Preparation of aromatic systems

Removal of water from certain compounds is often used for synthesis of aromatic rings. Adams and Hufferd977 obtained mesitylene from acetone by treatment with sulfuric acid, and Lyle et al.91s give the following generalized procedure for preparation of 1,3,5-triarylbenzenes  

A substituted acetophenone (10 g) is added to anhydrous ethanol (50 ml) previously saturated with gaseous hydrogen chloride. After storage for 30 days the mixture is filtered and the solid triarylbenzene is washed with cold ethanol. The yield is improved if the mother-liquors are poured into water, and the solid product is then collected this material is somewhat impure and must be washed several times with ethanol. Depending on the substituent in the acetophenone, the yield varies between 20% and 85%. 

The Knoevenagel reaction can also be applied to synthesis of aromatic rings thus 2,3-dimethyl-l,4-naphthalenedicarbonitrile is obtained in 65% yield when biacetyl and 0-phenylenediacetonitrile are condensed in anhydrous piperidine with initial cooling and later 3 hours storage.979 

Diaryl ketones carrying a methyl or a methylene group in the ortho-position to the carbonyl group often cyclize with loss of water on pyrolysis (the Elbs reaction), e.g.  

This reaction has some importance for the preparation of derivatives of anthracene,980 dibenz[a,h]anthracene,981 benz[ anthracene,982 and chol-anthrene.983 

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