Deck 26: Orbitals and Organic Chemistry: Pericyclic Reactionse

Question 1

Exhibit 30-5
To answer the following question(s), consider the reaction below:
Refer to Exhibit 30-5. Propose a mechanism for the reaction that fully accounts for the formation of both products.

Answer 1

This reaction occurs with suprafacial stereochemistry that would be expected for a photochemical [2 + 2] cycloaddition. However, suprafacial cycloaddition can occur in two different ways⎯with the methyl groups of both alkenes on the same side or on opposite sides.

Question 2

To answer the following question(s), consider the π molecular orbitals of a conjugated diene shown below:
Refer to Exhibit 30-1. Which molecular orbital of a ground-state conjugated diene is the highest occupied molecular orbital (HOMO)?

A) ψ₁
B) ψ₂
C) ψ₃*
D) ψ₄*

Answer 2

ψ₂

Question 3

Exhibit 30-3
To answer the following question(s), consider the reaction below:
Refer to Exhibit 30-3. Explain the stereochemistry of compound Y.

Answer 3

Thermal electrocyclic opening of the cis-substituted cyclobutene yields (1E, 3Z, 5Z)-cyclodecatriene (X). Symmetry-allowed thermal cyclization of the cyclodecatriene occurs in a disrotatory fashion to yield the observed compound Y.

Question 4

Exhibit 30-2
Consider the reaction below to answer the following question(s):
Refer to Exhibit 30-2. Has this reaction taken place in a conrotatory manner or in a disrotatory manner?

Question 5

To answer the following question(s), consider the π molecular orbitals of a conjugated diene shown below:
Refer to Exhibit 30-1. Which molecular orbital of an excited-state conjugated diene is the lowest unoccupied molecular orbital (LUMO)?

A) ψ₁
B) ψ₂
C) ψ₃*
D) ψ₄*

Question 6

Exhibit 30-6
Consider the reaction sequence below to answer the following question(s):
Refer to Exhibit 30-6. Explain the stereochemistry of product B.

Question 7

Exhibit 30-5
To answer the following question(s), consider the reaction below:
Refer to Exhibit 30-5. How many pairs of electrons are involved in this pericyclic reaction?

A) two
B) four
C) eight
D) sixteen

Question 8

To answer the following question(s), consider the π molecular orbitals of a conjugated diene shown below:
Draw the product(s) you would expect to obtain from photochemical cyclization of (2Z, 4E)-hexadiene.

Question 9

Exhibit 30-4
Consider the reaction below to answer the following question(s):
Refer to Exhibit 30-4. Would you expect this reaction to occur under photochemical or thermal conditions?

Question 10

To answer the following question(s), consider the π molecular orbitals of a conjugated diene shown below:
Refer to Exhibit 30-1. Which molecular orbital of a conjugated diene contains two nodes between nuclei?

A) ψ₁
B) ψ₂
C) ψ₃*
D) ψ₄*

Question 11

Exhibit 30-2
Consider the reaction below to answer the following question(s):
Refer to Exhibit 30-2. Under what conditions, thermal or photochemical, would you carry out this reaction? Explain.

Question 12

Exhibit 30-3
To answer the following question(s), consider the reaction below:
Refer to Exhibit 30-3. The thermal reaction which generates X is an example of:

A) an electrocyclic reaction
B) a sigmatropic rearrangement
C) a reverse cycloaddition reaction
D) a cycloaddition reaction

Question 13

Exhibit 30-4
Consider the reaction below to answer the following question(s):
Refer to Exhibit 30-4. What type of pericyclic reaction is involved in this transformation?

A) sigmatropic rearrangement
B) reverse cycloaddition reaction
C) electrocyclic reaction
D) cycloaddition reaction

Question 14

Exhibit 30-5
To answer the following question(s), consider the reaction below:
Refer to Exhibit 30-5. What type of pericyclic reaction is involved in this transformation?

A) sigmatropic rearrangement
B) reverse cycloaddition reaction
C) electrocyclic reaction
D) cycloaddition reaction

Question 15

Exhibit 30-3
To answer the following question(s), consider the reaction below:
Refer to Exhibit 30-3. Two pericyclic reactions are involved in this synthesis of compound Y. Draw the structure of intermediate X and name it using IUPAC nomenclature.

Question 16

Exhibit 30-4
Consider the reaction below to answer the following question(s):
Refer to Exhibit 30-4. How many pairs of electrons are involved in this pericyclic reaction?

A) two
B) four
C) eight
D) sixteen

Question 17

Exhibit 30-6
Consider the reaction sequence below to answer the following question(s):
Refer to Exhibit 30-6. Step 1 is an example of:

A) an electrocyclic reaction
B) a cycloaddition reaction
C) a sigmatropic rearrangement
D) a reverse cycloaddition reaction

Question 18

The mnemonic phrase "The Electrons Circle Around", TECA, assists in predicting stereochemistry of pericyclic reactions. Describe application of this mnemonic.

Question 19

Exhibit 30-4
Consider the reaction below to answer the following question(s):
Refer to Exhibit 30-4. Explain the stereochemistry of this reaction.

