Salicylaldehyde

To a solution of 122 g. of pure salicylaldehyde (Note 1) in 1000 cc. of normal sodium hydroxide solution at room temperature, is added 1420 g. of 3 per cent hydrogen peroxide. The mixture darkens slightly in color and the temperature rises to 45-50°. The solution is allowed to stand for fifteen to twenty hours, whereupon a few drops of acetic acid are added in order to neutralize any excess alkali, and the solution evaporated to complete dryness on the water bath under reduced pressure. 

A considerably lower yield is obtained (50 per cent or less) if technical salicylaldehyde (not purified through the bisulfite compound) be employed. 

The above process is applicable to almost all hydroxyalde-hydes in which the hydroxyl and carbonyl groups occupy ortho or para positions relatively to each other 1 in the latter case derivatives of hydroquinone are produced. When the hydroxyl and carbonyl groups occupy the meta position with respect to each other, no reaction takes place, as is also the case with certain ortho and para compounds containing nitro groups and iodine atoms. o-Hydroxyacctophenone and -hydroxyaceto-phenone are also capable of yielding catechol and hydroquinone respectively under the above conditions. 

Catechol may also be produced from salicylaldehyde by the use of certain derivatives of hydrogen peroxide, such as persulfates or sodium peroxide, but the method is far less convenient. 

To isolate the p-hydroxybenzaldehyde, 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. 

P-Hydroxy-a-naphthaldehyde, Equip a 1 litre three-necked flask with a separatory funnel, a mercury-sealed mechanical stirrer, and a long (double surface) reflux condenser. Place 50 g. of p-naphthol and 150 ml. of rectified spirit in 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. II, 41, 1, but retain the stirrer in the central aperture. Treat the residue, with stirring, dropwise with concentrated hydrochloric acid until 

This dismutation or disproportionation reaction is known as the Cannizzaro reaction. 

The mechanism of the reaction probably involves the production, by into -action of the aldehyde with hydroxide ions, of two reducing anions, the first (I) more easily than the second (II). Either of these anions may transfer a hydride ion to a carbonyl carbon atom in another aldehyde molecule  

The production of benzyl benzoate from benzaldehyde, which may be isolated under special conditions (low temperature and absence of excess of alkali), is explained by assuming that when some benzyloxide ions (CgHj—CHjO s RCH O ) are formed in the alkaline solution, these can replace hydroxide ions thus  

CgHjCHO + CsHjCHO — C.HsCHjOH + C.HjCOOK Benzaldehyde Benzyl alcohol Potassium benzoate 

Cannizzaro reaction. Aromatic aldehydes (and other aldehydes in which a-hydrogen atoms arc absent, e.g., formaldehyde, trimcthylacetaldehyde, and a-hydroxy-tso-butyraldehyde) under the influence of strong aqueous or alcoholio alkali undergo simultaneous oxidation and reduction yielding the alcohol and corresponding acid. Thus — 

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C, b.p. 290 C. Occurs in the Tonka bean, of which it is the odorous ingredient. Prepared synthetically by heating salicylaldehyde with elhanoic anhydride and sodium elhanoate. It... 

CfiHi 05 0 C6H4 CH20H. Colourless, bitter crystals, m.p. 20 PC soluble in water and alcohol, insoluble in chloroform. It occurs in the leaves, bark and twigs of species of willow and poplar. On oxidation with dilute nitric acid it is converted into helicin, the glucoside of salicylaldehyde, which has been made the starting point of further syntheses. Gives populin with benzoyl chloride. 

Coumarin is usually prepared by heating salicylaldehyde with acetic anhydride and sodium acetate (i.e., the Perkin cinnamic acid synthesis, p. 23 6), whereby the 0" hydroxy-cinnamic acid (I) undergoes cyclisation to coumarin. Coumarins having substituents in the benzene ring can often be similarly prepared. 

Aldehydes. Formaldehyde, metaformaldehyde, acetaldehyde, paraldehyde, chloral hydrate, benzaldehyde, salicylaldehyde (and other substituted benzaldehydes). 

Benzaldehyde, C HjCHO, and salicylaldehyde, HOC3H4CHO, are liquids insoluble in water. Benzaldehyde has a characteristic odour of bitter almonds salicylaldehyde has a faint but also characteristic odour, resembling that of phenol. Salicylaldehyde stains the skin yellow. 

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. 

Aliphatic aldehydes reduce Fehling s solution rapidly, benzaldehyde very slowly and indecisively, salicylaldehyde does not reduce it. 

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. 

It does not reduce Fehling s solution, but turns it a pale green colour. (Note that salicylaldehyde turns ordinary copper sulphate solution a pale green.)... 

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

TEST Formaldehyde (solution) Acetal d yde (solution) Paraldehyde Chloral hydrate Benzaldehyde Salicylaldehyde... 

In the strongly basic medium, the reactant is the phenoxide ion high nucleophilic activity at the ortho and para positions is provided through the electromeric shifts indicated. The above scheme indicates theorpara substitution is similar. The intermediate o-hydroxybenzal chloride anion (I) may react either with a hydroxide ion or with water to give the anion of salicyl-aldehyde (II), or with phenoxide ion or with phenol to give the anion of the diphenylacetal of salicylaldehyde (III). Both these anions are stable in basic solution. Upon acidification (III) is hydrolysed to salicylaldehyde and phenol this probably accounts for the recovery of much unreacted phenol from the reaction. 

Basic catalysts other than alkali acetates have been employed in the Perkin reaction thus salicylaldehyde condenses with acetic anhydride in the presence of triethylamine to yield coumarin (tlie lactone of the cis form of o-hydroxy-cinnamio acid) together with some of the acetyl derivative of the trans form (o-acetoxycoumaric acid) ... 

Coumarin. In a 250 ml. round-bottomed flask, provided with a small reflux condenser and a calcium chloride drying tube at the top, place 2 1 g, of salicylaldehyde, 2 0 ml. of anhydrous triethylamine and 5 0 ml. of acetic anhydride, and reflux the mixture gently for 12 hours. Steam distil the mixture from the reaction flask and discard the distillate. Render the residue in the flask basic to litmus with solid sodium bicarbonate, cool, filter the precipitated crude coumarin at the pump and wash it with a little cold water. Acidify the filtrate to Congo red with... 

This substance is readily obtained by the interaetion of salicylaldehyde with hydroxylamlne hydrochloride in the presence of alkali ... 

Method 1. Dissolve 25 0 g. of salicylaldehyde (Section IV,122) in 215 ml. of 2N sodium hydroxide solution, add 12 05 g. of hydroxylamine hydrochloride, and warm the mixture for 30 minutes on a water bath. Acidify with acetic acid and cool in ice the salicylaldoxime separates as a congealed oil. Recrystalhae from chloroform - light petroleum (b.p. 40-60°). The yield of salicylaldoxime (colourless crystals, m.p. 57°) is 5 g. 

Both carbonyl groups of terephthaldehyde are reported to react with the exocyclic nitrogen of 2-aminothiazole yielding 1.4-phenylene bis(2-methyleneamino)thiazole. The same report describes the reactions of 2-amino-4-phenylthiazole with terephth aldehyde and salicylaldehyde as yielding 64 and 65, respectively (Scheme 45) (215), whose structures are based on ultraviolet and infrared spectra. 

Co(II), Ni(n), Cu(n), and Zn(II) complexes of Schiff bases derived from 4-aryl-2-aminothiazoles and salicylaldehyde have been prepared, and structure 276 (Scheme 170) was established by magnetic susceptibility measurements and by infrared, electronic, and mass spectra (512). 

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