4-aminoazobenzene

2C3H3N2.SO4H + NaaSOi + qHjO. C H-.N.SO H + C.HsNH = CoH NiN.NHCoH- + HoSO.  

—The sulphuric acid, set free in the second phase of the reaction, acts upon the sodium nitrite, so that one molecule only is required. 

Properties.—Golden yellow plates (from alcohol) m. p. 98 5 insoluble in water it explodes when heated above its nieltinig point. 

Rciufion.—Dissolve a little of the substance in alcohol and add a drop or two of an alcoholic solution of silver nitrate. A red crystalline precipitate of CqHjNiN.NAg.CjjH is deposited. See Appendix., p. 285. 

Allow the salt to dry for about 5 minutes. Heat for 1 hour on a water-bath the mixture so that the temperature of the latter is between 50° and 60° stir frequently. Dissolve the product in 50 cc. of glacial acetic acid, and pour the solution slowly into 400 cc. of water, stirring vigorously. After about 5 minutes filter by suction, wash with water and set aside to dry. 

Convert a part of the aminoazobenzene into the chloride as follows Dissolve 2 grams of the compound in 10 cc. of alcohol and pour the solution into a boiling mixture of 20 cc. of concentrated hydrochloric acid and 200 cc. of water. Boil for 10 minutes and set aside to crystallize. Filter and wash the crystals with water containing some hydrochloric acid. 

Aminoazobenzene crystallizes in yellow prisms, melts at 127°, and distils undecomposed above 360°. It crystallizes well from hot water in which it is difficultly soluble. 

Phenylhydrazine is nearly colorless when pure it melts at 17.5°, boils at 242°, and has the specific gravity 1.097 at 23°. The yield in this preparation should be about 10 grams. 

Benzene, the product of the reaction, boils at 80.4°. The yield should be about 6 grams. 

---

CgHgNa. While crystals m.p. 147 C, b.p. 267"C, darken rapidly in air. Prepared by reducing p-nitroaniline or aminoazobenzene. Oxidizing agents convert it to quinone derivatives, hence it cannot be diazotized with nitric acid. 

C,H5N,NHC,Hs+HC1 = C,H 5N C1+H,NC,H5- C H5N NC H NH +HC1 gen atom of the aniline molecule, giving aminoazobenzene. Since this reaction is irreversible, whilst the former is freely reversible, the final result is the complete conversion of the diazoaminobenzene into the aminoazobenzene. (The intermediate formation of the benzenediazonium chloride can be demonstrated by adding dimethylaniline, with which the diazonium chloride couples preferentially, giving dimethylaminoazobenzene, C6HsN NC6HiN(CH3)i.)... 

The conversion of the diazoaminobenzene into aminoazobenzene is promoted by the addition of aniline hydrochloride even more readily than by that of free hydrochloric acid. The aniline hydrochloride dissociates in solution giving hydrochloric acid and aniline the former promotes the formation of the above equilibrium, and the latter by increasing the active mass of the free aniline further accelerates the condensation to aminoazobenzene,... 

Aminoazobenzene is a very weak base, and consequently it will not form salts with weak organic acids, such as acetic acid, although it will do so with the strong mineral acids, such as hydrochloric acid. Aminoazobenzene is a yellowish-brown compound, whilst the hydrochloride is steel blue. The colour of the latter is presumably due to the addition of the proton to the phenyl-N-atom, the cation thus having benzenoid and quinonoid forms ... 

Aminoazobenzene is freely soluble in methylated spirit, although insoluble in water. For recrystallisation, therefore, dissolve the crude substance in boiling methylated spirit, remove from the water-bath, and then add water drop by drop until the solution becomes just cloudy owing to the separation of the solute replace the solution momentarily on the water-bath until the cloudiness disappears, and then at once remove the solution, and allow it to cool slowly. (Alternatively, the crude dry material can be reciystallised from carbon tetrachloride in the usual way.) Aminoazobenzene is thus obtained as yellowish-brown crystals, m.p. 126° yield, 5 g. 

To prepare the hydrochloride, add about i g. of aminoazobenzene to 200 ml. of dilute hydrochloric acid and boil until nearly all the solid material has dissolved. Filter hot and allow to cool slowly. Aminoazobenzene hydrochloride separates as beautiful steel-blue crystals filter and dry. If a small quantity of the powdered hydrochloride is moistened with water and a few drops of ammonia added, the blue hydrochloride is converted back to the yellowish-brown base. 

Aminoazobenzene is a member of the large class of azo-compounds, all of which contain the characteristic grouping, C-N N-C. Azo-compounds are of considerable technical importance, as they are all coloured, and the majority possess considerable stability. They may be prepared by the following methods ... 

Chalcones yield orange-red to brown-colored zones [4] as do 4-aminoazobenzene derivatives, but their colors begin to change after 10 min and slowly fade [5]. Penicillic acid is visible as a greyish-black zone [18]. 

Aminoanthraqumone, 12 Aminoazobenzene, 12 Aminoazotoluene, 12 2-Amino-4-chlorophenol, 12 2 - Amino-5 -diethylaminopentane, 12 Aminoethanol, 12... 

Reaction.—Make a solution of 4 grams stannous chloride in TO c.c. cone, hydrochloric acid, add 2 grams aminoazobenzene, and boil for a few minutes. On cooling ciystals of the hydrochlorides of aniline and yi-phenylenediamine separate out. The liquid is filtered and washed with a little cone, hydrochloric acid to remove the tin salts. If the precipitate is dissolved in water and made alkaline with caustic soda, a mixture of liquid aniline and solid/-phenylenediamine is precipitated, from which the former may be removed by filtering, washing, and draining on a porous plate. 

If aminoazobenzene is sulphonated with fuming sulphuric acid, and the product again diazotised and coupled w ith /3-naplilhol, Bicbrich scarlet is formed. 

As an example, take the molecule aminoazobenzene, one of the solutes listed in Table 39. When colorimetric measurements were made at room temperature on very dilute aqueous solutions of HC1, containing a trace of this substance, it was found that neutral molecules and (BH)+ ions were present in equal numbers when the concentration of the HCl was 0.0016 molal.1 At this low concentration the activity coefficient of the HCl is very near unity, and we may use (216) to find how far the vacant proton level provided by the aminoazobenzene molecule in aque-... 

Phenylazoanilines. See under Aminoazobenzenes and Derivatives in Vol 1, A184-R to A185-L. ... 

---