1. Electronic effects and stability of reaction intermediates
Lecture 2

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

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

4. 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

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

6. Distribution of electron density
Inductive effect decays quickly along the chain

7. 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)

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

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

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

11. 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

12. 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

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