inductive effect . This effect is passed on to the subsequent bonds also but the effect decreases rapidly as the number of intervening bonds increases and becomes vanishingly small after three bonds. The inductive effect is related to the ability of substituent(s) to either withdraw or donate electron density to the attached carbon atom. Based on this ability, the substitutents can be classified as electron-withdrawing or electron donating groups relative to hydrogen.
Halogens and many other groups such as nitro (- NO ), cyano (- CN), carboxy (- COOH), ester (COOR), aryloxy (-OAr, e.g. – OC H ), etc. are electron-withdrawing groups. On the other hand, the alkyl groups like methyl (–CH ) and ethyl (–CH –CH ) are usually considered as electron donating groups.
Problem . Which bond is more polar in the following pairs of molecules: (a) H C-H, H C-Br (b) H C-NH , H C-OH (c) H C-OH, H C-SH (a) C–Br, since Br is more electronegative than H, (b) C–O, (c) C–O Problem . In which C–C bond of CH CH CH Br, the inductive effect is expected to be the least? Magnitude of inductive effect diminishes as the number of intervening bonds increases.
Hence, the effect is least in the bond between carbon- and hydrogen. . . Resonance Structure There are many organic molecules whose behaviour cannot be explained by a single Lewis structure.
An example is that of benzene. Its cyclic structure containing alternating C–C single and C=C double bonds shown is inadequate for explaining its characteristic properties. As per the above representation, benzene should exhibit two different bond lengths, due to C–C single and C=C double bonds. However, as determined experimentally benzene has a uniform C–C bond distances of pm, a value intermediate between the C–C single( pm) and C=C double ( pm) bonds.
Thus, the structure of benzene cannot be represented adequately by the above structure. Further, benzene can be represented equally well by the energetically identical structures I and II. benzene cannot