Cannizaro Reaction

Overall:2 RCHO

conc. KOH

heatRCO2

- + RCH2OH

Restriction: no hydrogens in the aldehydes.

H3C

O

CHO

H

CHO

hydrogens No hydrogens

Why the restriction? The hydrogens are acidic leading to ionization.

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.

R

O

H

HO-:

R

O

H

OH

R

OH

R

O

OH

+

R

OH

H

R

O

O

+R

OHH

H

Attack of nucleophilic HO-

Re-establish C=O and eject H- which is immediately received by second RCHO

Acid-base

Experimental Evidence

2 RCDO RD2OH + RCO2-

KOH, H2O

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

Kinetic vs Thermodynamic Contol of a Reaction

Examine Addition of HBr to 1,3 butadiene

HBrH

Br

+

Br

H

1,2 product 1,4 product

Mechanism of reaction.

H-Br

HH

Allylic resonance

1,2 product 1,4 product

Br Br

H

BrBr

H

But which is the dominant product?

HBrH

Br

+

Br

H

1,2 product 1,4 product

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?

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.

HBrH

Br

+

Br

H

1,2 product 1,4 product

Nature of the product mixture depends on the temperature.

Product mixture at -80 deg 80% 20%Product mixture at + 40 deg 20% 80%

Thermodynamic ControlMore stable product

Kinetic Control

Product formed most quickly, lowest Ea

Formation of the allylic carbocation.

Can react to yield 1,2 product or 1,4 product.

Most of the carbocation reacts to give the 1,2 product because of the smaller Ea 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.

Most of the carbocation reacts to give the 1,2 product because of the smaller Ea 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.

Diels Alder Reaction/Symmetry Controlled Reactions

Quick Review of formation of chemical bond.

HO- + H+ H - O - H

Electron donor

Electron acceptor

Note the overlap of the hybrid (donor) and the s orbital which allows bond formation.

HO- + H+ H - O H

For this arrangement there is no overlap. No donation of electrons; no bond formation.

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.

new bonds

A B

A

B

HOMOdonor

HOMOdonor

LUMOacceptor

LUMOacceptor

B HOMO donates into A LUMO

A HOMO donates into B LUMO Note the

overlap leading to bond formation

Note the overlap leading to bond formation

Try it in another reaction: ethylene + ethylene cyclobutane

new bonds

A B

A B

LUMO

HOMO

LUMO

HOMO

Equal bonding and antibonding interaction, no overlap, no bond formation, no reaction

Br

Br

excess sodium methoxide

Reaction Problem

HO

OEt

using only compoundshaving two carbons as the source of all carbons in the target molecule

Synthesis problem

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

aq. acid

heatOEt

OHO

+ EtOH

Mechanism Problem