERS Electrocyclic reactions are a type of pericyclic reaction where a conjugated pi-
electron system forms a ring by the formation of a sigma bond between the ends of the system.
These reactions are characterized by their stereospecificity and can be either ring-opening or
ring-closing, influenced by the electronic nature of the system (thermal or photochemical
conditions).
How does Frontier Molecular Orbital Theory apply to organic reactions? - CORRECT
ANSWERS FMO Theory focuses on the interactions between the highest occupied
molecular orbital (HOMO) of one reactant and the lowest unoccupied molecular orbital (LUMO)
of another. In reactions, the HOMO of a nucleophile typically interacts with the LUMO of an
,electrophile, guiding the direction and type of chemical reaction based on orbital energies and
symmetries.
Why are the coefficients of molecular orbitals important in determining chemical reactivity? -
CORRECT ANSWERS The coefficients of molecular orbitals indicate the contribution of
each atomic orbital to the molecular orbital. Larger coefficients at a particular atom in a
molecule's highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital
(LUMO) signify higher reactivity at that atom, guiding selective chemical reactions.
What factors influence orbital overlap in organic molecules? - CORRECT ANSWERS Orbital
overlap is influenced by three main factors: 1. Orbital Energy Alignment - Orbitals must be close
in energy for effective interaction. 2. Orbital Orientation and Symmetry - Orbitals must be
properly aligned and must have compatible symmetry to overlap efficiently. 3. Overlap Extent -
Greater overlap occurs when orbitals are larger and extend towards each other, facilitating
stronger bonding.
How does hybridization describe bonding in organic molecules? - CORRECT
ANSWERS Hybridization theory explains the bonding in organic molecules by mixing
atomic orbitals (s, p, and sometimes d) to form new hybrid orbitals (sp3, sp2, sp) that are
oriented optimally to form covalent bonds with other atoms. This provides a clear structure
predicting molecular geometry (e.g., tetrahedral for sp3, trigonal planar for sp2) and bonding
capabilities.
How is bonding described in extended conjugated systems? - CORRECT ANSWERS In
extended conjugated systems, p-orbitals overlap laterally to form a delocalized π-electron
system across multiple atoms. This delocalization lowers the energy, increases stability, and
contributes to unique chemical properties like absorption of visible light and enhanced
reactivity in aromatic compounds and polyenes.
What is the Diels-Alder reaction mechanism? - CORRECT ANSWERS The Diels-Alder
reaction is a [4+2] cycloaddition between a conjugated diene and a dienophile. The reaction
typically occurs through orbital interactions where the diene's highest occupied molecular
, orbital (HOMO) overlaps with the dienophile's lowest unoccupied molecular orbital (LUMO),
forming a new six-membered ring in a concerted process.
How does Lewis acid catalysis affect the Diels-Alder reaction? - CORRECT ANSWERS Lewis
acids can enhance the Diels-Alder reaction by coordinating to the electron pairs of the
dienophile, increasing its electrophilic character. This coordination stabilizes the LUMO,
increasing reactivity with the diene's HOMO and often improving the reaction rate and
selectivity.
Why is the conformation of the diene important in the Diels-Alder reaction? - CORRECT
ANSWERS The conformation of the diene is crucial as it affects the orbital alignment and
reactivity. S-cis diene conformation (where the double bonds are conrotatory) is necessary for
effective orbital overlap with the dienophile, enabling the cycloaddition process.
What determines the stereochemical outcome of the Diels-Alder reaction? - CORRECT
ANSWERS The stereochemistry of the Diels-Alder product is determined by the
substituent orientation on the diene and dienophile, the approach geometry, and
stereochemical demands of substituents. Endo preference—where substituents orient towards
the electron-withdrawing group on the dienophile—is common due to secondary orbital
interactions.
How do thermodynamic and kinetic control differ in chemical reactions? - CORRECT
ANSWERS Thermodynamic control conditions favour the formation of the most stable
product at equilibrium, typically at higher temperatures or longer reaction times. Kinetic control
conditions favour the formation of products that are formed fastest, which are not necessarily
the most stable and occur under lower temperatures or shorter reaction times.
How can one predict the product of a Diels-Alder reaction? - CORRECT ANSWERS Identify
the Dienophile and Diene: Recognize which compound serves as the diene (must be in s-cis
conformation for effective reaction) and which as the dienophile.
