Electronic effects and stability of reaction intermediates Lecture 2

Outline Distribution of electron density in molecules. Inductive effect Mesomeric effect. Resonance Reaction intermediates – radicals, cations, anions Relative stability of reactions intermediates

Distribution of electron density Electronegativity (Pauling, 1932) – the tendency of an atom to attract electron density

Distribution of electron density If the displacement of the electron density takes place along s-bonds, inductive effect (I) takes place - a dipole is induced Negative inductive effect: -I electron-withdrawing groups

Distribution of electron density Positive inductive effect: < +I electron-donating groups charged groups: -I +I very strong!

Distribution of electron density Inductive effect decays quickly along the chain

Mesomeric effect If the displacement of the electron density takes place along p-bonds, mesomeric effect (M) takes place – conjugation effect Conjugation – overlap of three or more adjacent p-orbitals – energetically favorable Atoms capable of conjugation: atoms that are part of a double/triple bond atoms with lone pairs atoms with empty p orbitals (such as carbocations) atoms with half-filled orbitals (radicals)

Mesomeric effect 1,3-butadiene delocalization of electron density (resonance): hybrid structure resonance structures delocalization always lowers the energy of the system

Mesomeric effect +M -M acrylic aldehyde p - electron-donating groups (+M-effect) p - electron-withdrawing groups (-M-effect)

Mesomeric effect decays very slowly along the conjugated chain Mesomeric effect is stronger than inductive and two more examples – phenol and benzoic acid

Reaction intermediates Intermediate – a particle with a lifetime appreciably longer than a molecular vibration that is formed (directly or indirectly) from the reactants and reacts further to give (either directly or indirectly) the products of a chemical reaction REACTANTS PRODUCTS INTERMEDIATE REACTANTS PRODUCTS TRANSITION STATE

Reaction intermediates Intermediates are usually formed when chemical bonds are broken The leaving group can leave with the pair of electrons, empty orbital, single electron carbocation heterolytic bond cleavage carbanion radical homolytic bond cleavage In general, intermediates are unstable, but can be stabilized by the delocalization of electron density

Reaction intermediates REACTANTS PRODUCTS-1 INTERMEDIATE-1 INTERMEDIATE-2 PRODUCTS-2 more stable major or the only Compare the relative stability of particles: