Chapter 16 - Conjugated Pi Systems and Pericyclic Reactions Flashcards
What is the primary function of most bleaching agents?
They react with colored compounds in stains to produce colorless compounds.
The stain remains but becomes invisible.
What are the three classifications of dienes based on the proximity of π bonds?
- Cumulated
- Conjugated
- Isolated
Each classification reflects the arrangement of π bonds in relation to σ bonds.
What is a cumulated diene?
A diene where the π bonds are adjacent.
Also known as allenes.
What distinguishes conjugated dienes from isolated dienes?
In conjugated dienes, π bonds are separated by exactly one σ bond.
Isolated dienes have π bonds separated by two or more σ bonds.
What special feature do conjugated dienes exhibit?
They contain one continuous system of overlapping p orbitals.
This overlapping results in unique properties and reactivity.
How can conjugated dienes be prepared from allylic halides?
Via an elimination process using a sterically hindered base.
This prevents competing SN2 reactions.
What is the relationship between the bond length of a conjugated diene and a typical C–C single bond?
The single bond in a conjugated diene is shorter than a typical C–C single bond.
This is due to the hybridization states involved.
What hybridization state is involved in the C–C bond of a conjugated diene?
sp2 hybridization.
This results in a bond that is shorter than sp3 hybridized bonds.
What indicates that conjugated double bonds are more stable than isolated double bonds?
The heat of hydrogenation for conjugated dienes is less than expected.
This suggests a stabilization energy of approximately 15 kJ/mol for conjugated dienes.
What are the two conformers of 1,3-butadiene?
- s-cis
- s-trans
These conformers differ in the dihedral angle between the π bonds.
Which conformer of 1,3-butadiene is favored at room temperature?
s-trans conformer.
Approximately 98% of the molecules exist in this conformation.
What is the activation energy for the conversion from s-cis to s-trans conformer of 1,3-butadiene?
15 kJ/mol.
This is the same as the stabilization energy associated with conjugated double bonds.
What are the two types of molecular orbitals produced from overlapping p orbitals according to MO theory?
- Bonding MO
- Antibonding MO
The bonding MO is lower in energy than the antibonding MO.
How many molecular orbitals are produced from the four overlapping p orbitals of 1,3-butadiene?
Four molecular orbitals.
These MOs have varying numbers of nodes.
What is the Highest Occupied Molecular Orbital (HOMO) in 1,3,5-hexatriene?
The highest energy among the bonding MOs.
It is occupied by π electrons.
What is the significance of the bond distance in conjugated dienes?
It is shorter due to the delocalization of π electrons over multiple carbon atoms.
This contributes to the observed stabilization energy.
What is the highest energy molecular orbital called?
Highest Occupied Molecular Orbital (HOMO)
What is the lowest energy unoccupied molecular orbital called?
Lowest Unoccupied Molecular Orbital (LUMO)
What are the most important MOs for conjugated polyenes?
HOMO and LUMO
What phenomenon allows conjugated π systems to interact with light?
The ability of a π electron in the HOMO to absorb a photon and promote to the LUMO
Who advanced the frontier orbital theory and when?
Kenichi Fukui in 1954
What is the effect of light on frontier orbitals?
It can cause a change in the identities of the frontier orbitals
What is the product of protonation at position C1 or C4 in butadiene?
Allylic carbocation (resonance stabilized)
What are the two types of addition products formed when butadiene reacts with HBr?
1,2-adduct and 1,4-adduct
What determines the regiochemical outcome of the addition reaction?
The stability of the carbocation formed during proton transfer
What is the major product at low temperature when butadiene reacts with HBr?
1,2-adduct
What is the major product at elevated temperature when butadiene reacts with HBr?
1,4-adduct
What is the term for a reaction where the product distribution is determined by the relative rates at which the products are formed?
Kinetic control
What is the term for a reaction where the product distribution is determined by the relative energy levels of the products?
Thermodynamic control
What happens to the product distribution as temperature increases in electrophilic addition?
