Delocalisation charge stabilisation

Pyridines form crystalline, frequently hygroscopic, salts with most protic acids. Pyridine itself, with pATa 5.2 in water, is a much weaker base than saturated aliphatic amines which have pATa values mostly between 9 and 11. Since the gas-phase proton affinity of pyridine is actually very similar to those of aliphatic amines, the observed solution values reflect relatively strong solvation of aliphatic ammonium cations this difference may in turn be related to the mesomerically delocalised charge in pyridinium ions and the consequent reduced requirement for external stabilisation via solvation. 

The valence-bond (resonance) description of the triphenylmethine dye Malachite Green (125) is illustrated in Figure 6.5. Comparison with Figure 6.4 reveals their structural similarity compared with cyanine dyes. Formally, the dye contains a carbonium ion centre, as a result of a contribution from resonance form II. The molecule is stabilised by resonance that involves delocalisation of the positive charge on to the p-amino... 

The stabilisation that can result by delocalisation of a positive or negative charge in an ion, via its n orbitals, can be a potent feature in making the formation of the ion possible in the first place (cf. p. 55). It is, for instance, the stabilisation of the phenoxide anion (23), by delocalisation of its charge via the delocalised n orbitals of the nucleus, that is largely responsible for the acidity of phenol (cf. p. 56) ... 

With alcohols there is no such factor stabilising the alkoxide anion ROe, relative to the alcohol itself, and alcohols are thus very much less acidic than carboxylic acids. With phenols, however, there is again the possibility of relative stabilisation of the anion (2), by delocalisation of its negative charge through interaction with the n orbitals of the aromatic nucleus ... 

Thus for hydrolysis in 50% aqueous acetone, a mixed second and first order rate equation is observed for phenylchloromethane (benzyl chloride, 10)—moving over almost completely to the SV1 mode in water alone. Diphenylchloromethane (11) is found to follow a first order rate equation, with a very large increase in total rate, while with triphenylchloromethane (trityl chloride, 12) the ionisation is so pronounced that the compound exhibits electrical conductivity when dissolved in liquid S02. The main reason for the greater promotion of ionisation—with consequent earlier changeover to the SW1 pathway in this series—is the considerable stabilisation of the carbocation, by delocalisation of its positive charge, that is now possible ... 

It is to be expected that attack by nucleophiles on an unsubstituted benzene nucleus will be much more difficult than attack by electrophiles. This is so (o) because the n electron cloud of the nucleus (p. 130) is likely to repel an approaching nucleophile, and (b) because its n orbital system is much less capable of delocalising (and so stabilising) the two extra electrons in the negatively charged (72), than the positively charged Wheland intermediate (73) ... 

The H atom flanked by the two 0=0 groups in (22) exhibits hardly any more acidic character than the analogous one in the corresponding hydrocarbon. The different behaviour of (22) stems from the fact that after proton removal, the carbanion s lone pair would be in an sp3 orbital more or less at right angles to the p orbitals on each of the adjacent carbonyl carbon atoms (cf. p. 259) no sp3/p overlap could thus take place, consequently there would be no stabilisation of the -ve charge through delocalisation, and the (unstabilised) carbanion does not, therefore, form. 

It is significant that the substituents involved at the far left-hand side of the plot (38 X, Z = MeO) are powerfully electron-donating, and thus capable of stabilising the carbocation (41a ++ 41b), developing in step , by delocalisation of its +ve charge. It is indeed... 

Related to these diphosphine dichalcogenides are the triphosphine trisulfides [R2P(S)]3CH (12) which can be prepared from lithiated methylene diphosphine disulfides upon treatment with thiophosphinic chlorides (Equation 9). Deprotonation of 12 with tBuLi gives a resonance-stabilised anion 13 containing a planar central carbanion whose charge is delocalised onto the three neighbouring phosphorus and sulfur atoms.32... 

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