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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 1
Chapter 25, Problem 1

Examine the following pericyclic reactions. For each reaction, tell whether it is an electrocyclic reaction, a cycloaddition reaction, or a sigmatropic rearrangement.
a. Illustration of pericyclic reactions with structures and arrows indicating reaction types.
b. Chemical structure transformation illustrating a pericyclic reaction with reactants on the left and product on the right.
c. Chemical structures illustrating a pericyclic reaction with reactants and products, including a heat symbol.
d. Chemical structures illustrating a pericyclic reaction with reactants and products labeled, including a heat symbol.

Verified step by step guidance
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Step 1: Understand the three types of pericyclic reactions. An electrocyclic reaction involves the opening or closing of a conjugated π-system to form a new σ-bond or break an existing one. A cycloaddition reaction involves the formation of a cyclic product by the combination of two π-systems. A sigmatropic rearrangement involves the migration of a σ-bond across a π-system, with the overall connectivity of the molecule remaining unchanged.
Step 2: Analyze reaction (a). Look for changes in the π-system and determine if a new σ-bond is formed (electrocyclic), if two π-systems combine to form a ring (cycloaddition), or if a σ-bond migrates across a π-system (sigmatropic rearrangement).
Step 3: Analyze reaction (b). Use the same criteria as in Step 2 to classify the reaction. Pay attention to the movement of electrons and the formation or breaking of bonds.
Step 4: Analyze reaction (c). Again, determine whether the reaction involves the opening/closing of a π-system (electrocyclic), the combination of two π-systems (cycloaddition), or the migration of a σ-bond (sigmatropic rearrangement).
Step 5: Analyze reaction (d). Follow the same process as in the previous steps to classify the reaction based on the changes in bonding and electron movement.

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Key Concepts

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

Pericyclic Reactions

Pericyclic reactions are a class of organic reactions that occur through a concerted mechanism, involving a cyclic transition state. These reactions typically involve the rearrangement of electrons in a way that conserves orbital symmetry, and they can be classified into three main types: electrocyclic reactions, cycloadditions, and sigmatropic rearrangements.
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Properties and Types of Pericyclic Reactions

Electrocyclic Reactions

Electrocyclic reactions involve the conversion of a conjugated system into a cyclic structure through the breaking and forming of sigma bonds. These reactions are characterized by the movement of electrons in a way that can be influenced by heat or light, and they follow specific rules based on the number of π electrons involved, as described by the Woodward-Hoffmann rules.
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Cycloaddition Reactions

Cycloaddition reactions are a type of pericyclic reaction where two or more unsaturated molecules or fragments combine to form a cyclic product. These reactions can be classified as [2+2] or [4+2] cycloadditions, depending on the number of π bonds involved. The stereochemistry and regioselectivity of these reactions are crucial for determining the structure of the final product.
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Related Practice
Textbook Question

a. For conjugated systems with two, three, four, five, six, and seven conjugated p-bonds, construct quick MOs (just draw the lobes at the ends of the conjugated system as they are drawn on pages 1220 and 1221) to show whether the HOMO is symmetric or antisymmetric.

b. Using these drawings, convince yourself that the Woodward–Hoffmann rules in Table 28.1 are valid.

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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?

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Textbook Question

a. How many MOs does 1,3,5,7-octatetraene have?

b. What is the designation of its HOMO (c1, c2, etc.)?

c. How many nodes does its highest energy MO have between the nuclei?

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