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 while on the southernmost projection of the modern Cape peninsula, whose remarkable highlands (Table Mountain, &c.) doubtless impressed him as the practical termination of the continent, he bestowed, says De Barros, the name of Cape of Storms (Cabo Tormentoso) in memory of the storms he had experienced in these far southern waters; this name (in the ordinary tradition) was changed by King John to that of Good Hope (Cabo da Boa Esperança). Some excellent authorities, however, make Diaz himself give the Cape its present name. Hard by this “so many ages unknown promontory” the explorer probably erected his last pillar. After touching at the Ilha do Principe (Prince’s Island, south-west of the Cameroons) as well as at the Gold Coast, he appeared at Lisbon in December 1488. He had discovered 1260 m. of hitherto unknown coast; and his voyage, taken with the letters soon afterwards received from Pero de Covilhão (who by way of Cairo and Aden had reached Malabar on one side and the “Zanzibar coast” on the other as far south as Sofala, in 1487–1488) was rightly considered to have solved the question of an ocean route round Africa to the Indies and other lands of South and East Asia.

No record has yet been found of any adequate reward for Diaz: on the contrary, when the great Indian expedition was being prepared (for Vasco da Gama’s future leadership) Bartolomeu only superintended the building and outfit of the ships; when the fleet sailed in 1497, he only accompanied da Gama to the Cape Verde Islands, and after this was ordered to El Mina on the Gold Coast. On Cabral’s voyage of 1500 he was indeed permitted to take part in the discovery of Brazil (April 22), and thence should have helped to guide the fleet to India; but he perished in a great storm off his own Cabo Tormentoso. Like Moses, as Galvano says, he was allowed to see the Promised Land, but not to enter in.

 DIAZO COMPOUNDS, in organic chemistry, compounds of the type R·N·2·X (where R＝a hydrocarbon radical, and X＝an acid radical or a hydroxyl group). These compounds may be divided into two classes, namely, the true diazo compounds, characterized by the grouping −N＝N−, and the diazonium compounds, characterized by the grouping N ⫶ N<.

The diazonium compounds were first discovered by P. Griess (Ann., 1858, 106, pp. 123 et seq.), and may be prepared by the action of nitrous fumes on a well-cooled solution of a salt of a primary amine,

C6H5NH2·HNO3 + HNO2＝C6H5N2·NO3 + 2H2O,

or, as is more usually the case (since the diazonium salts themselves are generally used only in aqueous solution) by the addition of a well-cooled solution of potassium or sodium nitrite to a well-cooled dilute acid solution of the primary amine. In order to isolate the anhydrous diazonium salts, the method of E. Knoevenagel (Ber., 1890, 23, p. 2094) may be employed. In this process the amine salt is dissolved in absolute alcohol and diazotized by the addition of amyl nitrite; a crystalline precipitate of the diazonium salt is formed on standing, or on the addition of a small quantity of ether. The diazonium salts are also formed by the action of zinc-dust and acids on the nitrates of primary amines (R. Mohlau, Ber., 1883, 16, p. 3080), and by the action of hydroxylamine on nitrosobenzenes. They are colourless crystalline solids which turn brown on exposure. They dissolve easily in water, but only to a slight extent in alcohol and ether. They are very unstable, exploding violently when heated or rubbed. Benzene diazonium nitrate, C6H5N(NO3)∶N, crystallizes in long silky needles. The sulphate and chloride are similar, but they are not quite so unstable as the nitrate. The bromide may be prepared by the addition of bromine to an ethereal solution of diazo-amino-benzene (tribromaniline remaining in solution). By the addition of potassium bromide and bromine water to diazonium salts they are converted into a perbromide, e.g. C6H5N2Br3, which crystallizes in yellow plates.

The diazonium salts are characterized by their great reactivity and consequently are important reagents in synthetical processes, since by their agency the amino group in a primary amine may be exchanged for other elements or radicals. The chief reactions are as follows:—

1. Replacement of -NH2 by -OH:—The amine is diazotized and the aqueous solution of the diazonium salt is heated, nitrogen being eliminated and a phenol formed.

