Chem 332: Advanced Organic Chemistry Exam
3 – Version A Questions And Answers With
Rationales/ Graded A+/2026 Update
Section I: Pericyclic Reactions (Questions 1–20)
1. The [4+2] cycloaddition of 1,3-butadiene and ethylene is classified as:
a) Thermally allowed, suprafacial/suprafacial under Hückel rules
b) Thermally forbidden, suprafacial/antarafacial
c) Photochemically allowed, suprafacial/suprafacial
d) Thermally allowed, antarafacial/antarafacial
Answer: a) Thermally allowed, suprafacial/suprafacial under Hückel rules
*Rationale: For thermal pericyclic reactions, the Hückel rule (4n+2 π electrons) applies. This is a 4π
(diene) + 2π (dienophile) system, totaling 6π electrons (n=1). The reaction proceeds suprafacially on
both components.*
2. Which of the following is the correct product of the thermal electrocyclic ring closure of (2E,4Z,6E)-
octatriene?
a) 1,3-cyclohexadiene with a methyl group
b) 1,3-cyclohexadiene with two methyl groups (disrotatory)
c) 1,3-cyclooctadiene
d) 1,3,5-cyclooctatriene
Answer: b) 1,3-cyclohexadiene with two methyl groups (disrotatory)
*Rationale: This is a 4n system (6π electrons, n=1.5? Actually 6π = 4(1)+2, so it's 4n+2). Wait correction:
6π is 4n+2. For thermal 4n+2 electrocyclic reactions, the rotation is conrotatory. However, the
stereochemistry depends on the substituents. The given triene is conjugated. Under thermal conditions
for 6π electrons, the rotation is conrotatory, yielding a cis-disubstituted product. The correct Answer
reflects the stereochemistry of the methyl groups based on the terminal substituents.*
,3. The [1,5] sigmatropic shift of hydrogen in (1Z,3E)-1,3-pentadiene is:
a) Thermally allowed, suprafacial
b) Thermally forbidden, suprafacial
c) Photochemically allowed, antarafacial
d) Thermally allowed, antarafacial
Answer: a) Thermally allowed, suprafacial
*Rationale: [1,5] sigmatropic shifts involve a suprafacial migration of hydrogen. This is a 4n+2 system (6
electrons in the transition state: 2 from the C-H bond and 4 from the π system), making it thermally
allowed.*
4. Which pericyclic reaction is used in the biosynthesis of vitamin D from 7-dehydrocholesterol?
a) Diels-Alder
b) Claisen rearrangement
c) Electrocyclic ring opening
d) [2,3] Sigmatropic rearrangement
Answer: c) Electrocyclic ring opening
*Rationale: The photochemical electrocyclic ring opening of the B-ring of 7-dehydrocholesterol (a 5,7-
diene) is the key step in vitamin D biosynthesis, activated by UV light.*
5. In the Diels-Alder reaction, the presence of an electron-withdrawing group on the dienophile and
an electron-donating group on the diene:
a) Decreases the reaction rate due to steric hindrance
b) Increases the reaction rate by lowering the LUMO of dienophile and raising the HOMO of diene
c) Has no effect on the rate
d) Causes the reaction to proceed via a biradical intermediate
Answer: b) Increases the reaction rate by lowering the LUMO of dienophile and raising the HOMO
of diene
Rationale: Normal electron-demand Diels-Alder reactions are accelerated by HOMO(diene)-
LUMO(dienophile) interactions. EWGs lower the LUMO of the dienophile, while EDGs raise the HOMO of
the diene, minimizing the energy gap.
6. What is the stereochemistry of the product when maleic anhydride (cis-dienophile) reacts with 1,3-
butadiene?
a) Racemic mixture of enantiomers
, b) Meso compound
c) Endo product with cis substituents
d) Exo product with trans substituents
Answer: c) Endo product with cis substituents
Rationale: The Diels-Alder reaction is stereospecific; cis-dienophiles yield cis-substituted cyclohexenes.
The endo rule favors the product where the dienophile’s substituents are oriented under the diene’s π
system, though "cis" refers to the relative stereochemistry on the ring.
7. Which of the following is a 6π electron system that would undergo a thermally allowed conrotatory
electrocyclic ring closure?
a) 1,3,5-hexatriene
b) 1,3-butadiene
c) Allyl cation
d) Cyclobutadiene
Answer: a) 1,3,5-hexatriene
*Rationale: 1,3,5-hexatriene has 6π electrons (4n+2). Under thermal conditions, 4n+2 systems undergo
disrotatory ring closure, not conrotatory. Correction: For 4n+2 (thermal), it is disrotatory. For 4n
(thermal), it is conrotatory. The question asks for conrotatory, which would require a 4n system (e.g.,
butadiene). However, among the options, only butadiene (4π) is thermally conrotatory. The correct
Answer should be butadiene, but the question says "6π electron system", which is a trap. Since the
prompt expects a correct Answer, I will clarify: For thermal conrotatory, you need 4n electrons. None
of the 6π systems undergo thermal conrotatory. The question might have a typo. If forced: 1,3,5-
hexatriene is thermally disrotatory. Butadiene is thermally conrotatory. I'll correct the Answer to
reflect the rule.*
*Corrected Answer for Q7: 1,3-butadiene (though not listed as 6π). Since 1,3,5-hexatriene is listed,
the correct rationale is that it is 6π and thermally disrotatory, not conrotatory. To align with standard
exams: 1,3,5-hexatriene undergoes thermal disrotatory ring closure.*
8. The Cope rearrangement of 1,5-hexadiene is an example of a:
a) [3,3]-sigmatropic rearrangement
b) [2,3]-sigmatropic rearrangement
c) [1,5]-sigmatropic rearrangement
d) Electrocyclic reaction
3 – Version A Questions And Answers With
Rationales/ Graded A+/2026 Update
Section I: Pericyclic Reactions (Questions 1–20)
1. The [4+2] cycloaddition of 1,3-butadiene and ethylene is classified as:
a) Thermally allowed, suprafacial/suprafacial under Hückel rules
b) Thermally forbidden, suprafacial/antarafacial
c) Photochemically allowed, suprafacial/suprafacial
d) Thermally allowed, antarafacial/antarafacial
Answer: a) Thermally allowed, suprafacial/suprafacial under Hückel rules
*Rationale: For thermal pericyclic reactions, the Hückel rule (4n+2 π electrons) applies. This is a 4π
(diene) + 2π (dienophile) system, totaling 6π electrons (n=1). The reaction proceeds suprafacially on
both components.*
2. Which of the following is the correct product of the thermal electrocyclic ring closure of (2E,4Z,6E)-
octatriene?
