Reactions and characterisation of aromatic aldehydes

Aromatic aldehydes usually have relatively high boiling points, but distil with little or no decomposition. The vapours burn with a smoky flame. They are easily oxidised on standing in the air into the corresponding acids the odours are often pleasant and characteristic. Aromatic aldehydes, by virtue of their high molecular weight, yield 

Aromatic aldehydes react with the dimedone reagent (Section 111,70,2). All aromatic aldehydes (i) reduce ammoniacal silver nitrate solution and (ii) restore the colour of SchifiF s reagent many react with sodium bisulphite solution. They do not, in general, reduce Fehling s solution or Benedict s solution. Unlike aliphatic aldehydes, they usually undergo the Cannizzaro reaction (see Section IV,123) under the influence of sodium hydroxide solution. For full experimental details of the above tests, see under Ali-phalic Aldehydes, Section 111,70. They are easily oxidised by dilute alkaline permanganate solution at the ordinary temperature after removal of the manganese dioxide by sulphur dioxide or by sodium bisulphite, the acid can be obtained by acidification of the solution. 

Dimedone derivatives. For experimental details, see under Aliphatic Aldehydes, Section 111,70,2. 

Oximes (compare Section III,74,B). The following procedure has wide application. Dissolve 0-5 g. of hydroxylamine hydrochloride in 2 ml. of water, add 2 ml. of 10 per cent, sodium hydroxide solution and 0-2 g. of the aldehyde (or ketone). If the latter is insoluble, add just sufficient alcohol to the mixture to give a clear solution. Heat the mixture under reflux for 10-15 minutes, and then cool in ice. If crystals separate, filter these off, and recrystallise from alcohol, dilute alcohol, benzene or light petroleum (b.p. 60-80°). If no solid separates on cooling, dilute with 2-3 volumes of water, filter the precipitated sohd, and recrystallise. 

Phenylhydrazones (compare Action III,74,C). Dissolve 0-5 g. (rf colourless phenylhydrazine hydrochloride and 0 8 g. of sodium acetate in 5 ml. of water, and add a solution of 0 2-0 4 g. of the aldehyde (or ketone) in a little alcohol (free from aldehydes and ketones). Shake the mixture until a clear solution is obtained and add a little more alcohol, if necessary. Warm on a water bath for 10-15 minutes and cool. Filter off the crystalline derivative, and recrystallise it from dilute alcohol or water sometimes benzene or light petroleum (b.p. 60-80°) may be used. 

---

Certain heterocyclic compounds are also important aromatic substances in wines, such as pyrazines in Cabernet Sauvignon and Sauvignon Blanc wines (see Section 8.2.11.1.7) and both enantiomers of 3-hydroxy-4,5-dimethyl-5if-furan-2-one (sotolon), which occur in white wines, sherries and are a key component of the typical aroma of aged Port wines. The precise chemical reactions leading to the formation of bouquet substances are not yet widely known. There are two types of reactions that produce bouquet constituents oxidation, which is characterised by the presence of aldehydes and acetals (e.g. in Madeira-type wines) and reduction (such as in quality table wines after a period of bottle maturation the flavour of low-quahty wines does not improve under the same conditions, but instead maturation often leads to a loss of freshness). During wine aging, glycosides of terpenic alcohols and... 

For all the aldehydes tested, warming up the reaction mixture resulted in the appearance of new peaks, belonging to the corresponding 1,2-addition product, simultaneously with the disappearance of the cuprate signals. When aromatic ketones and 1,3-diketones or keto-esters were tested, formation of the corresponding ir-complexes was detected, albeit with on average lower conversion and stability with respect to the aldehydes. The only exceptions were acetophenone and benzophenone, for which no ir-complex was detected. Fluorenone 99, in contrast, formed a remarkably stable complex 99a (Scheme 36) that was also characterised for the first time by X-ray diffraction [136]. 

---