The Oxidation of TolueneThe Oxidation of Toluene

Microscale Oxidation of Toluene to Benzoic Acid (Reaction of an Microscale Oxidation of Toluene to Benzoic Acid (Reaction of an Aromatic Side Chain)Aromatic Side Chain)

Organic Chemistry Lab II, Spring 2010Dr. Milkevitch

April 12 & 14, 2010

CH3Na2CO3

H2O

MnO2

COO-Na+

+ KMnO4+ H+

COOH

toluene sodium benzoate benzoic acid

Today’s ExperimentToday’s Experiment

Conduct an oxidation of a group Conduct an oxidation of a group attached to an aromatic ringattached to an aromatic ring– Convert an alkyl group on an aromatic ring to a Convert an alkyl group on an aromatic ring to a

carboxylic acid groupcarboxylic acid group

Purpose of the experimentPurpose of the experiment– Review aromatic compoundsReview aromatic compounds

Their structure, stability, reactivityTheir structure, stability, reactivity– Demonstrate an oxidation of an aromatic side Demonstrate an oxidation of an aromatic side

chainchainSelectively oxidize a group on an aromatic ringSelectively oxidize a group on an aromatic ringLeaving the aromatic ring intact (unoxidized)Leaving the aromatic ring intact (unoxidized)

Reactions of Benzene, (con’t)Reactions of Benzene, (con’t)There are many reactions of benzeneThere are many reactions of benzene– Last week: examined electrophilic aromatic substitutionLast week: examined electrophilic aromatic substitution

Today: Oxidize aromatic side chainToday: Oxidize aromatic side chain

Remember: Remember: – Benzene does not react like a conjugated cyclic trieneBenzene does not react like a conjugated cyclic triene– You can oxidize an alkene, but not a benzene ringYou can oxidize an alkene, but not a benzene ring

With potassium permanganateWith potassium permanganate

H

H

KMnO4, H2OOH

OH

H

H

KMnO4, H2Ono reaction

Oxidation of Benzene Side ChainsOxidation of Benzene Side Chains

Benzene ring resistant to oxidationBenzene ring resistant to oxidation– By strong oxidants like potassium permanganateBy strong oxidants like potassium permanganate– Not so with alkyl side chains on aromatic ringsNot so with alkyl side chains on aromatic rings

Reactions take place at benzylic carbonsReactions take place at benzylic carbons– Benzylic carbon must have at least one hydrogenBenzylic carbon must have at least one hydrogen– IntermediatesIntermediates: benzylic radicals or benzylic carbocations: benzylic radicals or benzylic carbocations– Mechanism not well understoodMechanism not well understood

This ExperimentThis Experiment

Oxidant: potassium permanganateOxidant: potassium permanganate

Substrate is tolueneSubstrate is toluene

CH3Na2CO3

H2O

MnO2

COO-Na+

+ KMnO4+ H+

COOH

toluene sodium benzoate benzoic acid

Cautions!!!Cautions!!!

Potassium permanganate Potassium permanganate – Strong oxidizerStrong oxidizer– Avoid contact!Avoid contact!– Will stain skin and clothing Will stain skin and clothing

Its Its seriouslyseriously purple! purple!

Into a 10 ml RB flask, place:Into a 10 ml RB flask, place:– 825 mg of potassium permanganate825 mg of potassium permanganate– 100 mg of sodium carbonate100 mg of sodium carbonate– 3 ml of water3 ml of water– Small spin barSmall spin bar

Attach reflux condenser to flaskAttach reflux condenser to flaskAttach heating mantleAttach heating mantleDissolve reagents with gentle stirring/heatingDissolve reagents with gentle stirring/heatingTurn off heat, let cool ~ 3 minTurn off heat, let cool ~ 3 minAdd 250 microliters of tolueneAdd 250 microliters of toluene– Approx 10 mg of detergent Approx 10 mg of detergent – Aids mixing of toluene and waterAids mixing of toluene and water

Reflux with stirring for 45 minReflux with stirring for 45 min– Do not overheat, for it can “bump”Do not overheat, for it can “bump”

Procedure IProcedure I

Procedure IIProcedure IIMost of the purple color should be gone by the end of the refluxMost of the purple color should be gone by the end of the reflux– Turn off water to condenserTurn off water to condenser

While still warm:While still warm:– Add sodium bisulfite until the purple color is goneAdd sodium bisulfite until the purple color is gone– Filter off MnOFilter off MnO22 using a Hirsch funnel containing Celite using a Hirsch funnel containing Celite

Celite is a filter aid Celite is a filter aid Keeps filter paper from clogging Keeps filter paper from clogging See the lab TA or myself for CeliteSee the lab TA or myself for CeliteRinse flask with 0.5 ml of ddHRinse flask with 0.5 ml of ddH22O, add this to filtrateO, add this to filtrate

Check filtrate:Check filtrate: – Filtrate is what you want!Filtrate is what you want!– If its still purple colored, add sodium bisulfite until the solution is colorlessIf its still purple colored, add sodium bisulfite until the solution is colorless– Filter solution again if more brown precipitate formsFilter solution again if more brown precipitate forms

Cool filtrate in ice Cool filtrate in ice Acidify with 1 ml of concentated HClAcidify with 1 ml of concentated HClCollect crystals by vacuum filtration using a Hirsch funnelCollect crystals by vacuum filtration using a Hirsch funnelWash crystals with 1 ml of Wash crystals with 1 ml of coldcold water waterRecrystalize from waterRecrystalize from waterDry crystals, weigh and take a melting pointDry crystals, weigh and take a melting point

Formal lab report requiredFormal lab report required