Ring carboxy acids

S3mthesis of Ring Carboxy Acids.—We have just discussed the synthesis of aromatic acids by the oxidation of benzene hydrocarbons containing a side chain. As would be expected from our description... 

From Hydrocarbons, Friedel-Craft.—The aromatic hydrocarbons yield ring carboxy acids by other reactions than those effecting oxidation of a side chain. Carbon dioxide may be introduced directly into a benzene ring, thus converting a hydrogen into.carboxyl. This may be accomplished in the presence of aluminium chloride, Friedel-Craft reagent. 

From Aryl Halides. Kekule Synthesis.—Aromatic halides in which the halogen is in the ring yield ring carboxy acids by the action of carbon dioxide and sodium. 

Ring Carboxy Acids by the Grignard Reaction.— Aromatic acids with carboxyl in the ring may also be prepared by the Grignard reaction... 

By Grignard Reaction.—Exactly analogous to the Grignard synthesis of ring carboxy acids is the synthesis of side-chain carboxy acids by the introduction of carbon dioxide into the Grignard reagent. 

Mono-basic ring carboxy acids of the benzene homologues need only to be mentioned. 

The only other ring carboxy acids to be considered in detail are the three phthalic acids. They all have the constitution of di-carboxy benzene, CeH4 = (COOH)2, and therefore three isomers are possible, orthoj meta and para. 

The general methods for synthesizing ring carboxy acids are applicable to the phthalic acids if we start with the corresponding di-substituted benzenes or with the mono-substituted benzoic acids, e.g.j... 

A few other poly-basic ring carboxy acids may be mentioned by name and formula only. Mesitylene, I-3-5-tri-methyl benzene, yields a di-carboxy and tri-carboxy acid together with a mono-carboxy acid already referred to. 

Let us now examine the conditions to be met in applying either of the two general methods just given to the formation of a substituted ring-carboxy acid in which the substitution also is to be in the ring. 

As both nitration and sulphonation take place with comparative ease, with either hydrocarbons or acids, and as the nitro compounds yield amino compounds and these yield diazo compounds, with their numerous reactions, in particular the Sandmeyer reaction for obtaining nitriles and as sulphonic acids are easily converted into phenols or nitriles it will be seen that by a combination of these synthetic reactions practically any desired ring-substituted mono-basic or poly-basic ring-carboxy acid may be obtained. 

HYDROXY AROMATIC ACIDS PHENOL RING-CARBOXY ACIDS... 

Ring-hydroxy ring-carboxy acids, CeH4 Salicylic acid... 

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