Chapter X

Halogen Compunds

129. Preparation of Methyl Iodide from Methyl Alcohol and Phosphorus Iodide (Sections 49, 204). - In a 200 cc. round-bottomed flask place 15 grams of methyl alcohol and 3.2 grams of red phosphorus. Have ready a reflux condenser with cork attached. Place the flask in cold water and add in small portions at a time 38 grams of iodine; the addition should take about 10 minutes. If the contents of the flask begin to boil, attach it to the reflux condenser; when reaction ceases add more iodine. Finally attach the flask to the condenser and let it stand for at least 4 hours (preferably over night). Distil through a condenser from a water-bath, as long as any liquid passes over. The receiver should be placed in cold water as methyl iodide is very volatile. Wash the distillate by decantation with a dilute aqueous solution of sodium hydroxide until the lower layer is colorless, and then once with water. Separate the methyl iodide carefully from the water using a separatory funnel, and transfer it to a small distilling flask. Add about 10 grams of anhydrous calcium chloride. Stopper the flask, place a cork over the end of the side-arm, and set aside until the liquid is quite clear. Place a thermometer in the flask, connect the latter with a condenser, and distil from a water-bath. Note the boiling-point and weight of the methyl iodide. Calculate the theoretical yield from the iodine used (why iodine?), and the percentage of this obtained.

Methyl iodide boils at 44°, and has the specific gravity 2.27 at 15°. The yield in the experiment should be about 80 per cent of the theory.

Note. - An excess of the alcohol is usually taken in preparing alkyl halides by the method illustrated above. The phosphorus and iodine are used in the proportions necessary to form phosphorus tri-iodide, PI₃. The chief reaction which takes place is represented by the following equation:

3CH₃OH + P + 3I = 3CH₃I + P(OH)₃

The hydriodic acid which is evolved is produced as the result of side-reactions, which are described in the case of phosphorus trichloride in Section 57, small print.

130. Properties of Methyl Iodide (Section 205). - (a) Action of light on methyl iodide. - Place in each of two small test-tubes 2 cc. of methyl iodide and close the tubes tightly with corks. Put one tube in the desk in the dark, and allow the other to stand in direct sunlight. At the end of an hour or two, examine the two tubes. To remove the iodine set free, add a drop of mercury or some copper wire and shake. The alkyl iodides should be kept in well-stoppered bottles of brown glass.

(b) Methyl iodide and silver nitrate. - To a few drops of colorless methyl iodide suspended in water add a solution of silver nitrate and shake. Repeat, using a solution of methyl iodide in alcohol and of silver nitrate in alcohol. (Eq.)

(c) Methyl iodide and sodium hydroxide. - Shake occasionally during half an hour a mixture of 1 cc. methyl iodide and 5 cc. of a dilute solution of sodium hydroxide. At the end of this time neutralize with dilute nitric acid, filter, and test a part of the filtrate for methyl alcohol according to experiment 77b, page 53, and a part for an iodide. For other reactions of alkyl halides, see the experiments with ethyl bromide below.

Note. - (a, b, and c) Methyl iodide is much more reactive than the other alkyl iodides. Reactions analogous to those illustrated in the above experiments take place with the higher iodides, but much more slowly. The bromides are less reactive than the iodides; the chlorides are, in most cases, very stable. Tertiary halides are much more reactive than the halides derived from secondary and primary alcohols.

131. Preparation of Ethyl Bromide from Alcohol, Potassium Bromide, and Sulphuric Acid (Section 204). - Add to a mixture of 60 grams of alcohol and 50 cc. of water, contained in a 1 liter flask, 100 cc. of concentrated sulphuric acid. Cool thoroughly in running water, and add 100 grams of finely powdered potassium bromide. Connect the flask by means of a bent glass tube and a rubber stopper with a long condenser through which a rapid stream of water is passing, and fit an adapter by means of a cork to the end of the condenser. Use a 300 cc. Erlenmeyer flask as a receiver, and place it in a beaker about one-half full of cold water. Fill the receiver half full of water and adjust the condenser so that the end of the adapter is below the surface of the water. These precautions are taken because ethyl bromide is a very volatile liquid (boiling-point 39°). Place a sand-bath under the flask containing the reaction-mixture, heat with a large flame, and distil as rapidly as possible. As distillation proceeds, the ethyl bromide drops to the bottom of the receiver, and the aqueous part of the distillate finally overflows into the beaker. Distil until drops of the insoluble bromide cease to appear. About one hour is required for the distillation if it is carried on rapidly.

