Pericyclic Reactions

Question 1

What 2 things define a pericyclic reaction?

Answer 1

1) Concerted Process - bonds are formed/broken simultaneously
2) Bond orbitals overlap in a continuous cycle

Question 2

When determining orbital phases of a molecule, what does n define?

Answer 2

n = the number of p-orbitals in a molecule

Question 3

For a tri-ene, what is n, and how many electrons does the molecule have?

Answer 3

n = the number of p-orbitals, hence n = 6.
The tri-ene has 6 electrons

Question 4

When determining orbital coefficients, how many dots are drawn on each line?

Answer 4

n + 2, when n is the number of p-orbitals

Question 5

Aside from the orbitals, what else is drawn when determining orbital coefficients?

Answer 5

A sine wave

Question 6

What happens at each node when determining orbital coefficients?

Answer 6

The phase of the orbital changes

Question 7

If a polyalkene has an EWG, what is the method when determining its orbital coefficients?

Answer 7

The average of the unsubstituted polyalkene and the polyalkene with the “ultimate EWG” are taken

Question 8

What is the “ultimate” EWG?

Answer 8

CH2 +

Question 9

What is the “ultimate” EDG?

Answer 9

CH2 -

Question 10

True or False? When determining the orbital coefficients for the HOMO and LUMO of a substituted polyalkene, all p-orbitals must be taken into consideration?

Answer 10

False, only the p-orbitals involved during the reaction should be taken into consideration (e.g. not the lone pair on a ultimate EWG fragment –> only the alkene fragment)

Question 11

How does a conjugated alkene effect the relative energies of molecular orbitals?

Answer 11

The longer the conjugation length, the smaller the HOMO-LUMO energy gap becomes

Question 12

How does an EDG affect the relative energies of molecular orbitals?

Answer 12

EDG’s raise the energy of the orbitals (electrons are easier to remove due to increased electron density)

Question 13

How does an EWG affect the relative energies of molecular orbitals?

Answer 13

EWG’s lower the energy of the orbitals (electrons are more difficult to remove due to decreased electron density)

Question 14

Does a positive charge count as a p-orbital?

Answer 14

YES!

Question 15

What determines if a reaction can occur in the frontier molecular orbital approach?

Answer 15

If the phases of the overlapping orbitals match

Question 16

For Woodward-Hoffman rules, what determines if a THERMAL pericyclic reaction can occur?

Answer 16

If there are an ODD number of qualifying components

Question 17

For Woodward-Hoffman rules, what determines if a PHOTOCHEMICAL pericyclic reaction can occur?

Answer 17

IF there are an EVEN number of qualifying components

Question 18

For Woodward-Hoffman rules, what determines if a suprafacial components qualifies?

Answer 18

If the reacting orbitals have 2,6,10,14,… electrons

Question 19

For Woodward-Hoffman rules, what determines if an antarafacial component qualifies?

Answer 19

IF the reacting orbitals have 4,8,12,16,… electrons

Question 20

Define a component

Answer 20

A continuous group of connected atoms with the SAME TYPE of orbital

Question 21

What are the three types of orbitals that are considered for Woodward-Hoffman rules?

Answer 21

pi, sigma and omega (lone pair)

Question 22

Using Woodward-Hoffman rules, what are the 5 steps in identifying whether a reaction is thermally or photochemically allow?

Answer 22

1) Draw a curly arrow mechanism
2) Identify the components
3) Draw a 3D shape to identify the overlapping components
4) Assign components are supra- or antarafacial
5) Identify the number of qualifying components

Question 23

What are the 4 types of pericyclic reaction?

Answer 23

1) Cycloadditons
2) Electrocylic reactions
3) Sigmatropic rearrangements
4) Group transfer reactions

Question 24

What type of pericyclic reaction is Diels-Alder?

Answer 24

A Cycloaddition

Question 25

What are the two molecules in a Diels-Alder reaction?

Answer 25

A dieneophile (alkene) and a diene

Question 26

Why is the least stable endo-isomer sometimes the major product of a reaction?

Answer 26

Weak secondary reactions are present only with the endo product

Question 27

Why might the more stable exo-isomer be the main product in a reaction?

Answer 27

If the reaction is reversible

Question 28

In a normal electron demand Diels-Alder reaction, which molecule provides the HOMO and which the LUMO?

Answer 28

HOMO - Diene
LUMO - Dieneophile (alkene)

Question 29

In an inverse electron demand Diels-Alder reaction, which molecules provides the HOMO and which the LUMO?

Answer 29

HOMO - Dieneophile (alkene)
LUMO - Diene

Question 30

In a normal electron demand Diels-Alder reaction, how can the reaction be sped up by substituents and why?

Answer 30

An EDG on the diene and an EWG on the dieneophile (alkene) would decrease the HOMO-LUMO gap.

Question 31

In an inverse electron demand Diels-Alder reaction, how can the reaction be sped up by substituents and why?

Answer 31

An EWG on the diene and an EDG on the dieneophile (alkene) would decreases the HOMO-LUMO gap.

Question 32

How can Lewis Acids be used in pericyclic reactions?

Answer 32

Lewis Acids (like FeCl3 or AlCl3) can coordinate to an EWG to decrease electron density resulting in a decrease in the HOMO-LUMO gap.

Question 33

In terms of the size of orbital coefficients, how do substituents effect the regiochemistry of a reaction?

Answer 33

Large orbital coefficients prefer to overlap with other large orbital coefficients, and likewise small with small. Drawing resonance forms show that the ionic charges align.

Question 34

If the starting materials are not chiral, and there are no enantiomerically enriched compounds in the reaction mixture, then what will the products be?

Answer 34

The products will be racemic

Question 35

How can enantionselectivity be induced in a Diels-Alder reaction?

Answer 35

By using an enantiomerically enriched compound, like a chiral Lewis acid catalyst

Question 36

How do the reacting orbitals in a photochemical reaction differ to those in a thermal reaction?

Answer 36

In a thermal reaction, a HOMO and a LUMO orbital overlap. In a photochemical reaction a HOMO and excited SUMO orbital overlap (or LUMO and excited SUMO*)

Question 37

What type of reagents are used in 1,3-dipolar cycloaddtions?

Answer 37

Nitrones or ozone
X=Y(+)-O(-)

Question 38

Define a cheletropic reaction

Answer 38

Two or more sigma bonds are broken

Question 39

What is an electrocylcisation reaction?

Answer 39

A ring formed from one molecule

Question 40

Define conrotatory and disrotatory movement

Answer 40

Conrotatory: Both orbitals rotate in the same direction
Disrotatory: Orbitals rotate in different directions

Question 41

Why is stereochemistry particularly important to consider during electrocylicastion reactions?

Answer 41

Con- or disrotartoy movement will move substituents in different directions resulting in different product stereochemistry

Question 42

Why is equilibria rarely achieved in an electrocylication reaction?

Answer 42

Equilibria means the the forward and backward reaction must both have fast rates - which is rare under given condtions

Question 43

Define a sigmatropic rearrangment

Answer 43

Migration of a sigma bond from one end of a pi-system to the other

Question 44

What is the nomenclature in a sigmatropic rearrangement?

Answer 44

[m,n] where m is less than or equal to n

Question 45

Match the SM: 1,5-diene, ether, carbonyl to the reaction: cope, oxy-cope and claisen

Answer 45

Cope: 1,5-diene
Oxy-cope: Carbonyl
Claisen: Ether

Question 46

What is the best way to visualise sigmatropic rearrangments?

Answer 46

By drawing a chair conformation

Question 47

Define a hydride shift?

Answer 47

The movement of a H atom and its electrons to a different position in a molecule