Thermal electrocyclic ring closure of (2Z, 4Z, 6Z)-2,4,6-octatriene yielding products via conrotatory and disrotatory cyclizations. Concept introduction: Conrotatory and disrotatory is based on the rotation at each end of the molecule. In conrotatory mode, both atomic orbitals of the end groups turn in the same direction ,such as both atomic orbitals rotating clockwise or counter-clockwise. In disrotatory mode, the atomic orbitals of the end groups turn in opposite directions ,one atomic orbital turns clockwise and the other counter-clockwise. The cis/trans geometry of the final product is directly decided by the difference between conrotation and disrotation. Whether a particular reaction is conrotatory or disrotatory can be accomplished by examining the molecular orbitals of each molecule and through a set of rules. Only two pieces of information are required to determine conrotation or disrotation using the set of rules: how many electrons are in the pi-system and whether the reaction is induced by heat or by light. This set of rules can also be derived from an analysis of the molecular orbitals for predicting the stereochemistry of electrocyclic reactions.

Question

When 3,3-dimethylbut-1-ene is treated with HBr alone, the major product is 2-bromo-2,3- dimethylbutane. When the same alkene is treated with HBr and peroxide, the sole product is 1-bromo-3,3-dimethylbutane. Explain these results by referring to the mechanisms.

Answer 1

Question

Chapter 30.3, Problem 2P

Interpretation Introduction

Interpretation:

Thermal electrocyclic ring closure of (2Z, 4Z, 6Z)-2,4,6-octatriene yielding products via conrotatory and disrotatory cyclizations.

Concept introduction:

Conrotatory and disrotatory is based on the rotation at each end of the molecule. In conrotatory mode, both atomic orbitals of the end groups turn in the same direction ,such as both atomic orbitals rotating clockwise or counter-clockwise. In disrotatory mode, the atomic orbitals of the end groups turn in opposite directions ,one atomic orbital turns clockwise and the other counter-clockwise. The cis/trans geometry of the final product is directly decided by the difference between conrotation and disrotation.

Whether a particular reaction is conrotatory or disrotatory can be accomplished by examining the molecular orbitals of each molecule and through a set of rules. Only two pieces of information are required to determine conrotation or disrotation using the set of rules: how many electrons are in the pi-system and whether the reaction is induced by heat or by light. This set of rules can also be derived from an analysis of the molecular orbitals for predicting the stereochemistry of electrocyclic reactions.

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