1. Cannizaro Reaction Overall: Restriction: no  hydrogens in the aldehydes.  hydrogens No  hydrogens Why the restriction? The  hydrogens are acidic leading to ionization.

2. Mechanism What can happen? Reactants are the aldehyde and concentrated hydroxide. Hydroxide ion can act both as Base, but remember we have no acidic hydrogens (no  hydrogens). Nucleophile, attacking carbonyl group. Attack of nucleophilic HO - Re-establish C=O and eject H - which is immediately received by second RCHO Acid-base

3. Experimental Evidence These are the hydrogens introduced by the reaction. They originate in the aldeyde and do not come from the aqueous hydroxide solution.

4. Kinetic vs Thermodynamic Contol of a Reaction Examine Addition of HBr to 1,3 butadiene

5. Mechanism of reaction. Allylic resonance But which is the dominant product?

6. Nature of the product mixture depends on the temperature. Product mixture at -80 deg 80% 20% Product mixture at + 40 deg 20% 80% Goal of discussion: how can temperature control the product mixture?

7. Thermodynamic Control: Most stable product dominates Kinetic Control: Product formed fastest dominates When two or more products may be formed in a reaction A  X or A  B Thermodynamic control assumes the establishing of equilibrium conditions and the most stable product dominates. Kinetic Control assumes that equilibrium is not established. Once product is made it no longer changes. Equilibrium is more rapidly established at high temperature. Thermodynamic control should prevail at high temperature where equilibrium is established. Kinetic Control may prevail at low temperature where reverse reactions are very slow.

8. Nature of the product mixture depends on the temperature. Product mixture at -80 deg 80% 20% Product mixture at + 40 deg 20% 80% Thermodynamic Control More stable product Kinetic Control Product formed most quickly, lowest E a

9. Formation of the allylic carbocation. Can react to yield 1,2 product or 1,4 product.

10. Most of the carbocation reacts to give the 1,2 product because of the smaller E a leading to the 1,2 product. This is true at all temperatures. At low temperatures the reverse reactions do not occur and the product mixture is determined by the rates of forward reactions. No equilibrium.

11. Most of the carbocation reacts to give the 1,2 product because of the smaller E a leading to the 1,2 product. This is true at all temperatures. At higher temperatures the reverse reactions occur leading from the 1,2 or 1,4 product to the carbocation. Note that the 1,2 product is more easily converted back to the carbocation than is the 1,4. Now the 1,4 product is dominant.

12. Diels Alder Reaction/Symmetry Controlled Reactions Quick Review of formation of chemical bond. Electro n donor Electron acceptor Note the overlap of the hybrid (donor) and the s orbital which allows bond formation. For this arrangement there is no overlap. No donation of electrons; no bond formation.

13. Diels Alder Reaction of butadiene and ethylene to yield cyclohexene. We will analyze in terms of the pi electrons of the two systems interacting. The pi electrons from the highest occupied pi orbital of one molecule will donate into an lowest energy pi empty of the other. Works in both directions: A donates into B, B donates into A. A B HOMO donor HOMO donor LUMO acceptor LUMO acceptor B HOMO donates into A LUMO A HOMO donates into B LUMO Note the overlap leading to bond formation

14. Try it in another reaction: ethylene + ethylene  cyclobutane LUMO HOMO LUM O HOMO Equal bonding and antibonding interaction, no overlap, no bond formation, no reaction

15. Reaction Problem

16. Synthesis problem

17. Give the mechanism for the following reaction. Show all important resonance structures. Use curved arrow notation. Mechanism Problem