Organic Chemistry Lab II, Spring 2010 The Oxidation of Toluene Microscale Oxidation of Toluene to Benzoic Acid (Reaction of an Aromatic Side Chain) Organic Chemistry Lab II, Spring 2010 Dr. Milkevitch April 12 & 14, 2010

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

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

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 Formal lab report required 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