Pericyclic Reactions and MO

Conjugation requires ______ orbitals on three or more adjacent atoms in a structure.

p

Select all of the statements that describe a conjugated system.

-Conjugation results in a more stable system
-Conjugation allows electrons to be delocalized

An allylic carbocation is an example of a(n) _____ system. (Select all that apply.)

-Conjugated
-Delocalized

Select all criteria that must be fulfilled to allow conjugation.

-p orbitals on three or more adjacent atoms
-Planar structure
-Three or more adjacent sp2 or sp hybridized atoms

Conjugation leads to electron delocalization, which ______ the energy of the system and makes it ______ stable.

-Decreases
-More

Which structural feature must a compound possess in order to be classified as an allylic carbocation?

A c-c double bond adjacent to a carbocation

Conjugation is only possible if the participating atoms are _____ or _____ hybridized, because conjugation requires the overlap of p orbitals on three or more adjacent atoms.

-sp
-sp2

What features distinguish conjugated dienes from isolated dienes? Select all that apply

-They undergo reactions differently than isolated dienes
-They are more stable than isolated dienes

When comparing a conjugated diene to a similar isolated diene, the conjugated diene has a ______ heat of hydrogenation, which means it is ______ stable than a similar isolated diene.

-lower
-more

What two classes of compounds are commonly involved in a Diels-Alder reaction?

-Alkene
-1,3-diene

The Diels-Alder reaction is initiated by _____, occurs in a single step, and always produces a _____-membered ring product.

-Heat
-Six

A Diels-Alder reaction is an _____ -type reaction that occurs between a 1,3-diene and an alkene, also known as a _____.

-Addition
-Dienophile

Select all statements that describe a general Diels-Alder reaction.

-The reaction is initiated by heat
-The reaction involves the breaking of three pi bonds
-The reaction is concerted

True or false: The product of a Diels-Alder reaction always contains at least one 6-membered ring.

True

In the Diels-Alder reaction, a cis dienophile will form a _____ substituted cyclohexene product while a trans dienophile will form a _____ substituted cyclohexene product.

-Cis
-Trans

In the Diels-Alder reaction between cyclopentadiene and ethylene, the product is a ______ bicyclic ring system.

bridged

To participate in a Diels-Alder reaction, the diene must be able to assume the _____ conformation.

s-cis

Electron donating group

decreases dienophile reactivity

Electron withdrawing group

increases dienophile reactivity

fused bicyclic

Two adjacent carbon atoms are shared between two rings.

bridged bicyclic

Two nonadjacent carbon atoms are shared between two rings.

In a Diels-Alder reaction, the stereochemistry of the dienophile is _____ in the 6-membered ring product.

retained

Cl

exo

CN

endo

If the dienophile in a Diels-Alder reaction is oriented as shown, the product will be _____, whereas if the dienophile is turned so that the chlorine atom is turned away from the diene, the product will be _____.

-endo
-exo

What is the first step needed in order to start the retrosynthetic analysis of a Diels-Alder adduct?

Locate the six-membered ring (cyclohexene).

For the bridged bicyclic system shown, the H is in the ___________ position and the CHO group is in the ___________ position.

-exo
-endo

Cyclopenta-1,3-diene is a(n) ______ diene that readily undergoes a Diels-Alder reaction with itself where one molecule will act as the diene and the other as a ______.

-reactive
-dienophile

In the structure shown, which of the rings (if any) was/were synthesized using a Diels-Alder reaction?

Ring B only

What are the main reasons why 1,3-cyclopentadiene dimerizes upon standing at room temperature?

-One molecule will act as the diene and one as the dienophile.
-The constrained diene is extremely reactive.

Electrons are moved to _____ energy states when molecules absorb electromagnetic radiation and move to _____ energy states when molecules emit electromagnetic radiation.

-higher
-lower

Rank each structural feature according to increasing energy difference between the ground state and excited state electrons. (Place the lowest energy choice at the top of the list.)

1. conjugated polyene
2. conjugated diene
3. isolated diene
4. saturated hydrocarbon

UV-A

320-400 nm

UV-B

290-320 nm

UV-C

< 290 nm

To move from a lower energy state to a higher energy state, an electron needs to _____ energy of a specific _____.

-absorb,
-wavelength

As the number of conjugated π bonds in a compound increases, the energy difference between the ground and excited electronic states _____ and the compound absorbs light of _____ wavelengths.

