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Compunds Containing Two Unlike Substituents
140. Preparation and Properties of Trichloracetic Acid (Section 231). - (a) Hood. - In a 250 cc. flask place 25 grams of chloral hydrate and add 15 cc. of fuming nitric acid (sp. gr. 1.5). Heat the flask over a small flame until reaction begins; then remove the flame. When the reaction has apparently ceased, heat gently until the liquid becomes almost colorless. (Eq.) Transfer the liquid to a small distilling flask and distil very slowly, using an air-condenser. When the temperature reaches 150°, change the receiver, using this time a small distilling flask, and collect what comes over up to 200°. Redistil slowly and collect the fraction boiling at 193°-195°. Weigh the product and calculate the percentage yield.
Trichloracetic acid melts at 55° and boils at 195°. The amount obtained in this preparation should be about 10 to 12 grams.
(b) Dissolve in a test-tube about 1 gram of the acid in water, and add 5 cc. of a dilute solution of sodium hydroxide. Attach to the tube by means of a cork a piece of glass tubing about 3 feet long, clamp the tube over a wire gauze, and heat gently so that the solution just boils. At the end of 5 minutes cool the tube, examine the contents and note the odor. (Eq.)
141. Preparation and Properties of Lactic Acid (Section 239). - (a) In a 750 cc. flask dissolve 50 grams of cane-sugar or anhydrous glucose in 500 cc. of water; add 20 cc. of sour milk and 20 grams of precipitated calcium carbonate. Set the flask in a warm place (temperature about 40°) and allow the fermentation to proceed for one week. The flask should be shaken at least twice a day. At the end of the time add a small amount of calcium carbonate, heat to boiling, filter, pour the solution into a flat dish, and let it evaporate spontaneously. Filter off the solid by suction, dissolve it in a small amount of boiling water, filter if necessary, and set the solution aside to crystallize. Separate the crystals as before and let them dry on a porous plate. The yield is about 35 grams.
(b) Lactic acid and ferric chloride: test for an a-hydroxy acid. - The solution of lactic acid required for this and the following experiments can be made from lactic acid, or prepared from the calcium lactate obtained in (a) above, as follows: Dissolve about 2 grams of the salt in 20 cc. of boiling water and add a dilute solution of sulphuric acid, drop by drop, as long as a precipitate is formed. If it is difficult to determine when the correct amount of acid has been added, filter onto a watch-glass a few drops of the solution and add to the filtrate a drop of sulphuric acid. If an excess of sulphuric acid has been used, add to the original solution a little of a solution of calcium lactate. When all the calcium has been precipitated, warm the solution and filter. In a small evaporating dish add to 5 cc. of the solution 1 drop of a 10 per cent aqueous solution of ferric chloride. Repeat the test, using 5 cc. of water instead of the solution of the acid. Compare the color in the two cases.
(c) Solubility of lactic acid. - Determine whether lactic acid can be extracted from water by ether. Describe fully how you made the test.
(d) Application of the iodoform reaction to lactic acid. - Determine whether lactic acid gives the iodoform reaction (experiment 81b, page 57).
Note. - (a) The sour milk used in the preparation contains the lactic acid bacteria. The milk should be used before the curd has separated. The latter is needed for the development of the bacteria during fermentation. Calcium carbonate is added to neutralize the lactic acid as it is formed, since the fermentation stops if the acid accumulates. Toward the end of the fermentation butyric acid is usually formed. The odor produced is due to this cause. When the sugar has been fermented the calcium carbonate passes into solution. If there is a large residue at the end of a week the milk was not satisfactory. The zinc salt of lactic acid is characteristic. It can be prepared by digesting a strong solution of the acid with zinc carbonate, filtering hot, and evaporating to crystallization.
142. Properties of Tartaric Acid and Tartrates (Sections 251-254). - (a) Behavior of tartaric acid and tartrates on heating. - Heat a little tartaric acid in a test-tube. Note the change in appearance and the odor produced. What products are formed? Repeat, using Rochelle salt.