Question 20

There are three major classes of pericyclic reactions. Which of the following types of reactions is not an example of a pericyclic process?

A) sigmatropic rearrangements
B) cycloaddition reactions
C) annulation reactions
D) electrocyclic reactions

Question 21

Define a pericyclic reaction.

Question 22

The photochemical cycloaddition of the structure below is related to DNA mutations causing skin cancer. Draw the structure of this dimer. Atoms other than carbon and hydrogen are labeled.

Question 23

Exhibit 30-8
Classify each of the following sigmatropic reactions by order [x, y].
26. ANSWER:
[3, 3] POINTS:
1
27. ANSWER:
[1, 7] POINTS:
1
28. ANSWER:
[1, 5] POINTS:
1
Exhibit 30-9
Vitamin D₃, cholecalciferol, is synthesized under the skin by the photochemical pericyclic process shown below. Answer the following question(s) about these reactions.
Refer to Exhibit 30-9. The second step in this process is a(n):

A) sigmatropic rearrangement
B) cycloaddition reaction
C) annulation reaction
D) electrocyclic reaction

Question 24

How many π MO's does the following compound have?

A) 2
B) 4
C) 6
D) 8

Question 25

Under photochemical conditions, the following substance will undergo:

A) disrotatory cyclization.
B) conrotatory cyclization.
C) no reaction.

Question 26

In which type of pericyclic reaction is a σ bond broken and a new σ bond formed?

A) cycloaddition
B) photochemical electrocyclic
C) thermal electrocyclic
D) sigmatropic rearrangement.
E) all of the above

Question 27

Exhibit 30-8
Classify each of the following sigmatropic reactions by order [x, y].
26. ANSWER:
[3, 3] POINTS:
1
27. ANSWER:
[1, 7] POINTS:
1
28. ANSWER:
[1, 5] POINTS:
1
Exhibit 30-9
Vitamin D₃, cholecalciferol, is synthesized under the skin by the photochemical pericyclic process shown below. Answer the following question(s) about these reactions.
Refer to Exhibit 30-9. The first step in this process is a(n):

A) sigmatropic rearrangement
B) cycloaddition reaction
C) annulation reaction
D) electrocyclic reaction

Question 28

Exhibit 30-6
Consider the reaction sequence below to answer the following question(s):
Refer to Exhibit 30-6. Step 2 is an example of:

A) an electrocyclic reaction
B) a cycloaddition reaction
C) a sigmatropic rearrangement
D) a reverse cycloaddition reaction

Question 29

Exhibit 30-7
Consider the Sommelet-Hauser rearrangement below to answer the following question(s):
Refer to Exhibit 30-7. The Sommelet-Hauser rearrangement is an example of a [2, 3] sigmatropic rearrangement. How many pairs of electrons are involved in this reaction?

A) two
B) three
C) four
D) five

Question 30

Under thermal conditions, the following substance will undergo:

A) disrotatory cyclization.
B) conrotatory cyclization.
C) no reaction.

Question 31

For an odd number of electron pairs (double bonds), for cycloadditions reactions which of the following rules is correct?:

A) thermal conditions - antarafacial, photochemical conditions - suprafacial
B) thermal conditions - suprafacial, photochemical conditions - antarafacial

Question 32

Predict the product of the following reaction.

Question 33

Draw the structure of the product when the following substance reacts with maleic anhydride. Classify the reaction according to the number of π electrons that interact. Atoms other than carbon and hydrogen are labeled.

Question 34

Consider the following MO diagram for a conjugated triene. When irradiated with light of the appropriate wavelength, how many electrons will be in bonding MO's?

A) 6
B) 5
C) 4
D) 3

Question 35

Which of the following is not an electrocyclic reaction?

A)

B)

C)

D)

E) All are electrocyclic reactions.

Question 36

Exhibit 30-6
Consider the reaction sequence below to answer the following question(s):
Refer to Exhibit 30-6. Show how C is formed from B.

Question 37

For an even number of electron pairs (double bonds), which of the following rules is correct?:

A) thermal conditions - controtatory, photochemical conditions - disrotatory
B) thermal conditions - disrotatory, photochemical conditions - conrotatory

Question 38

Exhibit 30-7
Consider the Sommelet-Hauser rearrangement below to answer the following question(s):
Refer to Exhibit 30-7. Would you expect the Sommelet-Hauser rearrangement to be antarafacial or suprafacial? Explain.

Question 39

Exhibit 30-6
Consider the reaction sequence below to answer the following question(s):
Refer to Exhibit 30-6. Explain the stereochemistry of product C.

Question 40

Predict the product (including stereochemistry) when the following substance reacts under thermal conditions.

Question 41

Classify the following reaction.

A) electrocyclic reaction
B) cycloaddition
C) sigmatropic rearrangement

Question 42

Consider the reaction given below. Which of the following is correct?

A) [1,5] antarafacial
B) [3,3] antarafacial
C) [1,5] suprafacial
D) [3,5] antarafacial

Question 43

Classify the following reaction.

A) electrocyclic reaction
B) cycloaddition
C) sigmatropic rearrangement

Question 44

Classify the following reaction.

A) electrocyclic reaction
B) cycloaddition
C) sigmatropic rearrangement