Carboxylate anion, basicity anhydrides

In basic solution the nucleophile is the excellent methoxide ion, which attacks the anhydride to give a tetrahedral intermediate having the carboxylate anion as its best leaving group. This carboxylate aamot be protonated and is not attacked by methoxide anion. 

The base-induced reaction of the symmetrical dipropanoic anhydride (15) illustrates the mechanism of base hydrolysis with anhydrides, in a reaction that gives two equivalents of propanoic acid. Acyl addition of the nucleophilic hydroxide gives tetrahedral intermediate 16. In this case, the best leaving group is the carboxylate anion 18, which gives propanoic acid, 17. Because 17 is formed in a basic solution, reaction with hydroxide gives 18. A second step is required to convert 17 to 18, using aqueous acid (aqueous sulfuric acid or aqueous HCl). 

The reactivity of anhydrides is similar to that of acid chlorides. A carboxylate anion is the learing group in a variety of syntheses of acyl derivatives, all of which are examples of the addition-elimination process (Fig. 18.29). One reaction of this kind is the basic hydrolysis of phthalic anhydride to phthalic acid. Here the leaving group is an internal carboxylate anion. 

Esters are less reactive than acid chlorides and anhydrides in addition reactions, but more reactive than amides. Esters can be converted into their parent carboxylic acids under either basic or acidic aqueous conditions in a process called, logically enough, ester hydrolysis. In base, the mechanism is the familiar addition-elimination one (Fig. 18.31). Hydroxide ion attacks the carbonyl group to form a tetrahedral intermediate. Loss of alkoxide then gives the acid, which is rapidly deproto-nated to the carboxylate anion in basic solution. Notice that this reaction, saponification (p. 862), is not catalytic. The hydroxide ion used up in the reaction is not regenerated at the end. To get the carboxylic acid itself, a final acidification step is necessary. 

E. Jacobson in 1882 fused phthalic anhydride with quinoline bases obtained from coal tar, which also contained quinaldine (136). He thus received quinophthalone (137). Quinophthalone derivatives bearing sulfonic or carboxylic acid functions represent suitable anionic dyes. Derivatives carrying basic side chains containing quarternary nitrogen, on the other hand, provide cationic dyes. The compounds are used especially as disperse dyes [1]. 

Quinophthalone was first synthesised by E. Jacobsen in 1882 by fusing phthalic anhydride with quinoline bases derived from coal tar. Derivatives of quinophthalone carrying sulfonic or carboxylic acid groups provide anionic dyes, those with basic side chains such as quaternary nitrogen produce cationic dyes. However, quinophthalones are too soluble in various media to be... 

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