Basicity, substituents effect

Another example of enhanced sensitivity to substituent effects in the gas phase can be seen in a comparison of the gas-phase basicity for a series of substituted acetophenones and methyl benzoates. It was foimd that scnsitivtiy of the free energy to substituent changes was about four times that in solution, as measured by the comparison of A( for each substituent. The gas-phase data for both series were correlated by the Yukawa-Tsuno equation. For both series, the p value was about 12. However, the parameter r" ", which reflects the contribution of extra resonance effects, was greater in the acetophenone series than in the methyl benzoate series. This can be attributed to the substantial resonance stabilization provided by the methoxy group in the esters, which diminishes the extent of conjugation with the substituents. 

Be sure to remind students that these frequencies are gas phase data and arc thus not the same as the more-faniiliar solution spectra (we will treat solvated systems in Chapter 9). Even so, such gas phase calculations make excellent discovery-based exercises, For example, students may be asked to explain the substituent effects observed tising basic chemistry knowledge. 

The reactivities of compounds of type 6 with aniline in acetone correlate quite well with substituent effects, and autocatalysis is unimportant here. In the less polar tetrahydrofuran, where the hydrochloride is only partly soluble, the reaction shows autocatalysis when aniline and -chloro aniline are reactants but not when the more basic -toluidine is involved. In these cases the solubility of the acidic product may also influence the differential behavior observed. 

In spite of the potential complexity of the general problem, even when restricted to the reagent family of amines, the nucleophilicities of such series as meta- and pom-substituted pyridines and anilines appear to correlate very closely with the expected substituent effects and with the basicities. This has been verified in the following cases (i) The reaction of pyridines (R = H, m- andp-CHs) with 2-chloro-3-nitro-, 2-chloro-5-nitro-, and 4-chloro-3-nitro-pyridines. ... 

W-OCH3 group should lower the basicity of naphthoic acids (both a and j8) and the nucleophilic reactivity of aza-benzene derivatives. In terms of a, the substituent effect for alkoxy groups wiU be negative at all positions. The w-SCHg group behaves similarly. 

The effects of the nucleophile on aromatic substitution which are pertinent to our main theme of relative reactivity of azine rings and of ring-positions are brought together here. The influence of a nucleophile on relative positional reactivity can arise from its characteristics alone or from its interaction with the ring or with ring-substituents. The effect of different nucleophiles on the rates of reaction of a single substrate has been discussed in terms of polarizability, basicity, alpha effect (lone-pair on the atom adjacent to the nucleophilic atom), and solvation in several reviews and papers. ... 

Quinoxalines, because of the 1 4 arrangement of their ring nitrogen atoms, are only weakly basic. The effect of substituents on basic strength is illustrated in Table II thus -Me, -NH2, a-NHMe, and... 

There has been a HO study of the stereoelectronic effects in methy1phosphines (90). Steric effects were found to concentrate in the HOHO and accounted for half the substituent effects on the pK values, whilst electronic effects on the HOHO was minimal.242 The relative basicities of polymethoxytriarylphosphines have been measured. Tris(2,4,6-trimethoxyphenyl)phosphine was considerably more basic than piperidine.243 The basicities of P-N compounds have been reviewed and their correlation with P-N bond lengths invest igated.2 4 4... 

The reaction of 3,4-bis(benzenesulfonyl)furoxan with alcohols and thiols in basic media affords a variety of alkoxy-and alkylthio-substituted (benzenesulfonyl)furoxans. For these derivatives a paramount problem is to determine the position (3- or 4-) of the substitution in the furoxan ring. The structures of these derivatives were assigned on the basis of both chemical and NMR evidence. In particular, 13C NMR substituent constants were obtained by NMR study of suitable furoxan models. By assuming a complete additivity of the substituent effects at the furoxan ring, these values were used for structural determination <1997FES405>. 

It was also found that both the Lewis acidity and basicity depend not only on specific electronic properties of the central group 13 and group 15 elements, but significantly on substituent effects.24 Electron-withdrawing substituents increase the Lewis acidity but decrease the Lewis basicity, whereas electron-donating substituents decrease the Lewis acidity and... 

Substituent effects hae been observed in a series of substituted malonato complexes (167). The 9Be resonance frequencies move to higher field as the basicity of the ligand increases, as can be seen in Table XI. Quadrupolar broadening is considerable in complexes of the hydrolyzed trimer, so much so that unless a species of this sort is present in high concentration relative to the others, its signal may be buried in the baseline noise. The tetrahedron is significantly distorted from Td symmetry in these compounds. 

In general, there is always an attenuation in effects from gas phase to solution (slope < 1), but this has been found to be particularly large for substituent effects of the basicity of alkylamines48 and pyridines49 in water. 

The accurate determination of gas-phase basicities and gas-phase acidities opened the way to analyses of the effect of solvation on proton acidities, and on hydrogen-bond acidities and basicities, as well as on substituents effects. 

Substituent effects have fascinated organic chemists for generations and their study is still an active area of research. The generalization of the influence of substituents is expected to lead to an understanding of physical properties, structures, equilibria and reactions in organic chemistry (Schleyer, 1987). Substituents can be considered as perturbations of a given standard system and it is often believed that their character remains basically unaltered from one molecular situation to another, i.e. an invariable universal nature of a substituent is assumed. 

A kinetic smdy has been reported of substituent effects on the reactions of 2-phenoxy- and 2-(4-nitrophenoxy)-3-nitro-5-X-thiophenes with benzylamine and with A-methylbenzylamine in benzene as solvent. The intramolecularly hydrogen-bonded intermediate (14) is postulated. Reactions of the 5-unsubstimted thiophenes (X = H) are not base-catalysed, indicating that nucleophilic attack is rate limiting, and the more basic secondary amine shows higher reactivity than the primary... 

These p/Tg-values show that the inductive effect of the C=C bond accounts for about one half of the reduction in the basicity of the enamine nitrogen atom, the other half being due to the mesomeric effect (Stamhuis et al., 1965). The localization energy of the lone pair on the nitrogen, calculated from the second Ap/iTa, is only about T6-1-7 kcal mole (6 7-7 l kj mol ). There are indications from the substituent effects on the stretching frequency of the C=CH2 bond that this group is an electron sink of the same kind... 

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