Preparing solution

Meanwhile, during the cooling of the cuprous chloride solution, prepare a solution of benzenediazonium chloride by dissolving 20 ml. (20-5 g.) of aniline in a mixture of 50 ml. of concentrated hydrochloric acid and 50 ml. of water, and after cooling to 5°, adding slowly a solution of 17 g. of sodium nitrite in 40 ml. of water. Observe carefully the general conditions for diazotisation given in the preparation of iodobenzene (p. 184). 

Place 10 ml. of 1% starch solution (prepared as described above) in a boiling-tube, add 2 ml. of 1% sodium chloride solution and place the tube in a water-bath maintained at 38-40 . Place about 5 ml. of water in a series of test-tubes and to each add a few drops of 1% iodine solution. Now add 4 ml. of the diluted saliva solution to the starch solution, mix well and note the time. At intervals of about 30 seconds transfer 2 drops of the reacting mixture, by means of a dropping tube, to one of the test-tubes, mix and note the colour. As in the previous experiment, the colour, which is blue at first, changes to blue-violet, red-violet, red-brown, pale brown, and finally disappears at this stage the solution will reduce Fehling s solution. If the reaction proceeds too quickly for the colour changes to be observed, the saliva solution should be diluted. 

A solution prepared by dissolving 2 g. of biomine in 100 g. of carbon tetra. chloride is satisfactory. Carbon tetrachloride is employed because it is an excellent solvent for bromine as well as for hydrocarbons it possesses the additional advan. tage of low solubility for hydrogen bromide, the evolution of which renders possible the distinction between decolourisation of bromine due to substitution or due to addition. 

In a 1-litre three-necked flask, fitted with a mechanical stirrer, reflux condenser and a thermometer, place 200 g. of iodoform and half of a sodium arsenite solution, prepared from 54-5 g. of A.R. arsenious oxide, 107 g. of A.R. sodium hydroxide and 520 ml. of water. Start the stirrer and heat the flask until the thermometer reads 60-65° maintain the mixture at this temperature during the whole reaction (1). Run in the remainder of the sodium arsenite solution during the course of 15 minutes, and keep the reaction mixture at 60-65° for 1 hour in order to complete the reaction. AUow to cool to about 40-45° (2) and filter with suction from the small amount of solid impurities. Separate the lower layer from the filtrate, dry it with anhydrous calcium chloride, and distil the crude methylene iodide (131 g. this crude product is satisfactory for most purposes) under diminished pressure. Practically all passes over as a light straw-coloured (sometimes brown) liquid at 80°/25 mm. it melts at 6°. Some of the colour may be removed by shaking with silver powder. The small dark residue in the flask solidifies on cooling. 

Pour the reaction mixture cautiously into 400 g. of crushed ice and acidify it in the cold by the addition of a solution prepared by adding 55 ml. of concentrated sulphuric acid to 150 ml. of water and then coohng to 0°. Separate the ether layer and extract the aqueous layer twice with 50 ml. portions of ether. Dry the combined ethereal solutions over 50 g. of anhydrous potassium carbonate and distil the filtered solution thror h a Widmer column (Figs. II, 17, 1 and II, 24, 4). Collect separately the fraction boihng up to 103°, and the dimethylethynyl carbinol at 103-107° Discard the high boiling point material. Dry the fraction of low boihng point with anhydrous potassium carbonate and redistil. The total 3 ield is 75 g. 

Dissolve 46-5 g. (45-5 ml.) of aniUne in a mixture of 126 ml. of concentrated hydrochloric acid and 126 ml. of water contained in a 1-htre beaker. Cool to 0-5° in a bath of ice and salt, and add a solution of 36-5 g. of sodium nitrite in 75 ml. of water in small portions stir vigorously with a thermometer (1) and maintain the temperature below 10°, but preferably at about 5° by the addition of a httle crushed ice if necessary. The diazotisation is complete when a drop of the solution diluted with 3-4 drops of water gives an immediate blue colouration with potassium iodide - starch paper the test should be performed 3-4 minutes after the last addition of the nitrite solution. Prepare a solution of 76 g. of sodium fluoborate (2) in 150 ml. of water, cool, and add the chilled solution slowly to the diazonium salt solution the latter must be kept well stirred (1) and the temperature controlled so that it is below 10°. Allow to stand for 10 minutes with frequent stirring. Filter... 

Concurrently with the preparation of the phenyldiazonium chloride solution, prepare a cold suspension of sodium arsenite. Place 250 ml. of water in a 3-htre round-bottomed flask equipped with a mechanical stirrer. Heat the water to boding, add 125 g. of anhydrous sodium carbonate, and, as soon as the carbonate has dissolved, introduce 62 5 g. of pure arsenious oxide and 3 g. of crystallised copper sulphate with stirring. When all the solids have dissolved, cool the solution with stirring under a stream of tap water until the temperature has fallen to 15°. 

For many reductions it is not necessary to distil the reagent. Dilute the dark solution, prepared as above to the point marked with an asterisk, to 1 htre with dry isopropyl alcohol this gives an approximately one molar solution. Alternatively, prepare the quantity necessary for the reduction, using the appropriate proportions of the reagents. 

