The Resonance Effect.
the unshared electron pair, symbolized by two dots, would reside entirely on the chlorine atom. The electron density surrounding the chlorine atom would be comparable to that in
However, the real structure of vinyl chloride is not
In terms of resonance, the difference in basicity is due to the fact that in NH3, the unshared electron pair shown on nitrogen is restricted entirely to the nitrogen atom. if the real structure of aniline were
then the electron density due to the unshared electron pair would reside entirely on nitrogen only and aniline would be expected to have basicity (ability to donate an electron pair) comparable to’ that of ammonia. But the real structure of aniline is a hybrid of the following structures:
The contribution of the structures 3, 4; 5 implies that the unshared electron pair of the nitrogen atom is delocalized over the ring; the electron density on nitrogen is decreased while that on the ring is correspondingly increased due to the resonance effect. The nitrogen atom is said to be electron donating by resonance. The decreased electron density on the nitrogen atom of aniline relative to ammonia (where there is no resonance) explains why aniline is a weaker base than ammonia. The contribution of structures 3, 4, 5 also explains why aniline is more reactive than benzene in aromatic electrophilic substitution reactions such as nitration, bromination, sulphonation, and the Friedel-Craft alkylation and acylation, the ring having been made richer in electron density relative to benzene by delocalization of the nitrogen electrons over it.
Consider again the molecule of vinyl chloride which is a hybrid of structures such as
The chlorine atom is electron-withdrawing due to the inductive effect; i.e., the carbon chlorine bond will be polarized as follows:
However, the observed dipole moment of vinyl chloride is smaller than that of ethyl chloride. The contribution of the structure
shows that due to the resonance effect the chlorine atom is electron-donating in the direction of the arrow. Since the resonance effect as opposed to the inductive effect, in this case, the dipole moment is understandably smaller than that of a normal C – Cl bond. Note the dipole moments in the following cases:
The larger dipole moment of nitrobenzene is due to the contribution of structures 2,3 and 4 which have greater separation of charges. The nitro group is, thus, electron-withdrawing inductively as well as by resonance. The two effects add together to increase the dipole moment of nitrobenzene relative to nitromethane (inductive effect only).
Similarly, the dipole moments of aniline, phenol, chlorobenzene, and methoxy benzene are smaller than those of their aliphatic analogs due to the significant contributions of the following structure.
It is of interest to compare the dipole moments of the following compounds:
indicates greater electron density between carbon and chlorine than expected from one bonding electron pair only (as in CH, – CH, – Cl). The increased electron density between the two atoms enables them to come closer, as nucleus-nucleus repulsions would be minimized due to greater electron density between them (recall that a triple bond is shorter than a double bond which is, in turn, shorter than a single bond). In resonance language, it is said that the carbon-chlorine bond in vinyl chloride (or in chlorobenzene) has some double-bond- character. Due to a similar reason the C- C bond in vinyl chloride is not a full double bond; the electron density between the two carbon atoms is less than that expected for an isolated double bond. The C-C bond is therefore slightly longer (1.38A∘) than a normal carbon-carbon double bond (1.34 A∘).