Chapter VII

Aldehydes and Ketones

106. Formation and Properties of Formaldehyde (Sections 142, 143). - (a) Prepare a dilute solution of formaldehyde by oxidizing methyl alcohol as directed in experiment 77b, page 53, and make the following tests:

(b) Perform experiment 77b. Repeat the experiment, using 6 drops of a cold saturated alcoholic solution of gallic acid in place of the solution of resorcin.

(c) Formaldehyde in milk. - Mix 2 cc. of the formaldehyde solution prepared in (a) above with 5 cc. of milk, add 3 drops of a solution of ferric chloride, hold the tube in a slanting position, and pour down it slowly 5 cc. of concentrated sulphuric acid in such a way that the acid forms a layer at the bottom of the tube. Repeat the test with a sample of milk free from formaldehyde, which has been diluted with an equal volume of water.

(d) Reducing action of formaldehyde. - Clean a test-tube thoroughly by boiling a strong solution of sodium hydroxide in it, and then washing with water. Place in the test-tube about 5 cc. of a dilute solution of silver nitrate, and add a dilute solution of ammonia, drop by drop, until the precipitate first formed is just dissolved. Add about 1 cc. of the dilute solution of formaldehyde and put the tube into warm water.

(e) Formaldehyde and Schiff's reagent. - Add a few drops of the solution of formaldehyde to 5 cc. of Schiff's reagent.

(f) Formaldehyde and Fehling's solution. - Add 1 cc. of the dilute solution of the aldehyde to 5 cc. of Fehling's solution and heat to boiling. The preparation of Fehling's solution is described in the Appendix. (See note below and Section 151.)

(g) Polymerization of formaldehyde. - Evaporate on the steambath 10 cc. of formalin (a 40 per cent solution of formaldehyde). Heat some of the residue over a free flame and note cautiously the odor.

(h) Formaldehyde and proteins. - Place in a solution of 1 cc. of formalin and 5 cc. of water a thin piece of gelatin and let it stand for from 10 to 20 minutes. Place the gelatin in water and heat to boiling. Try the action of boiling water on gelatin which has not been in contact with formalin.

Notes. - (b) In these tests very dilute solutions of formaldehyde should be used, since strong solutions give precipitates which obscure the colors that develop.

(c) A test similar to that described is made by pouring into the tube containing the milk, so that two layers are formed, concentrated hydrochloric acid containing a small amount of ferric chloride.

(e) It should be noted that alkalies or salts which give an alkaline reaction produce a pink color with Schiff's reagent; it also becomes colored when heated. The preparation of the reagent is described in the appendix.

(f) Fehling's solution is much used in testing for aldehydes. It consists of a solution prepared by dissolving copper sulphate, sodium hydroxide, and sodium potassium tartrate in water, the tartrate serving to keep in solution the copper hydroxide, which would otherwise precipitate. The aldehyde reduces the cupric compound in the alkaline solution to cuprous oxide.

107. Preparation of Acetaldehyde from Alcohol (Section 144). - A 500 cc. distilling flask containing 80 grams of powdered potassium or sodium bichromate is closed by a cork bearing a separatory funnel, and connected with a long condenser through which a rapid stream of water below 15° passes. The condenser is attached to one end of an adapter the other end of which extends to bottom of a receiver sunk to its neck in ice-water. Drop in slowly through a funnel a cold mixture of 160 cc. of water, 40 grams of alcohol, and 70 cc. of concentrated sulphuric acid. A vigorous reaction takes place; as this subsides the mixture is run in more rapidly. The flask is shaken occasionally to prevent the bichromate from forming a cake at the bottom. When all the mixture has been added, and the action has ceased, replace the funnel by a thermometer and heat the flask very gently; remove the flame as soon as reaction sets up again. Repeat the heating in this way until reaction ceases when the flame is withdrawn. Care should be taken to prevent the rise of the thermometer beyond 65°. Place the distillate in a small distilling flask provided with a long condenser and receiver cooled by ice-water as described above, put the flask in water, and heat the latter slowly up to 50°. Collect the portion which boils between 20° and 35°. Acetaldehyde boils at 21°. The product obtained contains some water, but can be used for the experiments described below.

108. Properties of Acetaldehyde (Sections 144-147). - (a) Aldehyde and sodium hydrogen sulphite. - Shake under water 2 cc. of the aldehyde prepared in the experiment just described with 5 cc. of a saturated solution of sodium hydrogen sulphite. (Eq.) Note the disappearance of the odor of aldehyde. Add 5 cc. of a strong solution of sodium carbonate and heat, noting the odor. (Eq.)