(b) Tartaric acid and ferric chloride. - Apply the test for an a-hydroxy acid as follows: Dissolve about 0.1 gram of tartaric acid in 100 cc. of water. To 20 cc. of this solution add 1 drop of a 10 per cent aqueous solution of ferric chloride. Compare the color with that produced by adding 1 drop of ferric chloride to 20 cc. of water, and in a second tube to 20 cc. of water containing 1 drop of acetic acid. The depth of color can be more readily observed by looking down the tubes placed on white paper.
(c) Potassium salts of tartaric acid. - Make a cold saturated solution of tartaric acid by shaking an excess of the acid with 5 cc. of water. Decant off the liquid and add, drop by drop, a 10 per cent solution of potassium hydroxide, until a crystalline precipitate forms (Eq.); then add an excess of the solution of potassium hydroxide. (Eq.)
(d) Calcium tartrate: test for tartrates. - Neutralize a solution of tartaric acid or of cream of tartar with ammonia, and add to the solution 1 cc. of a solution of calcium chloride. (Eq.) If a precipitate of calcium tartrate does not appear, rub a glass rod against the side of the tube under the surface of the liquid. Filter off the precipitate, and pour 5 cc. of a solution of sodium hydroxide through the filter into a clean test-tube. Heat the filtrate to boiling. This behavior of tartaric acid is characteristic, and is used in the identification of the acid and its soluble salts.
(e) Reducing action of tartrates. - To a dilute solution of Rochelle salt or sodium tartrate contained in a test-tube, which has been cleaned by boiling in a solution of sodium hydroxide and washing with water, add 1 cc. of a solution of silver nitrate. Dissolve the precipitate by adding ammonia, drop by drop, avoiding excess, and place the tube in hot water. Compare the results with those obtained with formaldehyde (experiment 106d). Is the formation of a silver mirror a positive test for an aldehyde?
(f) Fehling's solution. - Add to 1 cc. of a solution of copper sulphate 5 cc. of a solution of sodium hydroxide (Eq.); then add a solution of Rochelle salt or of sodium tartrate until the precipitate dissolves. (Eq.) Repeat, using glycerine in place of Rochelle salt. It is necessary to have a large excess of sodium hydroxide. What organic compounds other than acids dissolve copper hydroxide? Determine if Rochelle salt will dissolve the hydroxides of iron and aluminium.
Notes. - (b) This test for a-hydroxy acids must be made in the cold, as a solution of ferric chloride alone develops a decided yellow tint on being heated. In applying the test to an unknown substance a very dilute solution should be used, and the color developed should be compared with that formed in a solution of equal concentration of an a-hydroxy acid (tartaric acid). Many solutions of hydroxyl compounds when sufficiently concentrated give a yellow color with ferric chloride. The colors given by poly-basic and ketonic acids are usually more intense or of a different shade from those given by a-hydroxy acids.
(d) In making this test the calcium tartrate is usually filtered off before solution in sodium hydroxide, since the carbonate that the alkali contains produces a precipitate of calcium carbonate, which is insoluble. The presence of ammonium salts should be avoided in making the test, as they interfere with the precipitation of calcium tartrate.
143. Properties of Citric Acid and Citrates (Section 258). - (a) Decomposition of citric acid by heat. - Heat a little citric acid in a dry test-tube. Note the odor. What acids are present among the products of decomposition when citric acid is heated? (See Section 113.)
(b) Calcium citrate: test for citric acid. - Dissolve about 1 gram of citric acid in about 50 cc. of water. Neutralize 5 cc. of the solution with ammonia, avoiding an excess of the latter. If too much has been added, add a little of the solution of the acid until the reaction is neutral or slightly acidic. To the solution of ammonium citrate add about 2 cc. of a 10 per cent solution of calcium chloride. Is a precipitate formed? Compare the behavior of tartrates. Heat the solution to boiling and set aside. Examine in a few minutes. A precipitate of tricalcium citrate is formed. Is the precipitate of tricalcium citrate crystalline? Wash the salt by decantation three times with water, and determine its solubility in a solution of sodium hydroxide. Compare the result with that obtained with tartrates. Preserve the solution of citric acid for experiments (d) and (e) below.
