Replacement, sulfonic acid hydroxyl

Alkyl sulfonates are derivatives of sulfonic acids m which the proton of the hydroxyl group is replaced by an alkyl group They are prepared by treating an alcohol with the appropriate sulfonyl chloride usually m the presence of pyridine... 

The ring-chlorinated derivatives of toluene form a group of stable, industrially important compounds. Many chlorotoluene isomers can be prepared by direct chlorination. Other chlorotoluenes are prepared by indirect routes involving the replacement of amino, hydroxyl, chlorosulfonyl, and nitro groups by chlorine and the use of substituents, such as nitro, amino, and sulfonic acid, to orient substitution followed by their removal from the ring. 

Azolesulfonic acids frequently exist as zwitterions. The usual derivatives are formed, e.g. pyrazole-3-, -4- and -5-sulfonic acids all give sulfonyl chlorides with PCI5. The sulfonic acid group can be replaced by nucleophiles under more or less vigorous conditions, e.g. by hydroxyl in imidazole-4-sulfonic acids at 170 °C, and by hydroxyl or amino in thiazole-2-sulfonic acids. 

Compounds that are even better analogues of carboxylic acids are produced when an alkyl or aryl group replaces one of the hydroxyls in sulfuric acid. This provides compounds called sulfonic acids, which in turn give rise to a range of derivatives exactly comparable to those we have met as carboxylic acid derivatives (Table 7.4). 

Methyl substituents in 3,4 -bipyridines have been oxidized by permanganate to the corresponding carboxylic acids, which are readily decar-boxylated, and esters have been converted to amides. 3,4 -Bipyridine-3 -sulfonic acid on fusion with potassium ferricyanide affords 3 -cyano-3,4 -bipyridine. Cyano-substituted 3,4 -bipyridines have been hydrolyzed to the amide or carboxylic acid, " and the amides were converted to amino groups. Hydroxyl substituents in the 2, 6, 6-positions of 3,4 -bipyridine have been replaced by chloro groups with phosphorus... 

The most interesting feature of this method, reviewed by Stepanov,62 is the ease with which the halogen atom is replaced by a hydroxyl group during the metallization process. This was first observed as long ago as 1931 when Delfs63 obtained the copper complex of 2-(2-hydroxy-naphthyl-l-azo)phenol-4-sulfonic acid (47) by heating an aqueous solution of l-chloro-2-(2-hydroxynaphthyl-l-azo)benzene-4-sulfonic acid (48), copper sulfate, sodium hydroxide and ammonia at 80 °C for 1 hour. 

The replacement of a nuclear substituent such as hydroxyl (-OH), chloro, (-C1), or sulfonic acid (-S03H) with amino (-NH2) by the use of ammonia (ammonolysis) has been practiced for some time with feedstocks that have reaction-inducing groups present thereby making replacement easier. For example, l,4-dichloro-2-nitrobenzene can be changed readily to 4-chloro-2-nitroaniline by treatment with aqueous ammonia. Other molecules offer more processing difficulty, and pressure vessels are required for the production of aniline from chlorobenzene or from phenol (Fig. 3). 

As mentioned before, halogen atoms (practically only chlorine) as well as sulfo groups can be replaced by hydroxyl by means of alkali fusion. This reaction is accelerated by copper. Whether a given phenol can be prepared better from the sulfonic acid or from the chloro derivative depends on which of the two starting materials is more easily and cheaply made. 

The reaction of di- and polysulfonic acids can usually be carried out so that the replacement of the sulfo groups by hydroxyls takes place stepwise (partial alkali fusion). Thus, phenol-m-sulfonic acid is obtained from benzene-m-disulfonic acid under mild conditions, while resorcinol is formed under more vigorous conditions (see page 144) similarly, naphthalene-l,5-disulfonie acid yields, first, l-naphthol-5-sulfonic acid, then l,5 dihydroxynaphthalene, both valuable azo dye... 

Replacement of the sulfo group by hydrogen instead of by hydroxyl, simultaneously forming sulfate instead of sulfite. This reaction is usually only an unimportant side reaction, but in certain cases it may become the main reaction. Thus, alkali fusion of 2,3-dihydroxynaphthalene-6-sulfonic acid does not give the expected 2,3,6-trihydroxynaphthalene, but instead, 2,3-dihydroxynaphthalene. 

Replacement of the Sulfonic Acid Group by the Hydroxyl Group. 167... 

It is advisable, particularly for introduction of several nitro groups, to start from phenol-sulfonic acids and to exchange their sulfo groups for nitro groups (see page 546). It is also possible to replace the chlorine of chloronitrobenzenes by a hydroxyl group by means of boiling aqueous alkali. 

In some cases replacement of an alcoholic hydroxyl group, leading to sulfonic acid salts, can be effected by means of sodium hydrogen sulfite ... 

The replacement of labile groups, such as nitro, halogen, hydroxyl, and sulfonic acid by reaction with ammonia or ammonia progenitors, such as urea. 

As may be observed from the following examples, halogenations may involve reactions of (1) addition, (2) substitution, i.e., of hydrogen, (3) replacement, i.e., of groups, for example, the hydroxyl or sulfonic acid group. 

Ammonolysis of Aminoalkyl Hydrogen Sulfates. Later, in discusdng the replacement of —OH groups, it will be shown that, when hydroxyethane-sulfonic (isethionic) acid is treated with ammonia, taurine is obtained by the preferential replacement of the hydroxyl group. 

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