Organic Lecture 2/7

Basic Reactions Overview

  • Focus on various types of reactions that require symmetry adaptation.

  • Symmetry adaptation means both interacting species must be aligned properly.

  • More conjugation, smaller HOMO/LUMO gap, longer wavelength

  • Beer Lambert Law: A= ecl (A is absorbance, e is molar absorptivity/ m^-1cm^-1, c is concentration in M, l is path length in cm.

Symmetry Adaptation

  • Key idea: the interacting lobes of the species must match in orientation.

  • Example: species needing to rotate to interact effectively.

Pericyclic Reaction Properties

  • Little, if any, solvent effect

  • No nucleophiles/electrophiles

  • not generally catalyzed by Lewis acids

  • Highly stereospecific

  • Often photochemically initiated

  • Concerted process: all electrons moving at once

Role of Light in Reactions

  • Introducing light into the system changes the Highest Occupied Molecular Orbital (HOMO).

  • Transition from side three (initial state) to side four (new state) upon light exposure.

  • Only one form is produced, no mixture occurs.

Heat and Reaction Outcomes

  • Starting with E/E and light= cis product, starting with E/E only heat= trans product

  • Starting with E/Z and light= trans product, starting with E/Z only heat= cis product

  • Woodward-Hoffmann rules allow for understanding the reaction mechanisms based on symmetry considerations.

Electron Motion and Rotational Symmetry

  • When dealing with pi electrons, rotation is critical to maintain symmetry.

  • The term "conrotatory" describes synchronized rotation of paired lobes (like thumbs rotating together).

Types of Rotational Motion

  • Conrotatory Motion: Both groups rotate in the same direction, leading to either a cis or trans product.

  • Disrotatory Motion: Groups rotate in opposite directions.

Examples of Conrotatory and Disrotatory Motion

  • Starting with E configuration and performing motion:

    • Conrotatory yields a cis product.

    • Disrotatory yields a trans product.

  • With Z configuration, the same rules apply:

    • Conrotatory motion leads to trans product.

Cycloaddition Reactions

  • A 4n + 2 cycloaddition leads to a six-membered ring formation.

  • A 2n + 2 cycloaddition would produce a four-membered ring.

  • The configuration of the products (cis/trans) depends on the starting materials and motion applied in reactions. Diene must be able to adopt s-cis conformation

  • Diene= two double bonds, dienophile= something that wants to react with diene, reacts faster if electron-withdrawing groups are on it.

  • Diene reacts faster if something on it will donate electron density, like oxygen, methyl group by hyperconjugation, etc. Diene wants something to donate, dienophile wants an electron-withdrawing group.

  • Diels-Alder is stereospecific: if they start trans, they end trans; if they start cis, they end cis. Groups end up 1,2 or 1,4 to each other, never 1,3.

  • Endo product is favored in cycloaddition