Decant off most of the water from the ethyl bromide into a beaker, add a dilute solution of sodium hydroxide, shake, and decant. Wash again with water, and transfer to a separatory funnel; run off the lower layer into a small dry distilling flask, add anhydrous calcium chloride (about one-fourth the volume of the bromide), and close the flask tightly with corks. Set aside until the liquid is quite clear and no drops of water are visible; if the calcium chloride appears moist, add more and set aside again. Connect the flask with a long condenser and use as a receiver a dry flask surrounded by ice-water. Distil from a water-bath and collect the portion which boils at 35°-40°. Calculate the theoretical yield from the potassium bromide used (why from the bromide?) and the percentage yield obtained. Write equations for all reactions. Why does the solution turn red during the heating? (Eq.)

Ethyl bromide boils at 38.4° and has the specific gravity 1.47 at 13°. The yield in this preparation should be about 80 per cent of the theoretical.

Note. - The ethyl bromide prepared in this way contains a small amount of ether, from which it cannot be separated readily by distillation, since ether boils at 35° and ethyl bromide at 38.4°. In separating the two substances advantage is taken of the fact that ether is soluble in cold concentrated sulphuric acid, while ethyl bromide is not soluble. The bromide prepared by the method given above can be purified by adding it slowly with constant shaking, to an equal volume of concentrated sulphuric acid kept cold by immersion in ice-water. After this treatment the liquids are separated and the ethyl bromide shaken with water, dried, and distilled.

132. Properties of Ethyl Bromide (Sections 205, 206). - (a) Solubilities of ethyl bromide. - Using about 1 cc. in each test, determine whether ethyl bromide dissolves in the following liquids: water, alcohol, ether, petroleum ether, and cold concentrated sulphuric acid.

(b) Test for bromine in ethyl bromide. - Test ethyl bromide for halogen with a copper wire (§60 page 40).

(c) Ethyl bromide and silver nitrate. - Shake about 2 cc. of ethyl bromide with 5 cc. of water to remove any free hydrabromic acid which may be present. Separate the bromide by means of a pipette and dissolve it in 5 cc. of an alcoholic solution of silver nitrate. Is silver bromide precipitated? Stopper the test-tube, and allow it to stand until the next laboratory exercise. Is silver bromide precipitated? Compare the result with that obtained with methyl iodide.

(d) Ethyl bromide and alcoholic potassium hydroxide. - Shake a small piece of potassium hydroxide with 10 cc. of absolute alcohol. Filter off the liquid and dissolve in it 2 cc. of ethyl bromide. Boil for 1 minute. (Eq.) Dilute with water, acidify with a dilute solution of pure nitric acid, and add a solution of silver nitrate.

(e) Ethyl bromide and magnesium. - Just cover the bottom of a test-tube with magnesium powder. Add 2 cc. of ethyl bromide, 2 cc. of ether, and a very small piece of iodine; place the tube in warm water for 1 minute. Cool and add a little water. (Eq.)

(f) Ethyl bromide and sodium. - To 5 cc. of ethyl bromide in a dry test-tube add a piece of sodium the size of a pea. Close the tube with a cork carrying a small drying-tube containing calcium chloride, and set aside. Examine in an hour. (Eq.)

Notes. - (a) Some tertiary halides are decomposed when treated with concentrated sulphuric acid.

(d) This reaction is useful as a test to distinguish alkyl halides from certain halogen derivatives of benzene and other hydrocarbons, which are not decomposed readily by alcoholic potassium hydroxide. It is necessary to use a solution of potassium or sodium hydroxide free from halogen compounds. Since the alkalies usually contain chlorides, the solution is prepared by using absolute alcohol in which sodium chloride and potassium chloride are insoluble.