The 1,4-adduct predominates due to the establishment of equilibrium
What stabilizes the 1,4-adduct compared to the 1,2-adduct?
It exhibits a more substituted double bond, making it lower in energy
What is the first step of the mechanism for 1,2-addition and 1,4-addition?
Protonation of the conjugated diene to form a resonance-stabilized, allylic carbocation
Fill in the blank: The ability of light to affect the frontier orbitals is important in ______ reactions.
photochemical
True or False: The 1,2-adduct forms more rapidly due to a proximity effect.
True
What occurs at low temperatures that makes the 1,2-adduct the major product?
Insufficient energy to convert to the allylic carbocation
What is the expected allylic carbocation formed via protonation at C1?
A resonance-stabilized allylic carbocation
The carbocation can be attacked at either of two positions, leading to 1,2- or 1,4-addition.
Which adduct is expected to be the kinetic product in the reaction?
1,2-adduct
Due to the proximity effect.
Which adduct is expected to be the thermodynamic product in the reaction?
1,4-adduct
It exhibits a more highly substituted (tetrasubstituted) π bond.
Under what temperature conditions does the 1,2-adduct predominate?
Low temperature
The reaction is under kinetic control.
What is the process called that improves the elasticity of natural rubber?
Vulcanization
Discovered by Charles Goodyear in 1839.
What type of linkage is formed during vulcanization?
Disulfide linkages
These linkages help maintain rubber’s elasticity at high temperatures.
What is the primary polymer produced from the polymerization of isoprene?
Cis-1,4-polyisoprene
This is the main component of natural rubber.
What is the commercially important polymer produced from chloroprene?
Neoprene
Produced via free-radical polymerization.
What are the three major groups of pericyclic reactions?
- Cycloaddition reactions
- Electrocyclic reactions
- Sigmatropic rearrangements
These reactions proceed without ionic or radical intermediates.
What characterizes pericyclic reactions?
- Concerted process
- Ring of electrons in a closed loop
- Cyclic transition state
- Little impact of solvent polarity
Suggesting minimal partial charge in the transition state.
In a cycloaddition reaction, how many bonds are formed and broken?
- Bonds broken: 2 π bonds
- Bonds formed: 2 σ bonds
This results in the addition of two reactants to form a ring.
What is the significance of the Diels–Alder reaction?
It is a useful [4+2] cycloaddition reaction
Named after Otto Diels and Kurt Alder, who won the Nobel Prize in Chemistry in 1950.
What happens to the bonds during a Diels–Alder reaction?
Two C–C σ bonds are formed simultaneously
A ring is also formed in this cycloaddition process.
What is the effect of moderate temperatures on Diels–Alder reactions?
They favor product formation
High temperatures can lead to the reverse reaction, known as retro Diels–Alder.
What determines the sign of ΔG in a Diels–Alder reaction?
ΔG = ΔH + (–T ΔS)
The enthalpy term must be larger than the entropy term for ΔG to be negative.
What happens at high temperatures during a Diels–Alder reaction?
The entropy term dominates
Leading to a positive ΔG and favoring reactants over products.
What does a negative ΔG indicate in a Diels–Alder reaction?
Products are favored over reactants
The equilibrium constant K will be greater than 1.
At what temperature range are Diels–Alder reactions generally performed?
Between room temperature and 200°C
This range can vary depending on the specific case.
What is a dienophile?
A compound that reacts with a diene in a Diels–Alder reaction
How does the presence of an electron-withdrawing substituent affect the dienophile?
It increases the reaction rate and yield
What type of alkene produces a cis disubstituted ring in a Diels–Alder reaction?
A cis alkene
What is the product of a Diels–Alder reaction when a triple bond is used as a dienophile?
A ring with two double bonds (1,4-cyclohexadiene)
Fill in the blank: The Diels–Alder reaction only occurs when the diene adopts an _______ conformation.
s-cis
What happens to cyclopentadiene when allowed to stand at room temperature?
