Reduction + Aldol Condensation

Reduction

basic type of chemical reaction and is the opposite of oxidation. The process of gaining electrons.

• results in a decrease in oxidation number of a carbon atom 

• Reducing agents (LiAlH4 and NaBH4) produce H-

  • LiAlH4 is larger and has more electrons further from nucleus, more reactive

  • look at electronegativity differences to see strength, NaBH4 weaker but safer

• must be performed in aprotic anhydrous solvents

catalytic hydrogenation

used in ochem to reduce pi bonds in a variety of functional groups, involves addition of a molecule of dihydrogen across a carbon-carbon or carbon-heteroatom double or triple bond in the presence of a metal catalyst such as platinum, palladium, nickel, or rhodium that is often adsorbed on an inert solid support like carbon.

• nickel and palladium expensive

• H2 gas and high temperature unsafe

• very little waste so ideal for industry but not lab

Chemical reduction

metal hydride reducing agents that are used in the latter have differing reactivities toward specific functional groups

• LiAlH4 is very reactive

• NaBH4 is less reactive and more selective

  • can reduce up to 4 mole of fluorenone due ot 4 H’s present 

  • will react with water, don’t get on skin and rinse if contacted.

• nitro (NO2) is reduced to amino (NH2) group 

• reduction of aldehydes leads to primary alcohols and reduction of ketones leads to secondary alcohols 

Carbonyl group 

a rich source of many important reactions in organic chemistry

• polarization of the carbon-oxygen pi bond, owing to relatively high electronegativity of oxygen atom

• increases the acidity of the alpha hydrogen atoms (alpha carbon atoms can become nucleophilic through deprotonation to form an enolate ion).

nucleophiles 

lewis bases, oxygen atom in the carbonyl group.  

electrophiles

lewis acids

tautomerization

keto-enol equilibrium to switch between enolate and enols.

Reduction of 9-Fluorenone, synthesis

Look for disappearance of carbonyl peak and there will not be a large OH peak due to hydrogen bonding

Initial product is salt formation: R-C(=O)-R → (R-C(-O-B(-)-Na(+))-R) (4 moles of this)

Mechanism of Aldol condensation 

• multiple enolates can form

• multiple electrophilic carbons, can control to make enolate first

• typical reaction favors reactants but we can use a stronger base like LDA to shift to products.

• should see a carbonyl peak in the ketone position and no alkene or C-H stretch

• works with weaker base due to only one possible enolate because aldehyde reacts faster

  • extended conjugation of product is energetically favorable

9-fluorenone color

yellow

Fluorenol color

white

Specific mechanism for aldol condensation