Secondary amines and imines

Secondary amines and imines. Secondary amines shown only a single NH stretching absorption in dilute solution, although a second band at low frequencies is sometimes shown at higher concentrations when hydrogen bonding effects occur. 

The most detailed studies of secondary amines are due to Russell and Thompson [59], who have examined both the intensity and frequency of this band in a wide range of compounds. Both were found to be very sensitive to the nature of the siurounding structure. In aliphatic secondary amines the frequency falls in the range 3360—3310 cm with low intensity. In alkylaryl amines the frequency rises sharply to near 3450 cm and the intensity increases by a factor of 50. In pyrrole, indole and similar compounds 

A very substantial amount of data is available on NH stretching frequencies in heterocyclic ring systems. Indoles have been studied by Fuson et al [13], by Witkop [35,41, 42] and by Mirone and Vampiri [45]. Indole itself absorbs at 3491 cm but values as low as 3371 cm are quoted for some derivatives. However it is doubtful if this is wholly due to substituent effects as not all cases were studied in dilute solution and it is more probable that self-association is responsible for much of this shift. 

Pyrroles have been studied by Fuson etal. [13] and by Jones and Moritz [49]. They find an overall range of 3495 to 3431 cm and the latter authors have developed an equation which allows the calculation of this frequency in pyrroles variously substituted with methyl or carbethoxy groups in different positions. Marion et al [34] quote a range of 3480—3440 cm for related systems in alkaloids. 

Other heterocyclic systems studied include the phenazines [61], triazoles and imidazoles [89] and aziridenes [88]. In the last named the NH bands are more typical of alkyl amines and absorption is between 3360 and 3340 cm in the vapour. In the liquid state self-association lowers this to 3250—3240 cm for alkyl aziridenes. When acyl groups are substituted in the ring two NH bands appear corresponding to two different invertomers. Marked variations in both frequency and intensity have been noted by Barr and Haszeldine [60] in NH groups with fluorine substituents at the a-carbon atom. The NH frequency of Z /s-2-2-2-trifluoro-dimethylamine, for example, is 3460 cm , and the extinction coefficient is about 50 times greater than that of dimethylamine itself. 

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Ether, oxide, sulfide, sulfoxode, primary and secondary amine and imine + + 0 + + + +... 

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