Stereoselective Reduction of Ketones with Sodium Borohydride

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Transcript Stereoselective Reduction of Ketones with Sodium Borohydride

The Oxidation of Toluene
Microscale Oxidation of Toluene to Benzoic Acid (Reaction of an
Aromatic Side Chain)
COO-Na+
CH3
Na2CO3
MnO2
+ KMnO4
COOH
H+
+
H2O
toluene
sodium benzoate
Organic Chemistry Lab II, Spring 2010
Dr. Milkevitch
April 12 & 14, 2010
benzoic acid
Today’s Experiment
Conduct an oxidation of a group
attached to an aromatic ring
– Convert an alkyl group on an aromatic ring to a
carboxylic acid group
Purpose of the experiment
– Review aromatic compounds
Their structure, stability, reactivity
– Demonstrate an oxidation of an aromatic side
chain
Selectively oxidize a group on an aromatic ring
Leaving the aromatic ring intact (unoxidized)
Reactions of Benzene, (con’t)
There are many reactions of benzene
– Last week: examined electrophilic aromatic substitution
Today: Oxidize aromatic side chain
Remember:
– Benzene does not react like a conjugated cyclic triene
– You can oxidize an alkene, but not a benzene ring
With potassium permanganate
H
H
OH
KMnO4, H2O
H
OH
H
KMnO4, H2O
no reaction
Oxidation of Benzene Side Chains
Benzene ring resistant to oxidation
– By strong oxidants like potassium permanganate
– Not so with alkyl side chains on aromatic rings
Reactions take place at benzylic carbons
– Benzylic carbon must have at least one hydrogen
– Intermediates: benzylic radicals or benzylic carbocations
– Mechanism not well understood
This Experiment
Oxidant: potassium permanganate
Substrate is toluene
COO-Na+
CH3
Na2CO3
MnO2
+ KMnO4
COOH
H+
+
H2O
toluene
sodium benzoate
benzoic acid
Cautions!!!
Potassium permanganate
– Strong oxidizer
– Avoid contact!
– Will stain skin and clothing
Its seriously purple!
Procedure I
Into a 10 ml RB flask, place:
–
–
–
–
825 mg of potassium permanganate
100 mg of sodium carbonate
3 ml of water
Small spin bar
Attach reflux condenser to flask
Attach heating mantle
Dissolve reagents with gentle stirring/heating
Turn off heat, let cool ~ 3 min
Add 250 microliters of toluene
– Approx 10 mg of detergent
– Aids mixing of toluene and water
Reflux with stirring for 45 min
– Do not overheat, for it can “bump”
Procedure II
Most of the purple color should be gone by the end of the reflux
– Turn off water to condenser
While still warm:
– Add sodium bisulfite until the purple color is gone
– Filter off MnO2 using a Hirsch funnel containing Celite
Celite is a filter aid
Keeps filter paper from clogging
See the lab TA or myself for Celite
Rinse flask with 0.5 ml of ddH2O, add this to filtrate
Check filtrate:
– Filtrate is what you want!
– If its still purple colored, add sodium bisulfite until the solution is colorless
– Filter solution again if more brown precipitate forms
Cool filtrate in ice
Acidify with 1 ml of concentated HCl
Collect crystals by vacuum filtration using a Hirsch funnel
Wash crystals with 1 ml of cold water
Recrystalize from water
Dry crystals, weigh and take a melting point
Formal lab report required