133. Preparation of Ethyl Iodide from Ethyl Alcohol and Hydriodic Acid. - Place 60 grams of a strong solution of hydriodic acid and water (the constant-boiling mixture, which contains 57 per cent hydrogen iodide, and has the specific gravity 1.7) in a 200 cc. distilling flask connected with a condenser through which a rapid stream of cold water is passing. Connect an adapter to the condenser and use as a receiver a flask surrounded by water. Close the distilling flask with a stopper hearing a dropping funnel (see Fig. 16, page 27), the end of which reaches to the bottom of the flask. Heat the hydriodic acid to boiling, and drop in through the funnel 10 grams of alcohol at about the same rate as that at which the iodide distils over (about 1 drop per second). Add water to the receiver, shake, and decant off the water into a beaker. Separate the iodide by means of a separatory funnel, place it in a small distilling flask, and add about one-fourth its volume of anhydrous calcium chloride. Cover the side-arm of the flask with a cork, and place one in the neck of the flask. Let the iodide stand until it is quite clear, and distil from a water-bath. Calculate the percentage yield obtained.

Ethyl iodide boils at 72.3°, and has the specific gravity 1.944 (14°). The yield in this preparation is about 90 per cent of the theory, calculated from alcohol; a slight excess of the hydriodic acid is used.

134. Preparation of Isoamyl Bromide from Isoamyl Alcohol and Hydrobromic Acid. - Distil slowly from a 250 cc. distilling flask, connected with a water condenser, a mixture of 20 grams of isoamyl alcohol and 76 grams of the constant-boiling mixture of hydrobromic acid and water, which contains 48 per cent of acid and has the specific gravity 1.49. When the bromide ceases to distil over, add water, separate the two layers in a separatory funnel, and shake the bromide with twice its volume of concentrated hydrochloric acid, which dissolves out from the bromide the alcohol that has distilled unchanged. Wash twice with water, separate, and dry with calcium chloride. Pour off the liquid through a funnel, which contains a bit of cotton wool, into a distilling flask and distil. Save the liquid in the flask, and put it in the bottle reserved for hydrobromic acid residues. Weigh the product and calculate the yield.

Isoamyl bromide boils at 118°, and has the specific gravity 1.219 at 15°. The yield is about 60 per cent of the theoretical calculated from the alcohol used.

Note. - It is frequently advantageous to prepare the alkyl bromides and iodide from the alcohols and aqueous solutions of the halogen acids (the so-called constant-boiling mixtures with water). This is particularly the case when a very pure product is sought, or when an expensive synthetic alcohol is used. In the latter case it is well, in order to increase the yield, to use a large excess - two or three times the theoretical amount - of the acid; the latter can be recovered readily. In making the lower alkyl halides, a good yield can be obtained by using slightly more than the theoretical amount of the halogen acid; in the case of the higher alcohols a large excess is necessary. The alkyl halides prepared in this way are contaminated in certain eases with a trace of an unsaturated hydrocarbon; this can be removed by shaking them with a dilute solution of potassium permanganate.

135. Preparation of Chloroform (Section 213). - Place in a liter flask 150 grams of fresh bleaching powder and 400 cc. of water. Shake vigorously in order to break up the lumps of bleaching powder, and connect the flask with a reflux condenser. Through the condenser add, drop by drop, from a separatory funnel a mixture of 12 grams of acetone and 50 cc. of water. Arrange the flask for distillation with steam (see §28, page 18) and distil as long as the chloroform comes over. Wash the chloroform by decantation twice with water, separate, and place it in a small dry distilling flask with anhydrous calcium chloride. When the liquid is quite clear and no drops of water are visible, distil from a water-bath, using a condenser. Weigh the product, determine its specific gravity (§56, page 37), and calculate the percentage yield obtained.

Chloroform boils at 61° and has the specific gravity 1.498 at 15°. The yield should be about 20 grams.

Note. - The yield of chloroform is affected greatly by the quality of the bleaching powder used. The acetone is added slowly as the reaction is a vigorous one, and frothing is apt to occur if the flask becomes hot.

136. Properties of Chloroform (Section 213). - (a) Odor and solubility of chloroform. - Note the odor of chloroform and determine whether it is soluble in water, ether, alcohol, benzene, petroleum ether, and concentrated sulphuric acid.

(b) Chloroform as a solvent. - Determine if the following substances are soluble in chloroform: iodine, butter, vaseline, olive oil, a bit of black rubber tubing. In the latter case, let the liquid stay in contact with the rubber for some time; then pour off the liquid on to a watch-glass, and let it evaporate.

