pembuatan nitrobenzene

NITROBENZENE

There are various methods of nitrating organic substances. The compound in question is sometimes stirred into an excess of either concentrated or fuming nitric acid; the acid itself serves as the solvent, for it usually dissolves the nitro compound if not the starting material. At the end of the reaction the mixture is

poured onto ice or water to precipitate the product. Another method is to dissolve the substance in glacial acetic acid and add just the calculated quantity of concentrated nitric acid; when the reaction is over the nitro compound, if it has suitable properties,will crystallize on cooling the solution. Concentrated sulfuric acid is a still better solvent for a great many organic compounds, particularly those containing nitrogen or oxygen

atoms, and it is used extensively in nitrations. Besides exerting a solvent action it serves another very useful purpose, namely,to absorb the water formed in the nitration:

C6II« + HONO2 V C6Hj\O2 + H2O

The sulfuric acid promotes the reaction because it prevents the water formed in the course of the nitration from diluting the nitric acid and decreasing its potency. It is not necessary to use a large excess of nitric acid or to elevate the temperature in order to push the reaction to completion. " Mixed acid," that is a mixture of nitric and sulfuric acids, is employed in the procedure given below. Nitrations usually are run at a fairly low temperature, often at o°. Otherwise there may be a considerable destruction of the organic material as the result of the oxidizing action of nitric acid. There are ways of increasing the intensity of the reaction

other than by elevating the temperature. The concentrated  nitric acid may be mixed with fuming sulfuric acid containing up to 60 per cent of sulfur trioxide and a still more potent mixture is made from fuming sulfuric acid and sodium nitrate, for the water introduced with concentrated nitric acid is thus eliminated.

It is sometimes inadvisable to use any but pure, colorless concentrated nitric acid, for the nitrous acid often present in the crude, yellow acid may give rise to secondary reactions. It is easily eliminated, however, by heating the crude acid with a small quantity of urea and passing in a stream of air:

2HSO2 + CO(NH2)2 -> CO2 + 2N2

Procedure. — To 40 cc. of concentrated sulfuric acid contained in a 500-cc. round-bottomed flask add in portions and with shaking 35 cc. of concentrated nitric acid (sp. gr. 1.4). Cool the nitrating mixture to room temperature and add, in portions of 2-$ cc, 30 cc. (26 g.) of benzene. Shake well after each addition

and keep the temperature at 50-600 throughout the reaction, cooling the flask when necessary in a pan of cold water (the reaction is exothermic). The addition of benzene takes about half an hour. To complete the reaction attach a reflux air condenser to prevent volatilization of the benzene and heat the flask in a pan of water at 6o° (bath temperature) for one-half hour. Shake well during this period in order to bring the immiscible reacting layers into intimate contact. At the end of the time specified, cool the mixture under the tap, pour it into

a separatory funnel, and draw off and discard the lower acid layer. To remove adhering acid shake the crude nitrobenzene thoroughly with about 100 cc. of water, separate the product (lower layer) and wash it with a second portion of water to which about 20 cc. of 10 per cent sodium hydroxide solution is added. After a final washing with water, separate the nitrobenzene carefully, pour it into a small Erlenmeyer flask, and dry

it with 6-8 g. of calcium chloride. Since the liquid retains water tenaciously, it is well to warm the mixture on the steam bath with shaking, for this increases the efficiency of drying. After a time the original cloudiness due to emulsified water should disappear. Distil the dried product from a small flask on a wire gauze over a flame, using an air condenser. There may be a fore-run containing benzene and a trace of water. After this is

removed, clean the condenser tube before continuing the distillation. Collect the fraction boiling at 200-2070 and do not distil quite to dryness, for there may be a residue of m-dinitrobenzene and this may decompose explosively if the temperature rises much higher than the boiling point of the' chief product. The yield should be about 30 g. Pure nitrobenzene is a clear, straw-colored liquid boiling at 206-2070 (uncor.). It is somewhat

poisonous; avoid inhaling the vapor or getting the material on the skin. Like other oxygen-containing compounds, nitrobenzene is soluble in concentrated sulfuric acid (oxonium salt). That it is not found dissolved in the sulfuric acid present at the end of the above nitration is because the acid is so diluted by the water present in the nitric acid and by that formed in the course of the reaction that the oxonium salt is not stable.

NOTE: It is sometimes noted that the product turns brown during the distillation with the evolution of oxides of nitrogen. This is because the nitric acid has not been removed completely by washing and reacts further to produce m-dinitrobenzene. In such a case it is best to stop the distillation at once and repeat the washing and drying of the product.

Experiments in organic chemistry.Fieser 1941