Skip to main content
Ch. 28 - Pericyclic Reactions
Bruice - Organic Chemistry 8th Edition
Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook
All textbooksBruice 8th EditionCh. 28 - Pericyclic ReactionsProblem 39
Chapter 25, Problem 39

Explain why two different products are formed from disrotatory ring closure of (2E,4Z,6Z)-octatriene, but only one product is formed from disrotatory ring closure of (2E,4Z,6E)-octatriene.

Verified step by step guidance
1
Understand the concept of disrotatory ring closure: In a disrotatory ring closure, the terminal p orbitals of the conjugated system rotate in opposite directions (one clockwise and the other counterclockwise) to form a new sigma bond. The stereochemistry of the substituents on the terminal carbons determines the stereochemistry of the product.
Analyze the stereochemistry of (2E,4Z,6Z)-octatriene: The (2E,4Z,6Z) configuration means that the substituents on C2 and C6 are on opposite sides of the double bonds. This configuration allows for two possible stereochemical outcomes during the disrotatory closure, depending on the direction of rotation of the terminal p orbitals.
Explain why two products are formed for (2E,4Z,6Z)-octatriene: During the disrotatory closure, the substituents on C2 and C6 can either both end up on the same side of the newly formed ring (cis) or on opposite sides (trans). This is because the initial stereochemistry allows for two distinct ways the p orbitals can rotate to form the ring, leading to two different stereoisomers.
Analyze the stereochemistry of (2E,4Z,6E)-octatriene: The (2E,4Z,6E) configuration means that the substituents on C2 and C6 are on the same side of the double bonds. This configuration restricts the possible outcomes of the disrotatory closure, as only one stereochemical arrangement is possible due to the initial geometry of the molecule.
Explain why only one product is formed for (2E,4Z,6E)-octatriene: In this case, the disrotatory closure leads to a single stereoisomer because the substituents on C2 and C6 are already aligned in a way that only one stereochemical outcome is possible. The geometry of the molecule does not allow for the formation of a second stereoisomer.

Verified video answer for a similar problem:

This video solution was recommended by our tutors as helpful for the problem above.
Was this helpful?

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Disrotatory and Conrotatory Mechanisms

Disrotatory and conrotatory mechanisms refer to the two types of stereochemical outcomes that can occur during cyclization reactions involving conjugated systems. In disrotatory closure, the substituents on the ends of the conjugated diene rotate in opposite directions, leading to the formation of stereoisomers. Understanding these mechanisms is crucial for predicting the products of ring closures in organic reactions.
Recommended video:
Guided course
04:32
General Mechanism

Stereochemistry of Dienes

The stereochemistry of dienes, particularly in terms of their geometric isomers (E/Z configurations), plays a significant role in determining the products of cyclization reactions. The specific arrangement of substituents around the double bonds influences the spatial orientation during the ring closure, which can lead to different stereochemical outcomes, as seen in the case of (2E,4Z,6Z)-octatriene versus (2E,4Z,6E)-octatriene.
Recommended video:
1:38
Polymer Stereochemistry Concept 1

Product Stability and Selectivity

The stability of the products formed during a reaction can greatly influence the selectivity of the reaction pathway. In the case of (2E,4Z,6Z)-octatriene, the formation of two different products suggests that both are relatively stable, allowing for their coexistence. Conversely, the formation of only one product from (2E,4Z,6E)-octatriene indicates that this product is more stable or favored, leading to a selective reaction outcome.
Recommended video:
Guided course
05:58
Determining Carbocation Stability
Related Practice
Textbook Question

cis-3,4-Dimethylcyclobutene undergoes thermal ring opening to form the two products shown. One of the products is formed in 99% yield, the other in 1% yield. Which is which?

123
views
Textbook Question

Propose a mechanism for the following reaction:

1094
views
Textbook Question

Draw the product of each of the following sigmatropic rearrangements:

d.

1139
views
Textbook Question

b. What would be the product if trans-2-butene were used instead of ethene?

989
views
Textbook Question

Draw the product of each of the following sigmatropic rearrangements:

c.

1127
views