(c) Inflammability of chloroform. - Try to ignite some chloroform. Light a match and pour on the flame the vapor from some chloroform which has been heated to boiling.

(d) Test for chlorine in chloroform. - Apply the test for halogen with a copper wire (§60, page 40). Shake some chloroform with a little water and test the aqueous solution with silver nitrate. If silver chloride is formed wash the chloroform until the wash-water no longer reacts with silver nitrate. Separate the chloroform with a pipette, dissolve it in alcohol and add an alcoholic solution of silver nitrate.

(e) Decomposition of chloroform in sunlight. - Place some chloroform which contains no free hydrochloric acid in a test-tube, close the tube with a stopper, and allow it to stand several days in a place where it will be in a strong light. Note the odor, shake with water, and test the aqueous solution with silver nitrate.

(f) Chloroform and sodium hydroxide. - Heat a few drops of chloroform for 1 minute with about 5 cc. of a very dilute solution of sodium hydroxide free from chlorides. (Eq.) Test half of the solution for chloride. Neutralize carefully the rest of the solution with hydrochloric acid, add 5 drops of a dilute solution of mercuric chloride and heat. (See note below and experiment 87d, page 62.)

(g) Carbylamine reaction (Section 198). - Hood. - Warm together 1 drop of aniline, which is a primary amine and has the formula C₆H₅NH₂, 1 drop of chloroform, and 1 cc. of an alcoholic solution of potassium hydroxide. (Eq.) On account of the very disagreeable odor produced, the test-tube should be washed in the hood with concentrated hydrochloric acid.

(h) Oxidation of chloroform. - Heat together a few drops of chloroform, a small crystal of potassium bichromate, and 2 cc. of concentrated sulphuric acid. (Eq.) Cautiously note the odor of the gas formed.

Note. - (f) The reaction between chloroform and sodium hydroxide is analogous to that between the alkali and other halogen compounds. In this case the three chlorine atoms are replaced by hydroxyl groups; the resulting compound loses water and formic acid is produced:

CHCl₃ + 3NaOH = CH(OH)₃ + 3NaCl

HC-(OH)₃ = HC=O-OH + H₂O

HCOOH + NaOH = HCOONa + H₂O.

137. Preparation of Ethylene Bromide (Sections 216, 29). - (a) With phosphorus pentoxide as dehydrating agent. - Weigh directly into a 200 cc. distilling flask 40 grams of phosphorus pentoxide; close the side-arm of the flask with a stopper and connect the flask with a reflux condenser (see Fig. 14, page 25). Place the flask in cold water, and with constant shaking add slowly through the condenser 30 grams of alcohol. Heat the flask carefully with a free, flame until the phosphorus pentoxide has dissolved. Remove the condenser, place the flask on a wire gauze at such a height that it can be heated with a burner, close with a stopper, and connect with the absorption apparatus which is arranged as follows: Fit three 8-inch test-tubes with tightly fitting stoppers each containing two holes. The stoppers should be covered with paraffin. This can be done by inserting them into paraffin melted in an evaporating dish. As the apparatus must be tight it is advisable to use stoppers of rubber. Prepare three tubes bent at a right angle of such a length that when passed through the holes in the stoppers they will extend to the bottom of the test-tubes; also three shorter right-angle tubes which just pass through the stopper. Put one long and one short tube into each stopper, and connect the tubes in such a way that a gas passing through the train when the test-tubes are in place, will pass to the bottom of each test-tube through the long tube. Put 10 cc. (30 grams) of bromine in each of two of the test-tubes, and cover with a layer of water about 2 cm. high, which largely prevents the volatilization of the bromine as the gas passes through. The three test-tubes are now placed in beakers containing cold water, and connected with the distilling-flask, the empty tube which serves to condense any liquid which distils over, being placed next to the flask. In order to prevent bromine from getting into the air, connect a drying tube containing sodalime with the tube farthest removed from the distilling flask. Test the apparatus to see if all joints are tight.