(c) Presence of citric acid in lemon juice. - Test 5 cc. of lemon juice or pineapple juice for citric acid as described above. Add ammonia to the filtered juice but have it slightly acid when making the test.
(d) Reducing power of citrates. - Neutralize 5 cc. of the solution of citric acid prepared in (b) above with sodium hydroxide, add 1 cc. of a solution of silver nitrate, and then ammonia drop by drop, until the precipitate is dissolved. Place the tube in boiling water for two minutes. Is a mirror of silver formed? (Compare with tartrates.) Heat the solution to boiling for two minutes. Set aside and examine later.
(e) Reduction of ferric citrate by light. - Add to 5 cc. of a solution of ferric chloride, until a clear green color is formed, a solution of ammonium citrate, prepared by neutralizing a solution of citric acid with ammonia. Moisten a piece of paper with the solution, dry the paper in the dark in your desk and then expose it to sunlight under a piece of thick paper in which a design has been cut. At the end of 10 minutes place the paper in a dilute solution of potassium ferricyanide. Write the formulas for all the substances involved in the reactions, starting with ferric chloride.
Note. - (b) In the presence of a large excess of sodium hydroxide, calcium chloride gives a precipitate in the cold with solutions of alkali citrates. Calcium citrate is soluble in acetic acid (calcium oxalate is insoluble).
144. Preparation of Acetoacetic Ester (Section 263). - (a) The ethyl acetate to be used in this preparation must be dried and distilled just before use. Place 250 cc. of ethyl acetate, which should be neutral, in a 500 cc. flask, and add anhydrous calcium chloride (about one-fourth of the volume of the ester). Let the mixture stand over night or longer. Distil directly on a waterbath from the calcium chloride, and collect the distillate in a dry receiver which is protected from the moisture of the air. Provide a reflux condenser for a 500 cc. flask, which has been carefully dried, and set the flask in cold water. Add 200 grams of the dried ethyl acetate and 20 grams of sodium in the form of wire or very thin shavings (see §41, page 27). Connect the condenser with the flask. When the evolution of hydrogen ceases, heat the flask on a water-bath at such a temperature that the contents boil gently. When all the sodium has dissolved, about 3 hours being required, cool the flask, and add slowly a mixture of 1 part of concentrated hydrochloric acid and 4 parts of water, until the solution shows an acid reaction after having been shaken thoroughly. Add an equal volume of a saturated solution of sodium chloride, transfer to a separatory funnel, and separate the ester as carefully as possible from water. Shake the ester with a little anhydrous calcium chloride until it becomes clear and no water can be seen, pour off from the drying agent, and distil the liquid slowly from a flask provided with a small Hempel tube (see Fig. 5, page 11). Discard the distillate which boils up to 160°; collect the following fractions: 160°-175°, 175°-185°, 185°-200°. The fraction boiling at 175°-185° is nearly pure acetoacetic ester. By redistilling the other fractions a further quantity can be obtained, but as acetoacetic ester decomposes slowly on boiling at atmospheric pressure, it is not advisable to carry out an extended fractionation. If a better yield is desired, the product of the reaction may be distilled under diminished pressure.
Acetoacetic ester boils at 182° at 760 mm., 74° at 14 mm., and 88° at 29 mm. It has the specific gravity of 1.03 at 15°. The yield in this preparation should be 35-40 grams.
(b) Compare the solubility of acetoacetic ester in water and in a solution of sodium hydroxide. Explain.
(c) Dissolve a few drops of acetoacetic ester in 1 cc. of alcohol and add a few drops of a solution of ferric chloride. What does the result indicate?