a) 1,3-cyclohexadiene with a methyl group
b) 1,3-cyclohexadiene with two methyl groups (disrotatory)
c) 1,3-cyclooctadiene
d) 1,3,5-cyclooctatriene
Answer: b) 1,3-cyclohexadiene with two methyl groups (disrotatory)
*Rationale: This is a 4n system (6π electrons, n=1.5? Actually 6π = 4(1)+2, so it's 4n+2). Wait correction:
6π is 4n+2. For thermal 4n+2 electrocyclic reactions, the rotation is conrotatory. However, the
stereochemistry depends on the substituents. The given triene is conjugated. Under thermal conditions
for 6π electrons, the rotation is conrotatory, yielding a cis-disubstituted product. The correct Answer
reflects the stereochemistry of the methyl groups based on the terminal substituents.*
,3. The [1,5] sigmatropic shift of hydrogen in (1Z,3E)-1,3-pentadiene is:
a) Thermally allowed, suprafacial
b) Thermally forbidden, suprafacial
c) Photochemically allowed, antarafacial
d) Thermally allowed, antarafacial
Answer: a) Thermally allowed, suprafacial
*Rationale: [1,5] sigmatropic shifts involve a suprafacial migration of hydrogen. This is a 4n+2 system (6
electrons in the transition state: 2 from the C-H bond and 4 from the π system), making it thermally
allowed.*
4. Which pericyclic reaction is used in the biosynthesis of vitamin D from 7-dehydrocholesterol?
a) Diels-Alder
b) Claisen rearrangement
c) Electrocyclic ring opening
d) [2,3] Sigmatropic rearrangement
Answer: c) Electrocyclic ring opening
*Rationale: The photochemical electrocyclic ring opening of the B-ring of 7-dehydrocholesterol (a 5,7-
diene) is the key step in vitamin D biosynthesis, activated by UV light.*
5. In the Diels-Alder reaction, the presence of an electron-withdrawing group on the dienophile and
an electron-donating group on the diene:
a) Decreases the reaction rate due to steric hindrance
b) Increases the reaction rate by lowering the LUMO of dienophile and raising the HOMO of diene
c) Has no effect on the rate
d) Causes the reaction to proceed via a biradical intermediate
Answer: b) Increases the reaction rate by lowering the LUMO of dienophile and raising the HOMO
of diene
Rationale: Normal electron-demand Diels-Alder reactions are accelerated by HOMO(diene)-
LUMO(dienophile) interactions. EWGs lower the LUMO of the dienophile, while EDGs raise the HOMO of
the diene, minimizing the energy gap.
6. What is the stereochemistry of the product when maleic anhydride (cis-dienophile) reacts with 1,3-
butadiene?
a) Racemic mixture of enantiomers
, b) Meso compound
c) Endo product with cis substituents
d) Exo product with trans substituents
Answer: c) Endo product with cis substituents
Rationale: The Diels-Alder reaction is stereospecific; cis-dienophiles yield cis-substituted cyclohexenes.
The endo rule favors the product where the dienophile’s substituents are oriented under the diene’s π
system, though "cis" refers to the relative stereochemistry on the ring.
7. Which of the following is a 6π electron system that would undergo a thermally allowed conrotatory
electrocyclic ring closure?
a) 1,3,5-hexatriene
b) 1,3-butadiene
c) Allyl cation
d) Cyclobutadiene
Answer: a) 1,3,5-hexatriene
*Rationale: 1,3,5-hexatriene has 6π electrons (4n+2). Under thermal conditions, 4n+2 systems undergo
disrotatory ring closure, not conrotatory. Correction: For 4n+2 (thermal), it is disrotatory. For 4n
(thermal), it is conrotatory. The question asks for conrotatory, which would require a 4n system (e.g.,
butadiene). However, among the options, only butadiene (4π) is thermally conrotatory. The correct
Answer should be butadiene, but the question says "6π electron system", which is a trap. Since the
prompt expects a correct Answer, I will clarify: For thermal conrotatory, you need 4n electrons. None
of the 6π systems undergo thermal conrotatory. The question might have a typo. If forced: 1,3,5-
hexatriene is thermally disrotatory. Butadiene is thermally conrotatory. I'll correct the Answer to
reflect the rule.*
*Corrected Answer for Q7: 1,3-butadiene (though not listed as 6π). Since 1,3,5-hexatriene is listed,
the correct rationale is that it is 6π and thermally disrotatory, not conrotatory. To align with standard
exams: 1,3,5-hexatriene undergoes thermal disrotatory ring closure.*
8. The Cope rearrangement of 1,5-hexadiene is an example of a:
a) [3,3]-sigmatropic rearrangement
b) [2,3]-sigmatropic rearrangement
c) [1,5]-sigmatropic rearrangement
d) Electrocyclic reaction
