Preparation of SALICYLALDEHYDE Equip a 1 litre three-necked flask with an efficient (double surface)reflux condenser, a mechanical stirrer, and a thermometer,
the bulb of which is within 2 cm. of the bottom of the flask. Place a warm solution of 80 g. of sodium hydroxide in 80 ml. of water in the flask, add a solution of 25 g. of phenol in 25 ml. of water, and stir. Adjust the temperature inside the flask to 60-65 (by warming on a water bath or by cooling, as may be found necessary) ; do not allow the crystalline sodium phenoxide to separate out. Introduce 60 g. ( 4 0 -5 ml.) of chloroform in three portions at intervals of 15 minutes by means of a dropping funnel fitted into the top of the condenser with a grooved cork (1). Maintain the temperature of the well-stirred mixture at 65-70 during the addition by immersing the flask in hot or cold water as may be required. Finally heat on a boiling water bath for 1 hour to complete the reaction. Remove the excess of chloroform from the alkaline solution by steam distillation (Fig. //, 41, 1). Allow to cool, acidify the orange-coloured liquid cautiously with dilute sulphuric acid, and again steam distil the almost colourless liquid until no more oily drops are collected. Set aside the residue in the flask for the isolation of p-hydroxybenzaldehyde. Extract the distillate at once with ether, remove most of the ether from the extract on a water bath (compare Fig. //, 13, 4). Transfer the residue, which contains phenol as well as salicylaldehyde, to a separatory funnel704 PRACTICAL ORGANIC CHEMISTRY [IV,or small glass-stoppered bottle, add about twice the volume of saturated sodium bisulphite solution, and shake vigorously (preferably mechanically) for at least half an hour, and allow to stand for I hour. Filter the paste of bisulphite compound at the pump, wash it with a little alcohol, and finally with a little ether (to remove the phenol). Decompose the bisulphite compound by warming hi a round-bottomed flask on a water bath with dilute sulphuric acid, allow to cool, extract the salicylaldehyde with ether, and dry the extract with anhydrous magnesium sulphate. Remove the ether (Fig. //, 13, 4) and distil the residue from an air bath. Collect the salicylaldehyde (a colourless liquid) at 195-197. The yield is 12 g. To isolate the phydroxybenzaldehyde, filter the residue from the steam distillation while hot through a fluted filter paper in order to remove resinous matter, and extract the cold filtrate with ether. Distil off the ether, and recrystallise the yellow solid from hot water to which some aqueous sulphurous acid is added. The yield of p-hydroxybenzaldehyde (colourless crystals), m.p. 116, is 2-3 g. Note. (1) If preferred, the chloroform may be added slowly during 30-40 minutes whilst the mixture is stirred and the temperature is maintained at 65-70. Purification of commercial salicylaldehyde. When comparatively large quantities of salicylaldehyde are required, it is more economical to purify the relatively inexpensive commercial product. This may be done either through the bisulphite compound (compare Section III,74,A) or by the following method. Add the commercial salicylaldehyde to a large excess of a hike-warm solution of copper acetate (previously saturated near the ooiling point), shake well, and allow to stand several hours in ice. Filter, wash the precipitate thoroughly first with alcohol and then with ether. Decompose the solid with dilute (10 per cent.) sulphuric acid, extract the aldehyde with ether, dry (anhydrous magnesium sulphate), and distil. The yield from a good commercial sample may be as high as 80 per cent.
p-Hydroxy-a-naphthaldehyde. Equip a 1 litre three-necked flask with a separatory funnel, a mercurysealed mechanical stirrer, and a long (double surface) reflux condenser. Place 50 g. of (3-naphthol and 150 ml. of rectified spirit hi the flask, start the stirrer, and rapidly add a solution of 100 g. of sodium hydroxide in 210 ml. of water. Heat the resulting solution to 70-80 on a water bath, and place 62 g. (42 ml.) of pure chloroform in the separatory funnel. Introduce the chloroform dropwise until reaction commences (indicated by the formation of a deep blue colour), remove the water bath, and continue the addition of the chloroform at such a rate that the mixture refluxes gently (about 1-5 hours). The sodium salt of the phenolic aldehyde separates near the end of the addition. Continue the stirring for a further 1 hour. Distil off the excess of chloroform and alcohol on a water bath ; use the apparatus shown in Fig. //, 41, 1, but retain the stirrer in the central aperture. Treat the residue, with stirring, dropwise with concentrated hydrochloric acid until122] AROMATIC COMPOUNDS 705 the contents of the flask are acid to Congo red paper (about 88 ml. are required) ; a dark oil, accompanied by a considerable amount of sodium chloride, separates. Add sufficient water to dissolve the salt, extract the oil with ether, wash the ethereal solution with water, dry with anhydrous magnesium sulphate, and remove the solvent. Distil the residue under reduced pressure and collect the slightly coloured aldehyde at 163-166/8 mm. (or at 177-180/20 mm.) ; it solidifies on cooling. Recrystallise the solid from 38 ml. of alcohol. The yield of p-hydroxy-anaphthaldehyde, m.p. 80, is 28 g.
