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In this experiment, you will deduce the structure of an unknown mono- or disubstituted benzene derivative, where the substituents could be Br-, Cl-, CH₃-, or CH₃O-, in any arrangement. The approach will be illustrated with o-methyl anisole.

To deduce the structure of the unknown aromatic, you will convert it to the corresponding acid chloride by chlorocarbonylation with oxalyl chloride and AlCl₃. Hydrolysis of the acid chloride will give the benzoic acid derivative, which you will titrate to determine the molecular weight. From the weight, you should be able to deduce the substituents on the ring. From the melting point of the recrystallized acid, you should be able to deduce the substitution pattern on the benzene ring.

1H NMR: After you have titrated your acid, prepare NMR samples both of your acid and of your starting unknown. Record the ¹H NMR spectra of each of these. Tape the NMR spectra into your notebook. Using the NMR's and the titration, deduce the structure of your starting unknown.

If you complete one unknown, and have time left, ask your TA for another.

ref: Osman, M.A. Helvetica Chim. Acta 1982, 65, 2448.

Acid Chloride: To 1.0 gm of the unknown aromatic in a 50 mL Erlenmeyer flask add 10 mL of a 10 weight % solution of oxalyl chloride in methylene chloride. Cool the mixture with stirring in an ice-water bath. Add 1.0 gm of anhydrous AlCl₃ in portions! (CAUTION: Corrosive! Do not breathe dust!), letting the reaction subside after each addition. If the AlCl₃ does not react, add the rest of it, then warm the mixture past room temperature until a reaction is observed. Most of the AlCl₃ should dissolve.

When the reaction has stopped bubbling, pour it over crushed ice in a beaker, rinsing the reaction flask with a little methylene chloride. Swirl until the orange color fades to a pale yellow. Separate the layers, extract the aqueous layer with 1 x 10 mL of methylene chloride, and concentrate the methylene chloride extracts on the rotary evaporator.

Acid Preparation: To the residue after rotary evaporation, add 20 mL of 1 M aqueous KOH, and stir for 15 minutes. Extract the KOH solution with 2 x 10 mL of CHCl₃, and set the extract aside to recover your unreacted unknown, if necessary. Acidify the aqueous phase with concentrated HCl to pH =1, and filter the precipitated acid. Wash the solid acid with water, and let it thoroughly air dry before titration. A portion of the acid should be recrystallized from ethanol/water to constant melting point.

If the acid does not precipitate directly on acidification, extract the aqueous layer with 3 x 10 mL of CHCl₃, dry (Na₂SO₄) and concentrate on the rotary evaporator.