Pericyclic Compound Lecture Handout

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Chapter 16.2 – Pericyclic Reactions


Pericyclic Reactions
Most organic reactions produce either ionic or radical intermediate




Another type of reaction does not involve those intermediates, rather proceed in a concerted process.




▪ All bond changes occur in one step
▪ No intermediate
▪ Electrons move in a closed loop
▪ The TS bears very little or no charge. The polarity of the solvent generally does not affect the rate or
yield or reaction (unlike SN1 or SN2 reactions)




Me waiting for class to start.

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Chapter 16.2 – Pericyclic Reactions


[4+2] Cycloaddition (Diels-Alder Reaction)




M 8

- H 11
C
-
i I
H




4 -
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Thermodynamic Consideration of Diels-Alder Reaction
spontaneous : negative o



⑦




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▪ Just like most reactions, Diels-Alder Reaction is in equilibrium.
▪ Going form two molecules to one molecule, ΔS term of Diels-Alder Reaction is negative which
makes -TΔS term positive.
▪ At lower temperature, -TΔS term is not large enough that ΔG is positive, favor the formation of
the product.
▪ At higher temperature (>200 oC), the -TΔS is highly positive. As a result, ΔG is positive and the
reaction favors the formation of the reactant.
▪ Therefore, one can perform “retro Diels-Alder reaction” under a temperature above 200 oC



Me waiting for class to start.

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Chapter 16.2 – Pericyclic Reactions


The Dienophiles




Electron-withdrawing effect through resonance



e

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Stereochemistry of Diels-Alder Reaction is Stereospecific




Alkyne can serve as a dienophile



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Me waiting for class to start.