The C═N 13C chemical shifts of two series of substituted benzaldehyde (2-hydroxycyclohexyl)-hydrazones were measured. The aromatic ring-substituent chemical shifts (SCS) of hydrazones were for the first time analyzed in terms of separate resonance and inductive effects through use of the dual substituent parameter (DSP) approach. The SCS values showed that electron-donating substituents cause downfield and electron acceptors upfield shifts, thus following the reverse trend. The inductive effects of the substituents are predominant, the inductive transmission coefficient for both para- and meto-substituted benzaldehyde (2-hydroxycyclohexyl)hydrazones having a general value of −6.0 ± 0.2. This reverse inductive contribution is consistent with the π-polarization mechanism. The resonance contribution to SCS varies from case to case and as expected is higher in the para series than in the meta series. The C═N 13C chemical shifts of hydrazones are appreciably more susceptible to substituent-induced electronic changes than those of imines, although both are more sensitive than the α-carbon 13C chemical shifts of styrenes, phenylacetylenes, benzonitriles, or the side-chain carbonyl derivatives of benzene to substituent effects.
ASJC Scopus subject areas
- Organic Chemistry