-decreases
-longer

Conjugated compounds are contained in many sunscreens. These molecules shield the skin from _____ radiation, which has a wavelength range of ______ nm.

-UV-B
-290-320

A concerted reaction that proceeds via a cyclic transition state is called a _____ reaction.

pericyclic

electrocyclic reaction

-involves ring closure or ring opening
-intramolecular reaction; the product contains one more σ bond and one fewer π bond than the reactant

cycloaddition

-results in the formation of a cyclic product
-a reaction between two different compounds with π bonds; a ring is formed

sigmatropic rearrangement

-involves the migration of π bonds, along with the breaking and formation of a σ bond
-intramolecular reaction; the number of π bonds does not change

MO theory describes bonds as the combination of the wave functions of atomic orbitals from two different atoms to form new orbitals called ______ orbitals.

molecular

ψ1 designates the

bonding (π) molecular orbital

ψ2 designates the

antibonding,(π*) molecular orbital.

The overlap of p orbitals with the same phase

forms the ψ1 orbital.

The overlap of p orbitals with opposite phases

forms the ψ2 orbital.

Which of the following options correctly describe pericyclic reactions? Select all that apply.

-Pericyclic reactions are mediated by light or heat.
-Pericyclic reactions are stereospecific.

Which of the following statements correctly describe the formation and characteristics of molecular orbitals? Select all that apply.

-Molecular orbitals are formed when atomic orbitals combine.
-The formation of molecular orbitals is viewed as the combination of the wave functions of the atomic orbitals.

When two p orbitals combine by sideways overlap they will form two _____ molecular orbitals, one bonding orbital and one _____ orbital.

-π
-antibonding

Which of the following options correctly describe the π molecular orbitals of 1,3-butadiene? Select all that apply.

-1,3-butadiene has four π molecular orbitals.
-The LUMO in the ground state of 1,3-butadiene is the ψ3* orbital.

Which of the following options correctly describe the π molecular orbitals of 2,4,6-octatriene? (Remember that the number of molecular orbitals formed is always equal to the number of atomic orbitals that combine.)

-There should be six π molecular orbitals.
-The highest energy π MO will have five nodes.
-The highest energy π MO is the ψ6* orbital.

Ring-closing

A new σ bond is formed and one π bond is lost.

Ring-opening

A new π bond is formed and one σ bond is lost.

Which of the following options correctly describe the stereochemical outcome of an electrocyclic reaction? Select all that apply.

-The stereochemical outcome depends on whether the reaction is thermally or photochemically mediated.
-These reactions show complete stereospecificity.

Which orbital and which electron configuration must be considered when determining the stereochemical outcome of a thermally mediated electrocyclic reaction? Select both parts of the answer.

- HOMO
- ground state configuration

Which of the following options correctly describe electrocyclic reactions? Select all that apply.

- Conjugated polyenes are frequently the starting material in this type of reaction.
- This type of reaction is reversible.
- Electrocyclic ring closure results in a product that has one more σ bond than the starting material.

Electrocyclic ring closure is a stereospecific reaction. This means that an electrocyclic ring closure of an acyclic starting material in which two new stereocenters are generated will generally produce either _____ or an achiral meso compound.

a racemic mixture of enantiomers

To determine the stereochemical outcome of a thermally mediated electrocyclic reaction, we need to consider the symmetry of the _____.

HOMO of the ground state electronic configuration

odd number of π bonds

disrotatory rotation

even number of π bonds

conrotatory rotation

Light causes an electron in a conjugated polyene to be promoted to a higher energy orbital; we therefore need to consider the HOMO of the ____________ state to determine the stereochemical outcome of this type of electrocyclic reaction.

excited

According to the Woodward-Hoffmann rules, a polyene with an even number of π bonds will show _____ rotation of outermost p orbitals in a thermal electrocyclic reaction, but _____ rotation in a photochemical reaction.

-conrotatory
- disrotatory

For a conjugated polyene in the ground state with an even number of π bonds, a ______ rotation of the outermost p orbitals will bring like phases together.

conrotatory

A [4 + 2] cycloaddition reaction _____. (Select all the options that complete the sentence correctly.)

-involves a total of 3 π bonds
-involves a total of 6 π electrons

In a photochemical electrocyclic reaction, we need to consider the symmetry of the ______ to determine the course of the reaction.

HOMO of the excited state

A polyene with an even number of π bonds

shows conrotatory rotation of p orbitals in a thermal reaction.

