Mesomeric factor

This was investigated by Schott and Langecker for the case of the hydrolysis of tetraalkyl and tetraaryldisilanes565 Substitution on the phenyl group changes the reaction rate. A relation between the rate constants and the Hammet constants was observed, but for linearity the Hammet constants have to be reduced to take into account a mesomeric factor. Special aSj constants are therefore necessary (there are similar differences in the a constants of other Si compounds16, 6281. 

Inductive activation (Scheme II) gives only about one-twentieth as great a rate increase as resonance activation from the adjoining ring. The pertinence of the jjom,pom-quinoid structures (342 and 343) or their para,ortho-isomers (not shown) to the observed inductive activation depends on the relative significance of the field and mesomeric factors in these inductive effects. The large decrease in effectiveness of intemuclear resonance activation suggests that the intemuclear mesomeric factor or resonance parameter 179,345 compounds... 

In addition to reaction sequences of type (66) -> (67), electrophilic reagents can attack at either one of the ring nitrogen atoms in the mesomeric anions formed by proton loss e.g. 70 71 or 72 see Section 4.02.1.3.6). Here we have an ambident anion, and for unsymmetrical cases the composition of the reaction product (71) + (72) is dictated by steric and electronic factors. 

Mesomerism involving sulfonyl groups is relatively weak, and in the case of the sulfonyliminopyridones (279) destabilization caused by the factors previously mentioned for the acyliminopyridones should be less important. Early ultraviolet spectral comparisons showed that acetylsulfapyridine and sulfapyridine (cf. reference 354) exist in aqueous solution as mixtures of comparable amounts of 279 and 280. A recent investigation of 2-, 3-, and 4-methanesulfonamido-... 

The marked acid-strengthening effect of p-NOy is usually attributed to the influence of the electron-attracting inductive effect (+/), augmented by a small electron-attracting mesomeric or resonance effect (+R). The smaller acid-strengthening effect of m-NOy is explained as the resultant of the inductive effect and a small relayed influence of the resonance effect. If op is regarded simply as a sum of oj and or (Section II.B) and a/ is taken as 0.67 (Section III.A), a value of 0.78 — 0.67 = 0.11 is indicated for or. The relay factor of 0.33 for the resonance effect accounts reasonably well for the value of om as ay + 0.33or = 0.67 + 0.04 = 0.71 cf 0.71 above. 

Much emphasis has been placed on the selectivity of quaternary ammonium borohydrides in their reduction of aldehydes and ketones [18-20]. Predictably, steric factors are important, as are mesomeric electronic effects in the case of 4-substituted benzaldehydes. However, comparison of the relative merits of the use of tetraethyl-ammonium, or tetra-n-butylammonium borohydride in dichloromethane, and of sodium borohydride in isopropanol, has shown that, in the competitive reduction of benzaldehyde and acetophenone, each system preferentially reduces the aldehyde and that the ratio of benzyl alcohol to 1-phenylethanol is invariably ca. 4 1 [18-20], Thus, the only advantage in the use of the ammonium salts would appear to facilitate the use of non-hydroxylic solvents. In all reductions, the use of the more lipophilic tetra-n-butylammonium salt is to be preferred and the only advantage in using the tetraethylammonium salt is its ready removal from the reaction mixture by dissolution in water. 

Since only atoms of opposite parity are covalently bonded, it follows that double bonds in these species must necessarily link starred and unstarred atoms. This fundamental feature of double bonding in alternant systems is an important factor governing the representation of mesomeric betaines. The relationship between mesomeric betaines and AHs will be developed later in the section. 

The key factor that accounts for differences in regioselectivity between the thiophene and the pyrrole is probably the greater mesomeric release of electrons from the nitrogen atom through the double bonds (89JHC397). 

The increase in stability of quasiphosphonium intermediates Ph P(0R)n-3MeX, with increasing n, is attributed mainly to inductive effects of the substituent groups. Phosphorus-carbon bond lengths for Ph - P and Me - P in the intermediates do not support the view that mesomeric release from phenyl to phosphorus is involved (1). On this basis, the stability should increase with decreasing n, if p -d interaction between oxygen and phosphorus is a significant factor (8), whereas exactly the opposite is observed. 

So far, only one detailed discussion of boron-11 nuclear magnetic resonance spectra of aminoborane systems has been reported 31>. It was found that the 1 lB chemical shifts of aminoborane systems can be described fairly well in terms of a set of additive substituent contributions. In consonance with earlier work on trisubstituted boron compounds 35> these contributions depend on the mesomeric effects of substituents rather than their electronegativity. 1,3,2-diazaboracycloalkanes can be considered as aminoborane derivatives and in the case of the known heterocycles the exocyclic boron substituent will govern primarily the boron chemical shifts and will do so by mesomeric effects. However, the available data are rather limited and it may be possible that additional factors must be considered. Steric effects appear to be negligible, however, since the heterocycles with either six or seven annular atoms have almost identical shifts (Table 5). 

The parent acids and alcohols, on the other hand, are not expected to display any significant mesomeric stabilisation, because this would involve the participation of some rather unreasonable Lewis structures with separated positive and negative charges. As a consequence, the tt-delocalisation in 1 and 2 is a factor that lowers the deprotonation energy of carboxylic acids and enols, thus reinforcing their acidity, according to standard organic-chemistry textbooks. 

Deuteriation of arsabenzene (9.12) in TFA/CH2C12 at 100°C has led to partial rate factors under standard conditions (TFA, 70°C) of /2 = 2500, /4 = 400 (81J0C881). The corresponding o+ values, -—0.39 and -0.30, compared with the 2-, 3-, and 4-o values (0.3, 0.4, 0.1) indicate very strong mesomeric electron release, though, unusually, this is more effective at the 2-position than at the 4-position. The strong electron release appears to be due to the high polarizability of the p orbitals of arsenic. 

The difference in electron influence of five-membered heterocycles which contain 0, S, or Se atoms is explained by the fact that the influence is a sum of the positive conjugation and negative induction thus it operates differently in the static (ground-state mesomeric) or dynamic state. In general, the influence depends on the systems in which it is observed and may be masked by space factors or field effects. 

---