Amino acids benzene-sulphonic acid

This compound is known as sulphanilic acid. It may also be considered as an amino aderivative of benzene sulphonic acid, i.e.j para-amino benzene sulphonic acid. Its name, sulphanilic acid, is in agreement with its relation to aniline and sulphuric acid. It is distinctly different from sulphonic acids of the hydrocarbons in being difficultly soluble in cold water. As we shall see later, it is of great importance in the preparation of dyes. It is a crystalline compound soluble in alkalies as the alkali salt, but is precipitated as the free acid on acidifying the solution of the salt. 

Reagent for Nitrites in Water.— An illustration of such a reaction is one which is the basis of the colorimetric determination of nitrites in water. When sulphanilic acid, para-amino benzene sulphonic acid, is diazotized and the resulting diazo compound treated with alpha-naphthylamine the benzene ring and the naphthalene ring become coupled as an azo compound which is red in color. 

Sulphanilic acid p-Amino benzene sulphonic acid... 

C) AMINO AROMATIC SULPHONIC ACIDS. Sulphanilic or p-amino-benzene sulphonic acid, NH2CJH4SO3H. 

In reality all carbon atoms share equally the pool of electrons which constitute the double bonds and benzene resists addition across the double bonds which would otherwise destroy its unique structure and stability. Single or multiple hydrogen atoms can be substituted to form a host of derivatives containing similar functional groups to those above, e.g. saturated and unsaturated aliphatic chains, amino, carboxylic acidic, halogeno, nitro, and sulphonic acid groups as shown in Table 3.6. 

Acid Yellow.—A mixture of the sodium salts of the mono- and di-sulphonic acid derivatives of amino azo benzene is a dye known as acid yellow or fast yellow, as above. 

Methyl Orange.— Another important dye compound related to the above is the well-known methyl orange which, though not used as a dye, is very widely used as an indicator. It is the mono-sulphonate derivative of di-methyl amino azo benzene. The alkali salts are orange yellow in color while the free acid is red violet, and these are the colors obtained when the indicator is used in acidimetric titrations. 

It is known also as helianthine and as tropaeolin D. It is not prepared by starting with amino azo benzene but with sulphanilic acid which is para-amino benzene sulphonic acid. This is diazotized and then coupled with di-methyl aniline yielding the azo compound. 

It is interesting that the para benzene di-sulphonic acid also yields the meta phenol sulphonic acid due to position rearrangement. This same rearrangement, it will be recalled, occurs in the stronger fusion of the para di-sulphonic acid, meta di-phenol being obtained (p. 522). Amino benzene sulphonic acid may also be converted into phenol sulphonic acid by diazotization and decomposition of the diazo compound with water. 

Diazotization.—Like aniline and other aromatic primary amines they undergo diazotization. The resulting diazo compounds undergo the various diazo reactions (p. 601) by means of which the naphthalene group becomes coupled as an azo compound with other naphthalene or benzene rings. These azo compounds are dyes. The most important dyes of this group are derived from mixed amino and sulphonic acid or mixed amino and hydroxyl derivatives of naphthalene and will be considered a little later. Not only, however, may the naphthylamines >deld diazo compounds and through them azo compounds but they may be coupled as azo compounds with a diazotized benzene compound. 

AI3-28529 2-Amino-5-chloro-4-methylbenzenesulf-onic acid 3-Amino-6-chlorotoluene-4-sulfonic acid 5-Amino-2-chlorotoluene-4-sulphonic xid Benzene-sulfonic acid, 2-amino-5-chlotxj-4-methyl- ... 

Cinnamic acid, like other unsaturated compounds, forms addition-products with the halogen hydrides, hypochlorous acid, bromine, and so forth. It yields esters, an acid chloride, and an amide. The hydrogen atoms in the benzene ring may be replaced by the halogens, nitro groups, amino groups, and the sulphonic acid group. 

Various dehydrating agents—concentrated sulphuric acid, zinc chloride, phosphorus pentoxide—can be used. Sulphuric acid, although perhaps the most convenient, has the disadvantage that it tends to sulphonate the aromatic substances employed. At a low temperature, however, diphenylmethane can be obtained from benzyl alcohol and benzene. At 140° phosphorus pentoxide condenses benzene and diphenylcarbinol to triphenylmethane (see B., 7,1204). Not only substituted benzyl alcohols, but even mandelic acid can be brought within the scope of the reaction, while in place of benzefte its nitro, amino or phenolic derivatives may be used. 

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