A polyene with an odd number of π bonds

shows disrotatory rotation of p orbitals in a thermal reaction.

A photochemical reaction for a polyene with an even number of π bonds

will result in disrotatory rotation of p orbitals.

A photochemical reaction for a polyene with an odd number of π bonds

will result in conrotatory rotation of p orbitals.

Which of the following options correctly describe a [4 + 2] cycloaddition reaction, such as the Diels-Alder reaction? Select all that apply.

-The ground state of both reactants allows bonding interactions between the p orbitals relevant to the reaction.
-A [4 + 2] cycloaddition involves an odd number of π bonds.

In a [2 + 2] cycloaddition

a total of 4 p orbital electrons are involved.

In a [4 + 2] cycloaddition reaction

a total of 6 p orbital electrons electrons are involved.

An odd number of π bonds is involved in

a [4 + 2] cycloaddition reaction.

An even number of π bonds is involved in

a [2 + 2] cycloaddition reaction.

Photochemical cycloadditions involving an even number of π bonds, such as ______ cycloadditions, proceed by a ______ pathway.

-[2 + 2]
-suprafacial

A suprafacial arrangement leads to a reaction when

like phases of the p orbitals of both reactants are on the same side of the π system.

An antarafacial arrangement leads to a reaction when

not all the p orbitals on the same side of the π system have the same phase.

No twisting occurs to allow like phases to interact in

a suprafacial mode of reaction.

Twisting occurs to allow like phases to interact in

an antarafacial mode of reaction.

A reaction involving an even number of π bonds

follows an antarafacial pathway under thermal conditions.

A reaction involving an odd number of π bonds

follows a suprafacial pathway under thermal conditions.

Under photochemical conditions, a suprafacial pathway is followed

for a reaction involving an even number of π bonds.

Under photochemical conditions, an antarafacial pathway is followed

for a reaction involving an odd number of π bonds.

A [4 + 2] cycloaddition reaction involves a ______ reaction under thermal conditions.

suprafacial

Which of the following options correctly describe the outcome of a sigmatropic rearrangement? Select all that apply.

- The position of a σ bond changes in this reaction.
- The total number of π bonds remains constant.

suprafacial rearrangement

A new σ bond forms on the same side of the π system as the original σ bond.

antarafacial rearrangement

A new σ bond forms on the opposite side of the π system to the original σ bond.

Which of the following options correctly describe a [2 + 2] cycloaddition reaction? Select all that apply.

- This type of reaction is always photochemically mediated.
- One reactant must be in the excited state to allow for a suprafacial reaction.

According to the Woodward-Hoffmann rules, a reaction involving an even number of π bonds will proceed via a suprafacial pathway under ______ conditions, whereas an antarafacial pathway will be followed under ______ conditions.

- photochemical
- thermal

Which of the following options correctly describe a sigmatropic rearrangement? Select all that apply.

- The total number of π bonds remains unchanged in this type of reaction.
- This type of reaction is intramolecular.

In an antarafacial sigmatropic rearrangement, the new σ bond is on the ______ side of the π system as/to the original σ bond, whereas in a suprafacial rearrangement the new σ bond will be on the ______ side of the π system.

- Opposite
- Same

When counting the electrons involved in a sigmatropic rearrangement, we count ______ the electrons in the π bonds that rearrange, as well as the electrons in the σ bond that ______.

- All
- BReaks

Both the Claisen and the Cope rearrangements are examples of a _____ sigmatropic rearrangement.

[3,3]

What type of compound is both the reactant and the product in a Cope rearrangement?

A 1,5-diene

The pathway and reaction conditions for a sigmatropic rearrangement are determined from the number of electron pairs involved. Which electrons are counted in order to make this determination?

- The electrons in all π bonds that rearrange
- The electrons in the σ bond that is broken

Which of the following options correctly describe the oxy-Cope rearrangement? Select all that apply.

- The rearrangement is followed by keto-enol tautomerization.
- The starting material is an unsaturated alcohol.
- High reaction yields can be obtained by deprotonating the starting material with strong base.

Cope rearrangement

isomerization of a 1,5-diene

Claisen rearrangement

conversion of an unsaturated ether to a γ,δ-unsaturated carbonyl compound

Which of the following options correctly describe the Cope rearrangement? Select all that apply.

- This reaction involves the isomerization of a 1,5-diene.
- This is a [3,3] sigmatropic rearrangement.
- This reaction occurs via a suprafacial pathway.