Preparation of Anthraquinone by Ring Closure

Introduction. Anthraquinone may be prepared by direct oxidation of anthracene with chromic acid. The hydrogen atoms at positions 9 and 10 are more reactive and are oxidized. Most of the anthraquinone used industrially for the preparation of alizarine is prepared from o-benzoylbenzoic acid. When this acid is heated with concentrated sulfuric acid at about 100° dehydration takes place with ring closure  

The hydrogen atom in the ortho position to the carbonyl group unites with the hydroxyl group of the carboxyl to effect the ring closure. The preparation of o-benzoylbenzoic acid from benzene and phthalic anhydride (Experiment 63) and the dehydration of this keto acid to anthraquinone considered in the present experiment illustrate the building up of polynuclear compounds from simpler benzene derivatives. 

As mentioned in the preceding experiment, derivatives of benzene may be used to combine with phthalic anhydride, thus forming substituted benzoylbenzoic acids. For example, chlorobenzene yields chlorobenzoylbenzoic acid. Substituted phthalic anhydrides may be employed in place of phthalic anhydride. These substituted benzoylbenzoic acids are important intermediates in the preparation of anthraquinone vat dyes. 

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