Bisulphite addition

To 2 ml. of the ester, add 2--3 drops of a saturated freshly prepared solution of scdium bisulphite. On shaking, a gelatinous precipitate of the bisulphite addition product (D) of the keto form separates, and on standing for 5-10 minutes usually crystallises out. This is a normal reaction of a ketone (see p. 344) hydrogen cyanide adds on similarly to give a cyanhydrin. 

Usually form bisulphite addition compounds, R CH(0H)S03Na. 

Bisulphite addition compound. Shake 1 ml. of benzaldehyde with about 0 5 ml. of saturated NaHSOj solution. The mixture becomes warm, and the white addition product separates (rapidly on cooling). 

Bisulphite addition compound. The formation of the bisulphite compound is delayed and appears only after 2-3 minutes shaking. 

Certain ketones give bisulphite addition compounds. 

Treat a small quantity of the bisulphite addition compound with 5 ml. of 10 per cent, sodium carbonate solution, and note the odour. Repeat the experiment with 5 ml. of dilute hydrochloric acid. 

Mandelic acid. This preparation is an example of the synthesis of an a-hydroxy acid by the cyanohydrin method. To avoid the use of the very volatile and extremely poisonous hquid hydrogen cyanide, the cyanohydrin (mandelonitrile) is prepared by treatment of the so um bisulphite addition compound of benzaldehj de (not isolated) with sodium cyanide ... 

Bisulphite compounds of aldehydes and ketones. These substances are decomposed by dilute acids into the corresponding aldehydes or ketones with the liberation of sulphur dioxide. The aldehyde or ketone may be isolated by steam distillation or by extraction with ether. Owing to the highly reactive character of aldehydes, the bisulphite addition compounds are best decomposed with saturated sodium bicarbonate solution so um carbonate solution is generally employed for the bisulphite compounds of ketones. 

Aldehydes and Ketones. The best derivative from which an aldehyde can be recovered readily is its bisulphite addition compound, the main disadvantage being the lack of a sharp melting point. The aldehyde (sometimes in ethanol) is shaken with a cold saturated solution of sodium bisulphite until no more solid adduct separates. The adduct is filtered off, washed with a little water, then alcohol. A better reagent is freshly prepared saturated aqueous sodium bisulphite solution to which 75% ethanol is added to near-saturation. (Water may have to be added dropwise to render this solution clear.) With this reagent the aldehyde need not be dissolved separately in alcohol and the adduct is finally washed with alcohol. The aldehyde is recovered by dissolving the adduct in the least volume of water and adding an equivalent quantity of sodium carbonate (not sodium hydroxide) or concentrated hydrochloric acid to react with the bisulphite, followed by steam distillation or solvent extraction. 

Ketones. Ketones are more stable to oxidation than aldehydes and can be purified from oxidisable impurities by refluxing with potassium permanganate until the colour persists, followed by shaking with sodium carbonate (to remove acidic impurities) and distilling. Traces of water can be removed with type 4A Linde molecular sieves. Ketones which are solids can be purified by crystallisation from alcohol, toluene, or petroleum ether, and are usually sufficiently volatile for sublimation in vacuum. Ketones can be further purified via their bisulphite, semicarbazone or oxime derivatives (see p. 51). The bisulphite addition compounds are formed only by aldehydes and methyl ketones but they are readily hydrolysed in dilute acid or alkali. 

Impurities resulting from storage can be removed by passage through chromatographic grade alumina. Furfural can be separated from impurities other than carbonyl compounds by the bisulphite addition compound. The aldehyde is steam volatile. 

To diminish its rate of oxidation, benzaldehyde usually contains additives such as hydroquinone or catechol. It can be purified via its bisulphite addition compound but usually distn (under nitrogen at reduced pressure) is sufficient. Prior to distn it is washed with NaOH or 10% Na2C03 (until no more CO2 is evolved), then with satd Na2S03 and H2O, followed by drying with CaSO MgS04 or CaCl2. 

Dried with MgSC>4, CaSC>4, NajSC or Linde type 13X molecular sieves, then distd. Cyclohexanol and other oxidisable impurities can be removed by treatment with chromic acid or dil KMnC>4. More thorough purification is possible by conversion to the bisulphite addition compound, or the semicarbazone, followed by decompn with Na2CC>3 and steam distn. [For example, equal weights of the bisulphite adduct (crystd from water) and NajCC are dissolved in hot water and, after steam distn, the distillate is saturated with NaCl and extracted with benzene which is then dried and the solvent evaporated prior to further distn]. 

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