CHEMICAL PROPERTIES OF FATTY ACIDS AND RELATED COMPOUNDS 127 



chloride, which is then reduced with stannous chloride. The resulting 

 stannic aldemonium chloride is then hydrolyzed to the aldehyde. 



b'. Properties of Aldehydes: The lower aldehydes are liquids which 

 have a fruity odor, while the higher members of the series are solids which 

 have a paraffin-like aroma when melted. In general both the melting 

 points and the boiling points are considerably lower than for the correspond- 

 ing acids. Darzens and L^vy**' found that Q:,a-substitution with methyl 

 groups lowers the melting point of the resultant aldehydes as compared 

 with the unsubstituted compounds. 



There are several series of constants for each aldehyde. The first is for 

 the free aldehyde itself, while the others are for the one or more polymers 

 into which the aldehydes may be spontaneously converted. The proper- 

 ties of the aldehydes vary according to the molecular form in which they 

 exist. In the case of the free aldehydes, the lower members are readily 

 soluble in most organic solvents, while the higher representatives dissolve 

 best in cold diethyl ether, chloroform, petroleum ether, benzene, ethyl 

 acetate, and hot ethanol. The polymers are quite insoluble in most organic 

 solvents in the cold. All the free aldehydes readily form oximes, semi- 

 carbazones, hydrazones, and such related compounds; the polymers do not 

 react in this way. The free aldehydes readily react with potassium per- 

 manganate; the polymeric forms are not oxidized by this substance, even 

 when boiled. On distillation under a vacuum, the polymers are quantita- 

 tively reconverted to the free aldehyde form. Although several types of 

 polymers are possible, LeSueur*^^ reported a trimolecular structure. The 

 physical constants of the saturated aldehydes are given in Table 45. 



Because of the ready polymerization of the aldehydes, the early data on 

 melting points are inaccurate. For example, heptadecanal, CH3(CH2)i5- 

 CHO, synthesized from a-hydroxystearic acid, was sho\vn^*^ to melt at 

 35-36°C.; however, a solution in hot ethanol deposited crystals melting 

 at 52°C.; after standing for 6 weeks, the compound melted at 55°C. 

 and, on recrystallization, yielded a product melting at 77-78°C. 



c'. Reactions of the Aldehydes: Aldehydes readily react with a 

 number of reagents, giving compounds which are crystalline, and which 

 have definite melting points as well as other characteristic physical proper- 

 ties. This renders it possible to identify most aldehydes from their deriva- 

 tives. This circumstance is fortunate because the direct identification of 

 aldehydes is extremely difficult. This is attributable to the impossibility 

 of carrying out determinations of physical properties without the danger of 

 polymerization having occurred. 



The derivatives most frequently employed are the semicarbazones, the 

 oximes, the 2,4-dinitrophenylhydrazones, the p-nitrophenylhydrazones, 



"^ G. Darzens and A. L6vy, Compt. rend., 196, 184-187 (1933). 

 ^« H. R. Le Sueur, J. Chem. Soc, 87, 1888-1906 (1905). 



