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Chapter XXI

Aromatic Acids

200. Preparation of Benzoic Acid from Benzyl Chloride (Section 462). - In a 500 cc. flask provided with a return condenser like that shown in Fig. 15, page 26, place 20 grams of benzyl chloride, 100 cc. of water, 40 cc. of concentrated nitric acid, and half a dozen small pieces from a broken porous plate to prevent bumping. Place the flask on a wire gauze and boil for about 4 hours. When the oxidation is complete, the oil, which at first floats, becomes heavier than the aqueous solution, and when the flame is removed for an instant sinks to the bottom of the flask. Cool under running water, shaking vigorously to avoid the formation of large lumps. Filter by suction and wash with cold water. Dissolve the crystals in the smallest amount of boiling water, about 900 cc., add 2 grams of bone-black, boil for 3 minutes, and filter hot; set aside over night to crystallize. Filter off the crystals by suction, and wash with cold water. Let the acid stand in the air until dry. Weigh and determine the melting-point. If the acid does not melt sharply, sublime a little of it (§35, page 23), and determine the melting-point again. Calculate the percentage yield. How could you readily distinguish a solution of phenol from one of benzoic acid? How could you distinguish a solid acid from a solid phenol? How could you obtain pure benzoic acid from sodium benzoate?

Benzoic acid melts at 121.4° and boils at 249°. One hundred parts of water at 17.5° dissolve 0.268 parts, and at 75° dissolve 2.19 parts of the acid. It is readily soluble in alcohol and in ether. The yield should be about 14 grams.

Note. - In oxidizing aromatic compounds with nitric acid, the latter is usually diluted with water in order to prevent nitration. An acid of the specific gravity 1.15 is ordinarily used.

201. Identification of Benzoic Acid: Conversion into Benzanilide - To definitely identify as benzoic acid an unknown substance which has the melting-point and physical properties of this acid, it should be converted into a solid derivative, and the properties of this compound determined. Benzanilide, C6H5CO.NHC6H5, is conveniently prepared and melts sharply. Proceed as follows: To about 0.2 gram of the acid in a dry test-tube add about 0.4 gram of phosphorus pentachloride; warm and stir the mixture until a homogeneous liquid is obtained. Cool, shake with about 5 cc. of water, decant off the latter, and add slowly about 1 cc. of aniline. Dissolve the product in a boiling mixture of 25 cc. of alcohol and 25 cc. of water. Filter the solution if necessary, cool, and filter off the crystals; wash with a little dilute alcohol, and dry the compound at 100°. Determine the melting-point. Benzanilide crystallizes in pearly, white scales which melt at 160°.

202. Preparation of Benzamide (Section 465). - Hood. - Place in a small flask 5 grams of benzoic acid and 9 grams of phosphorus pentachloride (weighed under the hood), and shake and warm on the steam-bath until a clear liquid is formed. Cool, and add the liquid to 150 grams of ice and 50 cc. of concentrated ammonia in a beaker. Stir occasionally until the liquid has changed to a solid. If the substance when pressed with a glass rod appears to be plastic, the reaction is not complete; it must be more or less granular. Filter by suction, wash with dilute ammonia, and crystallize from the smallest possible amount of boiling water (about 50 cc.). Weigh the product and determine its melting-point. A small amount of amide can be recovered from the mother-liquor.

Benzamide melts at 128°-130°. The yield is about 4 grams.

Determine whether benzamide shows the properties of an acid amide, and describe in your notes the test applied and the result obtained.

203. Preparation of p-Toluic Acid from the Nitrile (Section 467). - Hydrolyze the tolunitrile obtained in experiment 189, page 154, by heating it with sulphuric acid as follows: For each gram of nitrile use 6 grams of concentrated sulphuric acid and 2 cc. of water. Heat on a sand-bath the mixture contained in a flask provided with a wide-bore reflux condenser, until the acid sublimes freely into the condenser. Cool, dilute with about 3 volumes of water, filter off the crystals, and wash them several times with cold water. Dissolve the toluic acid in hot alcohol, add about 1 gram of bone-black, and boil for 5 minutes. Filter hot, and add boiling water until the solution clouds. Set aside the solution to crystallize. Filter off the crystals, wash with cold water, and dry on a porous plate. Determine the percentage yield. A further amount of acid can be obtained from the mother-liquor. Save the acid for a later experiment.

p-Toluic acid melts at 180° and boils at 275°. The yield of acid is the theoretical, although some of it is lost in the purification.