Heat the flask cautiously at first and then to such a temperature that ethylene is given off freely. After the gas begins to come off the apparatus needs little attention if it is so placed that the flame is not in a draught; if this is the case, use a chimney so that a steady heat may be obtained. Heat as long as ethylene is formed; from one-half hour to an hour is usually sufficient. If the bromine has not been used up when the ethylene ceases to be evolved, remove the tubes containing ethylene bromide, and under the hood transfer the liquids to a flask. Wash by decantation with a dilute solution of sodium hydroxide until the heavy oil is colorless, and then once with water. Separate the bromide, and dry it with calcium chloride. After standing an hour or more, until the liquid is clear and no water is visible, filter through a funnel, containing a bit of cotton wool, into a small distilling flask. Distil, using a thermometer and water condenser, and collect the fraction which boils at 130°-132°. Calculate the percentage yield both from the alcohol and bromine used.

Ethylene bromide melts at 8.4° and boils at 132°, and has the specific gravity of 2.189 at 15°. The yield in this preparation is about 55 grams.

(b) With sulphuric acid as dehydrating agent. - Mix cautiously in a 1 liter round-bottomed flask 90 cc. of alcohol and 135 cc. of concentrated sulphuric acid; then add about 50 grams of coarse sand and about 5 grams of aluminium sulphate. Place the flask on a wire gauze and connect it by means of a tightly fitting stopper, preferably one of rubber, with an absorbing train of test-tubes like that described in (a) above. In this case use four test-tubes which contain, respectively, beginning with the one nearest the flask, 20 cc. of a dilute solution of sodium hydroxide, 20 cc. of concentrated sulphuric acid, 10 cc. of bromine covered with water, and 10 cc. of bromine covered with water. Attach a drying tube containing soda-lime to the last tube. Place all of the tubes in cold water. Make a test to determine if all joints in the apparatus are tight.

Heat the flask at the lowest temperature at which ethylene is freely evolved. It is very essential to avoid over-heating, which results in frothing and the liberation of carbon. If this occurs, the contents of the flask should be poured out and ethylene should be prepared from a new mixture. When the bromine is decolorized, treat the product as directed in (a) above.

The yield is about 55 grams.

Note. - Sand is added to the flask to distribute the heat more evenly and, thus, to assist in preventing frothing. In the presence of aluminium sulphate, the decomposition of alcohol into ethylene takes place at about 140°, whereas in the absence of the sulphate a temperature of about 180° is required. At the lower temperature carbonization is not apt to occur.


Acyl Chlorides

138. Preparation of Acetyl Chloride (Section 219). - Connect a 200 cc. dry distilling flask with a separatory funnel and a condenser; for use as a receiver, attach to the latter, by means of a tightly fitting cork, a 200 cc. distilling flask or filter-bottle. Connect to the side-arm of the receiver a drying tube containing calcium chloride, which serves to protect the distillate from moisture. As a large quantity of hydrochloric acid is set free during the reaction between acetic acid and phosphorus trichioride, the drying tube should be fitted with a glass tube which reaches to just above the surface of some water contained in a flask. When the apparatus is arranged, place a water-bath containing cold water in such a position that the bulb of the distilling flask is covered with water. Put 50 grams of glacial acetic acid in the flask, and run in slowly through the funnel 40 grams of phosphorus trichloride. The latter should be weighed and transferred to the funnel in the hood. When the phosphorus trichloride has been added, heat the water in the bath to 40°-50°, until the evolution of hydrochloric acid slackens and the liquid has separated into two layers; then heat the water to boiling as long as any distillate comes over. Redistil the distillate, using a thermometer, a condenser, and a receiver protected from the air as directed above. Collect the fraction which boils at 50°-55°. Weigh the product, and calculate the theoretical yield and the percentage of this obtained.

Acetyl chloride boils at 51°. The experiment should yield about 40 grams.

139. Properties of Acetyl Chloride (Section 220). - (a) Acetyl chloride and water. - Cautiously add about 0.5 cc. of acetyl chloride to 2 cc. of ice-water. Observe if two layers form, and then shake carefully. (Eq.)

(b) Acetyl chloride and alcohol. - Cautiously add acetyl chloride, drop by drop, to 2 cc. of alcohol as long as reaction takes place. Is hydrogen chloride given off? Pour the product into 5 cc. of water, shake and note the odor. (Eq.)

(c) Acetyl chloride and an amine. - Add to 5 drops of aniline, C₆H₅NH₂, acetyl chloride, drop by drop, until reaction ceases. (Eq.) Add to the tube 10 cc. of water, heat to boiling, and filter hot. Set the tube aside to cool.