Cyano group electronic effects

The size and shape of the CN group make it unique in relation to other substituents. The cyano group is effectively a rod surrounded by a cylindrical cloud of ir electrons which can Interact with an adjacent ir system regardless ot rotatory orientation (Fig. 1). It has a strong dipole, oriented with the negative and toward the nitrogen. 

As to the electron-withdrawing substituents, the activating effect of a nitro group in the piperidino-dechlorination of 2-chloropyridine involves factors of 7.3 x 10 and 4.6 x 10 from the para and ortho positions, respectively. An ortho-cyano group was found to be... 

Riichardt originally made the assumption that substituents do not exert a noticeable electronic effect on the ground state of the radical precursor. He attributed the full electronic substituent power to the radical. On the basis of this presumption, the BDEs in [24] and [25] were interpreted in terms of radical stability. The original value for the BDE in [24] (Zamkanei et al., 1983) was later slightly modified (Birkhofer et al., 1987). The BDEs were compared with those where only one cyano- or one methoxyl group was incorporated. From Table 9 (Birkhofer et al., 1989) it can be derived that phenyl, cyano-, and methoxy-groups exert an additive substituent effect on... 

As points of reference, we will take two well-established hydrogen-bond donor/ acceptors, H2O and NH3. Their computed gas-phase Vs,max and Vs,mm are in Table 5, along with the same data for all of the molecules that have been discussed hydroxylamine (5), dimethylhydroxylamine (6), acetoxime (7), acetohydroxamic acid (8), and the isomeric pairs of oximes examined in the last section. Finally, we included an additional hydroxamic acid, 11, to see the effects of the strongly electron-withdrawing cyano group. 

The diatropism of diazapyraceheptylenes (106), which follows from a comparison of the PMR spectra with that of 105a (Table VI), may be due to the effect of the cyano groups on the electronic configuration of the 7r-system. The chemical shift difference between the inside and outside protons clearly reveals the expected diamagnetic ring current in the annulene derivatives (110). The coupling constants of the seven-membered rings of 110 show that this part of the molecule is planar and delocalized. 

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