It is converted into dicyclopentadiene
What is the endo preference in a Diels–Alder reaction?
The endo product is favored over the exo product
Why is the endo product typically favored in Diels–Alder reactions?
Due to favorable interactions between electron-withdrawing substituents and the developing π bond
What type of outcome is expected if the diene or dienophile is symmetrical?
One possible regiochemical outcome
What are the HOMO and LUMO in the context of a Diels–Alder reaction?
HOMO is the Highest Occupied Molecular Orbital; LUMO is the Lowest Unoccupied Molecular Orbital
What is required for the Diels–Alder reaction to occur regarding the phases of MOs?
The phases of the MOs must overlap and be symmetric
What does conservation of orbital symmetry refer to?
The requirement that the phases of the MOs overlap properly during a reaction
True or False: The Diels–Alder reaction is a symmetry-allowed process.
True
What is a hetero Diels–Alder reaction?
A variation where one or more carbon atoms of the diene and/or dienophile are replaced by other atoms such as oxygen or nitrogen
What happens when cyclopentadiene is allowed to stand at room temperature?
It rapidly dimerizes to form dicyclopentadiene
What is the effect of substituents on the reactivity of dienes in Diels–Alder reactions?
Substituents can either increase or decrease reactivity depending on their electron-withdrawing or electron-donating nature
What is a [2+2] cycloaddition?
A cycloaddition involving two different π systems, each associated with two atoms
What is the outcome of a Diels–Alder reaction when both the diene and dienophile are unsymmetrical?
Two possible regiochemical outcomes, resulting in constitutional isomers
What type of reaction is a [2+2] cycloaddition?
[2+2] cycloaddition is a cycloaddition involving two different π systems, each associated with two atoms.
What is the significance of frontier orbitals in determining the feasibility of a [2+2] cycloaddition?
The reaction is feasible if the HOMO of one compound can interact with the LUMO of the other.
What does it mean when a reaction is said to be symmetry forbidden?
It means that the phases of the molecular orbitals do not overlap properly, preventing the reaction from occurring.
Under what condition can a [2+2] cycloaddition become symmetry allowed?
When one of the compounds is subjected to UV light, promoting a π electron to a higher energy level.
What happens to the HOMO of ethylene when it is excited by UV light?
The HOMO is redefined to ψ2, allowing interaction with the LUMO of a ground-state molecule.
What is an electrocyclic reaction?
A pericyclic process where a conjugated polyene undergoes cyclization, converting one π bond into a σ bond.
What determines the stereochemical outcome of electrocyclic reactions?
The configuration of the reactant and the conditions of ring closure (thermal or photochemical).
What are the Woodward–Hoffmann rules for thermal electrocyclic reactions with four and six π electrons?
Four π electrons: Conrotatory; Six π electrons: Disrotatory.
What is the requirement for bond formation in electrocyclic reactions?
The lobes that interact must exhibit the same sign.
What type of rotation occurs in a disrotatory ring closure?
One set of lobes rotates clockwise, while the other rotates counterclockwise.
What is the stereochemical outcome of a thermal electrocyclic reaction involving six π electrons?
It occurs in a disrotatory fashion.
What occurs during photochemical electrocyclic reactions with six π electrons?
They undergo conrotatory ring closure.
What type of rotation occurs in a conrotatory ring closure?
Both sets of lobes rotate in the same direction.
What does the term ‘conjugated polyene’ refer to?
A compound with alternating double and single bonds, allowing for delocalization of π electrons.
What effect does temperature have on thermal electrocyclic reactions?
They can occur at or even below room temperature if sufficient heat is available.
How does the configuration of substituents affect the product of an electrocyclic reaction?
It determines whether the substituents are cis or trans in the product.
What is the relationship between the configuration of the reactant and the product in thermal electrocyclic reactions?
The configuration of the product is determined by the configuration of the reactant.
What happens to the HOMO during the excitation of an electron in photochemical reactions?
It redefines the identity of the HOMO, affecting the stereochemical outcome.