(b) Aldehyde and bromine. - To 2 cc. of the aldehyde add bromine, drop by drop, as long as the solution is decolorized. The bromine should be added slowly as the reaction at first is delayed. When the reaction is complete add about 10 cc. of water. Does a heavy oil separate? (Eq.)

(c) Polymerization of aldehyde. - Dip a glass rod into concentrated sulphuric acid, and remove as much of the acid as possible by shaking. Dip the rod into 2 cc. of aldehyde. Note the reaction. (Eq.) Add 5 cc. of water.

(d) Aldehyde resin. - Boil a few drops of aldehyde in 5 cc. of water with a few drops of a solution of sodium hydroxide. Note the appearance and odor of the product. Repeat the test using a solution of formaldehyde.

(e) Oxidation of aldehyde. - Add, in small portions, a dilute solution of potassium permanganate to a solution of 2 cc. of aldehyde in 5 cc. of water. (Eq.) Recall the action of an unsaturated hydrocarbon with a solution of potassium permanganate. Is the test with this salt a positive proof that a compound contains a double or triple bond between carbon atoms? How could you distinguish between a solution of an aldehyde and one of ethylene?

(f) Reducing action of aldehyde. - Warm 5 drops of aldehyde with 5 cc. of Fehling's solution.

(g) Tollen's reagent for aldehydes. - Test 3 drops of aldehyde with an ammoniacal solution of silver nitrate as described in experiment 106d, page 81, or without heating with Tollen's reagent[1]. (Eq.) Clean the test-tube to be used by boiling it out with a solution of sodium hydroxide.

(h) Schiff's reagent and aldehydes. - Test a few drops of the aldehyde with about 5 cc of Schiff's reagent[1].

109. Formation of Acetaldehyde from an Acetate (Section 144). - Mix together 5 grams of sodium formate and 6 grams of anhydrous sodium acetate in an 8-inch test-tube. Place the tube in an inclined position and attach by means of a cork a delivery-tube which dips under water contained in a test-tube. Heat for a few minutes. Test the resulting solution of aldehyde with Schiff's reagent.


Ketones

110. Formation of Acetone from an Acetate (Section 155) - Place in an 8-inch test-tube 7 grams of calcium acetate and 7 grams of anhydrous sodium acetate which have been intimately mixed by grinding together in a mortar. Clamp the tube in a horizontal position and connect it with a condenser. Tap the tube gently so that a channel is formed along its whole length. Heat until the acetates glow, turning the tube from time to time, so that all of the salts can be heated to a high temperature. Redistil the liquid and determine its boiling-point.

Acetone boils at 56°. The yield is from 3 to 4 grams.

Note. - Sodium acetate is mixed with calcium acetate in this preparation in order to facilitate the formation of acetone.

111. Properties of Acetone (Sections 155, 157, 158). - (a) Acetone and Schiff's reagent. - Add a few drops of acetone to 5 cc. of Schiff's reagent. Examine the solution after it has stood some time.

(b) Acetone and sodium hydrogen sulphite. - Mix 5 cc. of acetone with 5 cc. of a saturated solution of sodium hydrogen sulphite; shake and cool. (Eq.)

(c) Acetone and Tollen's reagent. - Test a few drops of acetone dissolved in 5 cc. of water with an ammoniacal solution of silver nitrate or with Tollen's reagent.

(d) Conversion of acetone into iodoform. - Apply the iodoform test to a dilute solution of acetone. (See experiment 81b, page 57.)

(e) Acetone and phosphorus pentachloride. - Hood. - To 5 cc. of acetone contained in a test-tube placed in cold water add, with constant shaking, small quantities of phosphorus pentachloride as long as any reaction takes place. Pour the contents of the tube slowly into cold water. Does a heavy liquid separate? (Eq.)

(f) Identification of acetone. - Acetone can be converted into a solid compound which is valuable in its identification, especially if the ketone is in a dilute aqueous solution. The substance prepared is formed as the result of the interaction of acetone and benzaldehyde, C₆H₅.CHO, in the presence of an alkali; it is called dibenzalacetone. The reaction is as follows:

2C₆H₅.CHO + (CH₃)₂CO = (C₆H₅.CH:CH)₂CO + 2H₂O

Place in a small flask 1 cc. of acetone, 4 cc. of water, 4 cc. of benzaldehyde, 20 cc. of alcohol, and 5 cc. of a 10 per cent solution of sodium hydroxide. Boil gently for 5 minutes. Cool and shake. Filter off the crystals and wash them with 20 cc. of cold alcohol. Recrystallize from 20 cc. of boiling alcohol. Let the solution cool. Filter and wash with 10 cc. of cold alcohol. Dry on a porous plate and determine the melting-point of the crystals. Dibenzalacetone crystallizes in yellow plates and melts at 111°-112°.


[1] See Appendix.