145. Acetoacetic Ester Synthesis (Section 264). - (a) Preparation of ethylacetoacetic ester. - Into a 500 cc. flask place 60 cc. of absolute alcohol. Weigh 4.6 grams of sodium and cut it into five or six pieces. Hold the flask under running water and add one piece of the metal. At the end of about a minute, when the violent reaction ceases, add the rest of the sodium and connect the flask by means of a tightly fitting cork or rubber stopper with a long reflux condenser through which a rapid stream of water is passing. When the metal has dissolved add slowly through the condenser 26 grams of acetoacetic ester, and then 24 grams of ethyl bromide. Heat slowly to boiling on a water-bath until a drop of the solution when mixed with a drop of water does not show an alkaline reaction. Avoid too rapid heating at first, as the reaction may become violent and ethyl bromide be lost. The time required is about 2 hours. When the reaction is complete, distil off the excess of alcohol on the water-bath, add 50 cc. of water to the residue, and shake until the salt has dissolved. Separate the oil in a small separatory funnel, dry over calcium chloride and distil. Collect the portion which boils at 192°-200°. Calculate the percentage yield. Write equations for all the reactions involved.
Ethylacetoacetic ester boils at 198°. The yield should be about 22 grams.
(b) Hydrolysis of ethylacetoacetic ester (Section 264). - Ketone hydrolysis. - In a 500 cc. flask dissolve 10 grams of sodium hydroxide in 100 cc. of water, add 15 grams of ethylacetoacetic ester, and connect with a reflux condenser. Place the flask in a bath containing boiling water for 2 hours. Cool, place the contents of the flask in a small separatory funnel, separate the oil, and dry over calcium chloride. Pour off the oil from the drying agent after a few hours and distil it. Collect what boils at 98°-103°.
Methyl propyl ketone, the product of the reaction, boils at 102°. The yield obtained is about 60 per cent of the theoretical.
Acid hydrolysis. - In a small flask dissolve 5 grams of sodium hydroxide in 15 cc. of water and add 5 grams of ethylacetoacetic ester. Shake vigorously and connect the flask with a reflux condenser. Place the flask in boiling water for 2 hours. At the end of this time distil with steam (§28, page 18) until about 100 cc. of water have passed over. (What else passes over?) Cool the flask through which steam has been passed, make the contents acid to litmus with dilute sulphuric acid, and then add about 50 cc. of the acid in excess to liberate the organic acids formed in the hydrolysis. Distill again with steam into a clean receiver until about 300 cc. of water have passed over. Note the odor of this second distillate. Neutralize the latter with sodium hydroxide and evaporate to dryness. The residue is a mixture of sodium acetate and sodium butyrate. Put a little of the solid on a watch-glass and add a few drops of concentrated sulphuric acid. Note the odor. Add to this a few drops of alcohol, and after warming gently note the odor.
The presence of butyric acid in this mixture can be shown in another way as follows: Dissolve about 0.5 gram of the salt in 5 cc. of water. Acidify with dilute sulphuric acid, warm to liberate any carbon dioxide present. Neutralize with ammonia, and add a solution of calcium chloride. If a precipitate is formed, it is calcium butyrate. Filter this off and heat the filtrate to boiling. Since calcium butyrate is slightly less soluble in hot water than in cold, the cold saturated solution deposits a precipitate when heated to boiling. Write equations for all reactions involved in the experiment.
Note. - (a) If the solution of sodium acetoacetate and ethyl bromide does not become neutral on boiling, it is evident that some of the latter has been lost. This may result from too active boiling at first or there may have been a leak in the stopper connecting the flask with the condenser. If this occurs add more of the bromide and heat again.
146. Properties of Chloral (Section 267). - (a) Preparation from chloral hydrate. - Shake in a small test-tube about 3 grams of chloral hydrate and 3 cc. of concentrated sulphuric acid. (Eq.) Separate by means of a pipette the oil which floats on the sulphuric acid, and add it to one-half its volume of water. (Eq.)
(b) Test for the aldehyde group. - Determine whether chloral hydrate forms a silver mirror with ammoniacal silver nitrate (experiment 106d, page 81), and whether it produces a color with Schiff's reagent.
(c) Decomposition of chloral hydrate by alkalies. - Dissolve about 1 gram of chloral hydrate in water and add a solution of sodium hydroxide. (Eq.) Test for chloroform.