204. Preparation of Cinnamic Acid by Perkin's Synthesis (Section 469). - Place in a small round-bottomed flask provided with a reflux air-condenser, in the end of which is a drying tube containing calcium chloride, 20 grams of freshly distilled benzaldehyde, 30 grams of acetic anhydride, and 10 grams of freshly fused and pulverized anhydrous sodium acetate. (See experiment 66, page 43.) Heat the mixture in an oil-bath at 170°-180° for 8 hours. Pour the mixture into 500 cc. of water, place about 100 cc. of a 10 per cent solution of sodium carbonate in the flask, and shake to dissolve most of the residue. Add this solution to the mixture of water and cinnamic acid, heat to boiling, and add solid sodium carbonate as long as anything dissolves and the solution has a strong alkaline reaction. Stir vigorously during the addition in order to bring the oil in contact with the sodium carbonate. A small insoluble residue is left. Filter the solution hot and add an excess of hydrochloric acid. Filter off the acid when the solution has cooled, and wash it with cold water. Dissolve the acid in boiling water, add about 2 grams of bone-black, boil for 5 minutes, filter hot, and set aside to crystallize. Weigh the acid, after drying on a porous plate, determine its melting-point, and calculate the percentage yield from the sodium acetate used.

Cinnamic acid melts at 134°; it is soluble in 3500 parts of water at 17° and in 100 parts at 1000. The yield is from 12 to 14 grams.

205. Properties of Cinnamic Acid (Section 469). - (a) Test for the double bond in cinnamic acid. - Dissolve about 0.5 gram of the acid in carbon tetrachloride and add, drop by drop, a solution of bromine in the same solvent. If no reaction takes place in 1 minute, warm the solution gently. (Eq.) How does the rate at which this reaction proceeds compare with the rate with which ethylene and bromine react?

(b) Oxidation of cinnamic acid. - Stir about 0.1 gram of the acid with about 5 cc. of a cold 10 per cent solution of potassium permanganate and note the odor. (Eq.) What acid would be obtained by the further oxidation of cinnamic acid?

Note. - (a) The rate at which bromine is added to a double bond is determined by the nature of the radicals linked to the carbon atoms joined by the double bond; negative atoms or groups markedly retard the reaction. In the case of certain compounds which contain four negative groups, for example tetraphenylethylene, (C6H5)2C = C(C6H5)2, addition does not take place at all.

206. Preparation of Terephthalic Acid from p-Toluic Acid (Section 476). - Dissolve 5 grams of the toluic acid prepared in experiment 203 in a solution of 3 grams of sodium hydroxide in 250 cc. of water. Heat the solution on a water-bath and add a solution of 12 grams of potassium permanganate in 250 cc. of water until the color of the permanganate persists after heating for 5 minutes. Add to the hot solution enough alcohol to react with the excess of permanganate, and filter hot. Wash the precipitate of manganese dioxide with a little hot water, and add an excess of concentrated hydrochloric acid to the filtrate. Filter off the terephthalic acid when the solution has cooled. Weigh the acid and calculate the yield. Sublime a little of the acid.

Terephthalic acid is almost insoluble in hot and cold water. It sublimes without melting. It can be identified most readily by converting it into its dimethyl ester as described below.

207. Preparation of Dimethyl Terephthalate. - In a dry test-tube warm together on the steambath or in hot water about 0.2 gram of terephthalic acid and 0.6 gram of phosphorus pentachloride. (Eq.) Cool the mixture and add in small portions, keeping the mixture cold in running water, 4 cc. of methyl alcohol. (Eq.) Warm the mixture cautiously for a short time until reaction is complete. Cool again, add 20 c. of water, filter by suction, and wash the precipitate with 10 cc. of water. Dissolve the dimethyl terephthalate in a boiling mixture of 8 cc. of methyl alcohol and 2 cc. of water. Filter hot, using a small funnel. Cool, filter off the crystals by suction, and wash them with a mixture of 3 cc. of methyl alcohol and 3 cc. of water. Dry the crystals on a porous plate and determine their melting-point.

Dimethyl terephthalate melts at 140°.

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