2. Replacement of -NH2 by halogens and by the -CN and -CNO groups:—The diazonium salt is warmed with an acid solution of the corresponding cuprous salt (T. Sandmeyer, Ber., 1884, 17, p. 2650), or with copper powder (L. Gattermann, Ber., 1890, 23, p. 1218; 1892, 25, p. 1074). In the case of iodine, the substitution is effected by adding a warm solution of potassium iodide to the diazonium solution, no copper or cuprous salt being necessary; whilst for the production of nitriles a solution of potassium cuprous cyanide is used. This reaction (the so-called “Sandmeyer” reaction) has been investigated by A. Hantzsch and J. W. Blagden (Ber., 1900, 33, p. 2544), who consider that three simultaneous reactions occur, namely, the formation of labile double salts which decompose in such a fashion that the radical attached to the copper atom wanders to the aromatic nucleus; a catalytic action, in which nitrogen is eliminated and the acid radical attaches itself to the aromatic nucleus; and finally, the formation of azo compounds.

3. Replacement of -NH2 by -NO2:—A well-cooled concentrated solution of potassium mercuric nitrate is added to a cooled solution of benzene diazonium nitrate, when the crystalline salt 2C6H5N2·NO3, Hg(NO2)2 is precipitated. On warming this with copper powder, it gives a quantitative yield of nitrobenzene (A. Hantzsch, Ber., 1900, 33, p. 2551).

4. Replacement of -NH2 by hydrogen:—This exchange is brought about, in some cases, by boiling the diazonium salt with alcohol; but I. Remsen and his pupils (Amer. Chem. Journ., 1888, 9, pp. 389 et seq.) have shown that the main product of this reaction is usually a phenolic ether. This reaction has also been investigated by A. Hantzsch and E. Jochem (Ber., 1901, 34, p. 3337), who arrived at the conclusion that the normal decomposition of diazonium salts by alcohols results in the formation of phenolic ethers, but that an increase in the molecular weight of the alcohol, or the accumulation of negative groups in the aromatic nucleus, diminishes the yield of the ether and increases the amount of the hydrocarbon formed. The replacement is more readily brought about by the use of sodium stannite (P. Friedlander, Ber., 1889, 22, p. 587), or by the use of a concentrated solution of hypophosphorous acid (J. Mai, Ber., 1902, 35, p. 162). A. Hantzsch (Ber., 1896, 29, p. 947; 1898, 31, p. 1253) has shown that the chlor- and brom- diazoniumthiocyanates, when dissolved in alcohol containing a trace of hydrochloric acid, become converted into the isomeric thiocyanbenzene diazonium chlorides and bromides. This change only occurs when the halogen atom is in the ortho- or para- position to the -N2- group.

Metallic Diazo Derivatives.—Benzene diazonium chloride is decomposed by silver oxide in aqueous solution, with the formation of benzene diazonium hydroxide, C6H5·N(OH)⫶N. This hydroxide, although possessing powerful basic properties, is unstable in the presence of alkalis and neutralizes them, being converted first into the isomeric benzene-diazotic acid, the potassium salt of which is obtained when the diazonium chloride is added to an excess of cold concentrated potash (A. Hantzsch and W. B. Davidson, Ber., 1898, 31, p. 1612). Potassium benzene diazotate, C6H5N2·OK, crystallizes in colourless silky needles. The free acid is not known; by the addition of the potassium salt to 50% acetic acid at −20° C., the acid anhydride, benzene diazo oxide, (C6H5N2)2O, is obtained as a very unstable, yellow, insoluble compound, exploding spontaneously at 0° C. Strong acids convert it into a diazonium salt, and potash converts it into the diazotate. On the constitution, of these anhydrides see E. Bamberger, Ber., 1896, 29, p. 446, and A. Hantzsch, Ber., 1896, 29, p. 1067; 1898, 31, p. 636. By the addition of the diazonium salts to a hot concentrated solution of a caustic alkali, C. Schraube and C. Schmidt (Ber., 1894, 27, p. 520) obtained an isomer of potassium benzene diazotate. These iso-diazotates are formed much more readily when the aromatic nucleus in the diazonium salt contains negative radicals. Potassium benzene iso-diazotate resembles the normal salt, but is more stable, and is more highly ionized. Carbon dioxide converts it into phenyl nitrosamine, C6H5NH·NO