The following procedures may be used for the preparation of ethereal solutions of diazomethane containing ethyl alcohol they differ slightly according to as to whether large or small quantities are required. The presence of alcohol is not harmful for many appUcatioiis of diazomethane. (It may be pointed out that ethereal diazomethane solution prepared from nitrosomethylurea is free from alcohol.)... 

Evidence from the viscosities, densities, refractive indices and measurements of the vapour pressure of these mixtures also supports the above conclusions. Acetyl nitrate has been prepared from a mixture of acetic anhydride and dinitrogen pentoxide, and characterised, showing that the equilibria discussed do lead to the formation of that compound. The initial reaction between nitric acid and acetic anhydride is rapid at room temperature nitric acid (0-05 mol 1 ) is reported to be converted into acetyl nitrate with a half-life of about i minute. This observation is consistent with the results of some preparative experiments, in which it was found that nitric acid could be precipitated quantitatively with urea from solutions of it in acetic anhydride at —10 °C, whereas similar solutions prepared at room temperature and cooled rapidly to — 10 °C yielded only a part of their nitric acid ( 5.3.2). The following equilibrium has been investigated in detail ... 

Certain features of the addition of acetyl nitrate to olefins in acetic anhydride may be relevant to the mechanism of aromatic nitration by this reagent. The rapid reaction results in predominantly cw-addition to yield a mixture of the y -nitro-acetate and y5-nitro-nitrate. The reaction was facilitated by the addition of sulphuric acid, in which case the 3rield of / -nitro-nitrate was reduced, whereas the addition of sodium nitrate favoured the formation of this compound over that of the acetate. As already mentioned ( 5.3. i), a solution of nitric acid (c. i 6 mol 1 ) in acetic anhydride prepared at — 10 °C would yield 95-97 % of the nitric acid by precipitation with urea, whereas from a similar solution prepared at 20-25 °C and cooled rapidly to —10 °C only 30% of the acid could be recovered. The difference between these values was attributed to the formation of acetyl nitrate. A solution prepared at room... 

The evidence outlined strongly suggests that nitration via nitrosation accompanies the general mechanism of nitration in these media in the reactions of very reactive compounds.i Proof that phenol, even in solutions prepared from pure nitric acid, underwent nitration by a special mechanism came from examining rates of reaction of phenol and mesi-tylene under zeroth-order conditions. The variation in the initial rates with the concentration of aromatic (fig. 5.2) shows that mesitylene (o-2-0 4 mol 1 ) reacts at the zeroth-order rate, whereas phenol is nitrated considerably faster by a process which is first order in the concentration of aromatic. It is noteworthy that in these solutions the concentration of nitrous acid was below the level of detection (< c. 5 X mol... 

Expts. II, /J, 16. Solutions prepared from pure nitric acid. 

Expts. 12, l, 18. Solutions prepared from fuming nitric acid. 

Despite the considerable amount of work which has been reported, our knowledge of the nitration of biphenyl is not in a satisfactory state. The 0 p-T3.tw varies considerably with the conditions of nitration, and the cause of the variation is not fully understood. Nitrations with solutions prepared from nitric acid and acetic anhydride have generally given o -ratios greater than unity, the most consistent value being 2-2, obtained at o °C. The corresponding partial rate factors are reported later. 

The nitration of nitro- and dinitro-biphenyls has been examined by several workers. i - As would be expected, nitration of the nitro-biphenyls occurs in the phenyl ring. Like a phenyl group, a nitrophenyl group is 0 -directing, but like certain substituents of the type CH CHA ( 9.1.6) it is, except in the case of w-nitrophenyl, deactivating. Partial rate factors for the nitration at o °C of biphenyl and the nitro-biphenyls with solutions prepared from nitric acid and acetic anhydride are given below. The high o p-v2X o found for nitration of biphenyl... 

Another method of aldehyde bromination, apart from Riehl s established method (432) from bromine at 20°C, is to use trimethylphenyl-ammonium bromide in tetrahydrofuran solution, prepared by Vorlander and Siebert s method (50). However, the yield of 5-phenylthiazole using this method with thioformamide dissolved in dioxane is only 8% (513). 

Source A. J. Bard, R. Parsons, and J. Jordan (eds.). Standard Potentials in Aqueous Solution (prepared under the auspices... 

Dextrin Polyethylene glycol 400 Use 5 mL of 2% aqueous solution of chloride-free dextrin per 25 mL of 0. IM halide solution. Prepare a 50% (v/v) aqueous solution of the surfactant. Use 5 drops per 100 mL end-point volume. 

Calculate the molarity of a potassium dichromate solution prepared by placing 9.67 g of K2Cr207 in a 100-mF volumetric flask, dissolving, and diluting to the calibration mark. 

A more challenging problem is to find the pH of a solution prepared from a polyprotic acid or one of its conjugate species. As an example, we will use the amino acid alanine whose structure and acid dissociation constants are shown in Figure 6.11. 

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