What is the outcome of a photochemical electrocyclic reaction involving four π electrons?
It occurs in a disrotatory fashion.
What is a sigmatropic rearrangement?
A pericyclic reaction in which one σ bond is formed at the expense of another.
What is a sigmatropic rearrangement?
A pericyclic reaction in which one σ bond is formed at the expense of another, changing the location of π bonds.
The term ‘sigmatropic’ comes from the Greek word ‘tropos’, meaning ‘change’.
What does the notation [3,3] in sigmatropic rearrangements indicate?
The number of atoms separating the bond that is forming and the bond that is breaking in the transition state.
The two numbers indicate the arrangement of atoms involved in the rearrangement.
What characterizes the transition state of a [3,3] sigmatropic rearrangement?
It is cyclic, with the breaking and forming bonds separated by two different pathways of three atoms each.
What is a Cope rearrangement?
A [3,3] sigmatropic rearrangement where all six atoms of the cyclic transition state are carbon atoms.
What does the equilibrium for a Cope rearrangement generally favor?
Formation of the more substituted alkene.
What is a Claisen rearrangement?
A [3,3] sigmatropic rearrangement commonly observed for allylic vinylic ethers.
Why does the Claisen rearrangement favor product formation?
Due to the formation of a C=O bond, which is thermodynamically more stable than a C=C bond.
What happens during the tautomerization process in allylic aryl ethers after a Claisen rearrangement?
It converts a ketone into an enol, regenerating the aromatic ring.
What is the main function of vitamin D in the body?
It increases the body’s ability to absorb calcium, essential for bone and teeth formation.
What is the difference between cholecalciferol and ergocalciferol?
Cholecalciferol (vitamin D3) and ergocalciferol (vitamin D2) differ in the identity of their side chain.
What is the process through which 7-dehydrocholesterol is converted into cholecalciferol?
Exposure to UV light.
What type of reactions convert precursors into vitamin D?
Two successive pericyclic reactions: an electrocyclic ring-opening reaction and a [1,7] sigmatropic rearrangement.
What does a UV-Vis spectrophotometer measure?
The absorbance of a sample as a function of wavelength.
What is the Beer-Lambert law formula?
A = log I0/I, where I0 is the intensity of the reference beam and I is the intensity of the sample beam.
What does λmax represent in UV-Vis spectroscopy?
The wavelength of maximum absorption for a compound.
What is a chromophore?
The region of the molecule responsible for absorption in a UV-Vis spectrum.
What are auxochromes?
Groups attached to the chromophore that can affect the value of λmax.
What do the Woodward–Fieser rules help predict?
The λmax for conjugated π systems.
What is the base value of λmax for a conjugated diene according to Woodward–Fieser rules?
217 nm.
How much does each additional double bond increase λmax?
+30 nm.
True or False: The λmax of a compound is independent of the extent of conjugation.
False.
Fill in the blank: Compounds with more highly conjugated systems require less energy to promote an electronic excitation because they have _______.
[smaller energy gaps between molecular orbitals].
What is the base value for λmax in the Woodward–Fieser rules?
217 nm
How much does each additional conjugated double bond add to the λmax?
+30 nm
What is the contribution of each auxochromic alkyl group to the λmax?
+5 nm
What is the contribution of one exocyclic double bond to the λmax?
+5 nm
What does a homoannular diene contribute to the λmax?
+39 nm
In the Woodward–Fieser rules, how many double bonds must be counted for the λmax estimation?
Count the number of conjugated double bonds
What is the predicted λmax for a compound with four conjugated double bonds and six auxochromic alkyl groups?
390 nm
True or False: UV-B radiation is less damaging than UV-A radiation.
False
What are the two types of damaging UV light?
- UV-A (315–400 nm) * UV-B (280–315 nm)
What can a deficiency in vitamin D cause?
Rickets
What is the primary photochemical reaction involved in vision?
Cis-trans isomerization
What type of cells in the retina are responsible for color vision?
Cones
Fill in the blank: The light-sensitive compound in rods is called _______.
