Textbook Question
a. Under thermal conditions, will ring closure of (2E,4Z,6Z,8E)-2,4,6,8-decatetraene be conrotatory or disrotatory?
b. Will the product have the cis or the trans configuration?
16. Conjugated Systems
Cumulative Electrocyclic Problems
Problem 38c
Textbook Question
Given the conditions, would you expect conrotatory or disrotatory ring closing/opening? Justify this on the basis of the molecular orbital picture.
(c) 
Verified step by step guidance1
Identify the type of pericyclic reaction involved. In this case, it is a ring-closing reaction of a hexatriene system, which is a type of electrocyclic reaction.
Determine the number of π-electrons involved in the system. The hexatriene system has 6 π-electrons.
Apply the Woodward-Hoffmann rules for electrocyclic reactions. For a thermal reaction involving 4n π-electrons (where n is an integer), the reaction proceeds via a conrotatory mechanism.
Visualize the molecular orbitals involved. In a conrotatory process, the terminal p orbitals rotate in the same direction, either both clockwise or both counterclockwise, to form the new σ-bond.
Justify the mechanism based on the symmetry of the highest occupied molecular orbital (HOMO). For 6 π-electrons, the HOMO is symmetric, which supports a conrotatory closure under thermal conditions.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Conrotatory and Disrotatory Motion
Conrotatory and disrotatory motions describe the stereochemical outcomes of electrocyclic reactions. In conrotatory motion, the substituents at the termini of the reacting pi system rotate in the same direction, while in disrotatory motion, they rotate in opposite directions. The type of motion depends on the number of pi electrons and whether the reaction is thermal or photochemical.
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Woodward-Hoffmann Rules
The Woodward-Hoffmann rules predict the stereochemistry of pericyclic reactions, including electrocyclic reactions, based on the conservation of orbital symmetry. For thermal reactions, a system with 4n pi electrons undergoes conrotatory closure, while a system with 4n+2 pi electrons undergoes disrotatory closure. These rules help determine the stereochemical outcome of the reaction.
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Molecular Orbital Theory
Molecular orbital (MO) theory explains the behavior of electrons in molecules in terms of orbitals that extend over the entire molecule. In electrocyclic reactions, the symmetry and phase of the frontier molecular orbitals (HOMO and LUMO) determine the allowed stereochemical pathway. The interaction of these orbitals during the reaction dictates whether the process is conrotatory or disrotatory.
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