Sulfur resonance effect

The mobility of the proton in position 2 of a quaternized molecule and the kinetics of exchange with deuterium has been studied extensively (18-20) it is increased in a basic medium (21-23). The rate of exchange is close to that obtained with the base itself, and the protonated form is supposed to be the active intermediate (236, 664). The remarkable lability of 2-H has been ascribed to a number of factors, including a possible stabilizing resonance effect with contributions of both carbene and ylid structure. This latter may result from the interaction of a d orbital at the sulfur atom with the cr orbital out of the ring at C-2 (21). 

A combination of steric and electrostatic factors is presumably decisive with regard to the form of the acid most stable in sulfuric acid solution. The simple protonated form XX of benzoic acid is stabilized by resonance structures sterically prohibited in mesitoic acids. The ortho methyl groups of mesitoic acid would interfere with a coplanar dihydroxymethylene group. On the other hand, the inductive and resonance effects of the methyl groups help stabilize the acylium ion form of mesitoic acid as in the formulae XXI. In the case of 2,4,6-tribromobenzoic acid the steric effect and its abetting electronic effects are not sufficient, and this acid behaves like benzoic acid.17 >177... 

The 13C NMR spectra of some parent A,B-diheteropentalenes and T(C,H) coupling constants are summarized in Tables 4 and 5. The sensitivity of C-3 to substituent effects is the same in both thieno[2,3-/>]thiophene (33) and thieno[3,2-6]thiophene (12) <76ACS(B)417>. The substituted carbon atoms also show comparable sensitivity. Differences were observed for the substituent transmittance effects to similar positions in these two systems. For example the resonance effect is much more efficiently transmitted over two sulfur atoms in derivatives of compound (12). On the other hand, for the 6a position the system (33) shows a great similarity with 2-substituted thiophenes. 

The ability of thienothiophenes and selenoloselenophenes to transmit substituent effects has been determined by H and l3C NMR, and also calculated by the CNDO/2 method [82CS(20)208 83CS(22)22]. As expected, resonance effects are better transmitted between the 2- and 5-positions in 8.185 than in 8.186, since only in the former is direct conjugation between these positions possible. Both resonance and inductive/field effects were greater in the sulfur- relative to the selenium-containing compounds, but in general, replacement of sulfur by selenium caused only minor perturbations of data, which follows from the fact that the main mode of transmission of substituent effects in five-membered heterocycles must be via the carbon chain (cf. Chapter 6, Section 9.C). 

Furan has a dipole moment of 0.70 D, while thiophene has a dipole moment of 0.51 D. The dipole moments of furan and thiophene are in the opposite direction of pyrrole due largely to the relatively strong inductive effect caused by the oxygen and sulfur in relation to weaker resonance effects. In the case of pyrrole, as described in Chapter 2, the resonance hybrids of the molecule result in the inversion of the dipole. 

NO2 substitution in meta (compound VI) resulted in lower NC values than the para analog (compound VII) (6.1 vs. 8.1, respectively Table I), resonance processes that would involve ring stabilization of a 4-substituted benzenesulphenyl ion may be implicated separation of inductive and resonance effects is complex and has not been attempted in our analysis. Nitro substitution in the ortho position (compound V) resulted in lesser activation (NC value 2.4 Table I) than expected or seen at the other positions. This effect could result from a stabilizing intramolecular H bond of the 2-nitro group that renders the thiophenol species less acidic, or from an inhibition of the approach to a negatively charged thiolate sulfur due to the... 

---