Rhodopsin
What vitamin is essential for the synthesis of 11-cis-retinal?
Vitamin A
What happens to a compound’s color when it is bleached?
It becomes colorless
What is the main function of sunscreens?
To prevent UV light from causing damage to DNA
What types of sunscreens exist?
- Inorganic * Organic
Which organic sunscreen is believed to offer the best protection against UV-A radiation?
Avobenzone
What compounds are responsible for the red color of tomatoes and the orange color of carrots?
- Lycopene * β-Carotene
What is the role of bleaching agents in the context of color?
They disrupt conjugated π systems
What wavelength range corresponds to the visible region of the electromagnetic spectrum?
400–700 nm
How do complementary colors relate to the appearance of a compound?
A compound is colored if it absorbs a specific color more strongly than its complementary color
What is the estimated chance of developing skin cancer for Americans?
10%
What is the configuration of 11-cis-retinal?
It has a cis double bond incorporated in a seven-membered ring preventing isomerization to the trans configuration.
What happens when Nakanishi’s analogue binds with opsin?
It produces a product similar to rhodopsin but unable to isomerize under light.
What is the first step in the chemistry of vision?
A photoisomerization reaction.
What initiates the cascade of enzymatic reactions in rhodopsin?
Excitation to the high-energy all-trans isomer changes rhodopsin’s shape.
What effect do calcium ions have on sodium ion channels?
They block channels that normally allow the passage of sodium ions.
What is the dark current?
The regular flow of sodium ions in the absence of light.
How sensitive is human vision to light?
The absorption of a single photon can prevent the flow of millions of sodium ions.
What are the three types of dienes?
- Cumulated
- Conjugated
- Isolated
What characterizes conjugated dienes?
They contain one continuous system of overlapping p orbitals.
How can conjugated dienes be prepared?
- From allylic halides
- From dihalides
What is the stabilization energy of conjugated double bonds compared to isolated double bonds?
Approximately 15 kJ/mol.
What conformations do conjugated dienes experience?
- s-cis
- s-trans
Which conformation of conjugated dienes is lower in energy?
s-trans conformation.
What do the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) refer to?
HOMO contains π electrons readily available for reactions; LUMO is the lowest energy MO that can accept electron density.
What is a photochemical reaction?
Reactions induced by light.
What are the major products when butadiene is treated with HBr?
- 1,2-adduct
- 1,4-adduct
What is the difference between kinetic control and thermodynamic control in reactions?
- Kinetic control: addition at low temperature
- Thermodynamic control: addition at elevated temperature
What are the classifications of pericyclic reactions?
- Cycloaddition reactions
- Electrocyclic reactions
- Sigmatropic rearrangements
What is the Diels–Alder reaction?
A [4+2] cycloaddition forming two C–C bonds simultaneously.
What conditions favor product formation in a Diels–Alder reaction?
Moderate temperatures.
What is favored over the exo cycloadduct when using cyclopentadiene in a Diels–Alder reaction?
The endo cycloadduct.
What is the significance of conservation of orbital symmetry in Diels–Alder reactions?
It requires MOs to have the same phases for effective overlap.
What is an electrocyclic reaction?
A pericyclic process where one π bond is converted into a σ bond during cyclization.
What determines the stereochemical outcome of an electrocyclic reaction?
The conditions (thermal vs. photochemical) and conservation of orbital symmetry.
What is a sigmatropic rearrangement?
A pericyclic reaction forming one σ bond at the expense of another.
What is the λmax in UV-Vis spectroscopy?
The wavelength of maximum absorption.
What is the relationship between molar absorptivity (ε) and absorbance?
Described by the Beer-Lambert law.
What is the chromophore in a molecule?
The region responsible for absorption in a conjugated π system.
What condition indicates that a compound will absorb visible light?
λmax between 400 and 700 nm.
What is rhodopsin responsible for?
Night vision.
What occurs when rhodopsin absorbs light?
A cis-trans isomerization occurs to give the all-trans isomer.
