← Back

Chapter 30: Orbitals and Organic Chemistry: Pericyclic Reactions

Every problem in the chapter beside the book’s answer, a chapter at a time as you scroll.

Chapter 30: Orbitals and Organic Chemistry: Pericyclic Reactions

12 problems · 8 figures

30-1 §30.1 source ↗

Look at Figure 30.2, and tell which molecular orbital is the HOMO and which is the LUMO for both ground and excited states of ethylene and 1,3-butadiene.
Figure 30.2 in the book ↗
Pi molecular orbitals of (a) ethylene and (b) 1,3-butadiene.

Answer

Ethylene: ψ1 is the HOMO and ψ2* is the LUMO in the ground state; ψ2* is the HOMO and there is no LUMO in the excited state. 1,3-Butadiene: ψ2 is the HOMO and ψ3* is the LUMO in the ground state; ψ3* is the HOMO and ψ4* is the LUMO in the excited state.

30-2 §30.3 source ↗

Draw the products you would expect from conrotatory and disrotatory cyclizations of (2Z,4Z,6Z)-2,4,6-octatriene. Which of the two paths would you expect the thermal reaction to follow?

Answer

Disrotatory: cis-5,6-dimethyl-1,3-cyclohexadiene;
conrotatory: trans-5,6-dimethyl-1,3-cyclohexadiene. Disrotatory closure occurs.

30-3 §30.3 source ↗

trans-3,4-Dimethylcyclobutene can open by two conrotatory paths to give either (2E,4E)-2,4-hexadiene or (2Z,4Z)-2,4-hexadiene. Explain why both products are symmetry-allowed, and then account for the fact that only the 2E,4E isomer is obtained in practice.

Answer

The more stable of two allowed products is formed.

30-4 §30.4 source ↗

What product would you expect to obtain from the photochemical cyclization of (2E,4Z,6E)-2,4,6-octatriene? Of (2E,4Z,6Z)-2,4,6-octatriene?

Answer

trans-5,6-Dimethyl-1,3-cyclohexadiene; cis-5,6-dimethyl-1,3-cyclohexadiene

30-5 §30.6 source ↗

What stereochemistry would you expect for the product of the Diels–Alder reaction between (2E,4E)-2,4-hexadiene and ethylene? What stereochemistry would you expect if (2E,4Z)-2,4-hexadiene were used instead?

Answer

cis-3,6-Dimethylcyclohexene; trans-3,6-dimethylcyclohexene

30-6 §30.6 source ↗

1,3-Cyclopentadiene reacts with cycloheptatrienone to give the product shown. Tell what kind of reaction is involved, and explain the observed result. Is the reaction suprafacial or antarafacial?

Answer

A [6 + 4] suprafacial cycloaddition

30-7 §30.7 source ↗

Classify the following sigmatropic reaction by order [x,y], and tell whether it will proceed with suprafacial or antarafacial stereochemistry:

Answer

An antarafacial [1,7] sigmatropic rearrangement

30-8 §30.8 source ↗

Draw a curved-arrow mechanism for the Cope arrangement just shown.

Answer

30-9 §30.8 source ↗

Propose a mechanism to account for the fact that heating 1-deuterioindene scrambles the isotope label to all three positions on the five-membered ring.

Answer

A series of [1,5] hydrogen shifts occur.

30-10 §30.8 source ↗

What product would you expect from Claisen rearrangement of 2-butenyl phenyl ether?

Answer

o-(1-Methylallyl)phenol

30-11 §30.8 source ↗

When a 2,6-disubstituted allyl phenyl ether is heated in an attempted Claisen rearrangement, migration occurs to give the p-allyl product as the result of two sequential pericyclic reactions. Explain.

Answer

Claisen rearrangement is followed by a Cope rearrangement.

30-12 §30.9 source ↗

Predict the stereochemistry of the following pericyclic reactions:
(a)
The thermal cyclization of a conjugated tetraene
(b)
The photochemical cyclization of a conjugated tetraene
(c)
A photochemical [4 + 4] cycloaddition
(d)
A thermal [2 + 6] cycloaddition
(e)
A photochemical [3,5] sigmatropic rearrangement

Answer (5 parts)

(a)
Conrotatory
(b)
Disrotatory
(c)
Suprafacial
(d)
Antarafacial
(e)
Suprafacial

Loading chapter 31…

Problems and answers are adapted from Organic Chemistry: A Tenth Edition by John McMurry, OpenStax, and are used under the CC BY-NC-SA 4.0 licence. Access for free at https://openstax.org/books/organic-chemistry/pages/1-why-this-chapter. Illustrations: Copyright Rice University, OpenStax, under CC BY-NC-SA 4.0 license. This adaptation is released under the same licence, is non-commercial, and is neither affiliated with nor endorsed by OpenStax or Rice University.