Salicylic acid acidity

Salicylsulfuric Acid. 2-(SutfooxyIbenzoic acid salicylic acid, acid sulfate salicylic acid sulfuric acid ester. C,H 0 S mol wt 218.18. C 38.53%, H 2.77%, O 44.00%. S 14.70%. Prepd by treating salicylic acid with chlorosulfonic acid in pyridine Loeper el at, Compt Rend. Six. Biol 135, 917 (1941) Chim. tnd. (Paris) 49, 99 (1943). 

C7H6O2 Oily liquid of aromatic odour b.p. 196°C. (t is prepared by the action of chloroform and caustic potash on phenol (the Reimer-Tiemann reaction) or by the oxidation of the glucoside salicin. It is easily reduced to salicyl alcohol or oxidized to salicylic acid. 

When the phenol contains a carboxylic acid group, e.g., m- or p-hydroxy-benzoic acid, the acetylated derivative will of course remain in solution as the sodium salt, but is precipitated when the solution is subsequently acidified. Salicylic acid, however, cannot be acetylated under these conditions. 

Required Salicylic acid, 10 g. pyridine, 7 ml. acetyl chloride. 

Dissolve 10 g. of salicylic acid (o-hydroxybenzoic acid) in 7 ml. of dry pyridine contained in a too ml. conical flask. Then without delay (since this solution if allowed to stand tends to become a semi-solid mass) run in 7 5 ml. (8 3 g.) of acetyl chloride, adding about i ml. of the chloride at a time, and shaking the mixture continuously during the addition. The heat of the reaction causes the temperature of the mixture to rise rapidly ... 

It should be emphasised that salicylic acid can be readily acetylated by Method 1, and that the above preparation of acetylsalicyclic acid is given solely as an illustration of Method 2. To employ Method 1, add 10 g. of salicylic acid to 20 ml. of a mixture of equal volumes of acetic anhydride and acetic acid, and boil gently under reflux for 30 minutes. Then pour into about 200 ml. of cold water in order to precipitate the acetylsalicylic acid (11 g.) and finally recrystallise as above. Method 2, however, gives the purer product. 

Reactions of Aspirin, (i) Distinction from Salicylic acid. Shake up with water in two clean test-tubes a few crystals of a) salicylic acid, (0) aspirin, a very dilute aqueous solution of each substance being thus obtained. Note that the addition of i drop of ferric chloride solution to (a) gives an immediate purple coloration, due to the free —OH group, whereas (b) gives no coloration if the aspirin is pure. 

In the following experiment, salicylic acid is reduced to o-hydroxybenzyl alcohol (or saligenin), which being crystalline is readily isolated the excess of hydride is destroyed by the addition of undried ether, and the aluminium hydroxide then brought into solution by the addition of sulphuric acid. 

Required Salicylic acid, 6 0 g. lithium aluminium hydride, 2 5 g. dry ether, 165 ml. 

Now cool the mixture thoroughly in ice-water, and run in over a period of 45 minutes a solution of 6 o g. of dry salicylic acid in 75 ml. of dry ether. When the addition of the acid to the stirred solution is complete, heat the mixture under reflux on the water-bath for 15 minutes to ensure completion of the reduction. Then thoroughly chill the mixture in ice-water, and hydrolyse any unused hydride by the slow addition of 50 ml. of ordinary undried ether, followed similarly by 75 ml. of dilute sulphuric acid. 

Violet coloration. (Note however that a dilute solution of salicylic acid will give this coloration without any preliminary neutralisation.)... 

I. Methyl salicylate test. Heat i ml. of methanol with 0 5 g. of sodium sdicylate (or free salicylic acid) and a few drops of cone. H2SO4 gently for i minute. Cool, pour into a few ml. of cold water in a boiling-tube, and shake. Note the odour of methyl salicylate (oil of wintergreen). 

Many of the reactions of phenols are frequently given also by their derivatives, e.g.f salicylic acid (p. 352) beha es like phenol towards a number of reagents. 

Salicylaldehyde gives a yellow coloration and forms salicylic acid very slowly. Cannizzaro s reaction is also given by formaldehyde but, owing to the difficulty in isolating the products, is not used as a test. 

Oxidation to acids. Varm together in a small conical flask on a water-bath for lo minutes a mixture of 0 5 ml. of benzaldehyde or salicylaldehyde, 15 ml. of saturated KMn04 solution, and 0-5 g. of NajCOj. Then acidify with cone. HCl, and add 25% sodium sulphite solution until the precipitated manganese dioxide has redissolved. On cooling, benzoic or salicylic acid crystallises out. 

Action of sodium hydroxide. Does not undergo the Cannizzaro reaction. It dissolves in dil. NaOH solution, giving a yellow solution from which the aldehyde is precipitated unchanged on acidification. If heated with cone. NaOH solution, salicylaldehyde slowly undergoes atmospheric oxidation to salicylic acid. 

Salicylic Acid. Ester has strong odour of oil of wintergreen. 

The methyl ester formed by substituting methanol for ethanol in the above reaction has an even stronger odour and should be prepared if salicylic acid is suspected. 

Acetylation. Boil i g. of salicylic acid with 4 ml. of an acetic anhydride-acetic acid mixture (equal volumes) under reflux for 10 minutes. Pour into water. Filter off the aspirin (p. 111), wash with water and recrystallise from aqueous acetic acid (1 1) m.p. l36 ... 

Phthalein formation Fuse together carefully in a dry test-tube a few crystals of salicylic acid or of a salicylate with an equal quantity of phthalic anhydride rnoistened with 2 drops of cone. HjSO. Cool, dissolve in water and... 

It is important to note that in these reactions salicylic acid is functioning as a phenol. 

Methyl, ethyl, n-propyl, isopropyl, n-hutyl, benzyl, cyclohexyl esters of formic, acetic, oxalic, succinic, tartaric, citric, benzoic, salicylic (and other substituted benzoic acids), phthalic and cinnamic acids phenyl esters of acetic, benzoic and salicylic acids. 

Phenolic carbolic) odour. Many phenols, some derivatives of salicylic acid e.g., salicylaldehyde). 

Substances suitable for the estimation acetanilide, sucrose, glucose, cinnamic acid, diphenyl amine, salicylic acid, vanillin, />"bromoacetanilide, toluene p-sul phonamide. 

Choice of solvent for recrystallisation. Obtain small samples (about 0 5 g.) of the following compounds from the storeroom (i) salicylic acid, (Li) acetanilide, (iii) m-dinitrobenzene, (iv) naphthalene, and (v) p-toluene-sulphonamide. Use the following solvents distilled water, methylated spirit, rectified spirit, acetone, benzene and glacial acetic acid. 

Optional or alternative experiments are the recrystaUisation of 3-0 g. of crude benzoic or salicylic acid from water. 

Salicylic acid, however, cannot be acetylated under these conditions. 

Salicylic acid. The preparation of salicylic acid by passing carbon dioxide into dry sodium phenoxide at 170-190° is the classical example of the Kolbe-Schmltt reaction. The latter is a method for introducing a carboxyl group directly into a phenol nucleus. 

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