Basicity inductive effect

Applying p/C Values in Organic Acid-Base Reactions 3. EFFECT QF STRUCTURE QN ACIDITY Effect of Periodic Trends on Acidity and Basicity Effect of Resonance on Acidity and Basicity Inductive Effects Effect of Hybridization on Acidity... 

The basic piSTa values, which have to be considered as equilibrium values, including those of anhydrous and hydrated species, reveal a destabilizing inductive effect of the 6- and 7-methyl group towards 3,4-hydrate formation, as do also the 2-methylamino and 2-dimethylamino groups for additional steric reasons. If the cation of 2-aminopteridine did not add water its value would be about 1.6, arrived at by substracting from the piSTa 2.6 of the essentially anhydrous 2-amino-4,7-dimethylpteridine cation 0.3 for the 7- and 0.7 for the 4-methyl group. The difference between the observed value of 4.29 and the... 

The basicities of the parent azole systems in water are shown in Table 1. When both heteroatoms are nitrogen, the mesomeric effect predominates when the heteroatoms are in the 1,3-positions, whereas the inductive effect predominates when they are in the 1,2-positions. The predominance of the mesomeric effect is illustrated by the pK value of imidazole (82 Z = NH), which is 7.0, whereas that of pyrazole (83 Z = NH) is 2.5 cf. pyridine, 5.2). An fV-methyl group is base-strengthening in imidazole, but base-weakening in pyrazole, probably because of steric hindrance to hydration. When the second heteroatom is oxygen or sulfur the inductive, base-weakening effect increases the pK of thiazole (82 Z = S) is 3.5 and that of isoxazole (83 Z = 0) is 1.3. 

This apparent characteristic enhancement in the basicity has been used quite frequently for the determination of the position of a double bond with respect to the nitrogen atom in unsaturated amines. The cases such as neostrychnine (134) and dehydroquinuclidine (139) in which the protonation at the 8-carbon atom cannot occur due to the lack of overlap between the electron pair on the nitrogen atom and the tt electrons of the double bond, since this would involve the formation of a double bond at the bridgehead— a violation of Bredt s rule—show a decrease in basicity. For instance the basicities of quinuclidine (140) and dehydroquinuclidine (139) have been shown by Grob et al. (82), to differ by 1.13 pK units in aqueous solution at 25. This decrease in basicity has been attributed to the electron-withdrawing inductive effect of the double bond. 

Recently Stamhuis et al. (33) have determined the base strengths of morpholine, piperidine, and pyrrolidine enamines of isobutyraldehyde in aqueous solutions by kinetic, potentiometric, and spectroscopic methods at 25° and found that these enamines are 200-1000 times weaker bases than the secondary amines from which they are formed and 30-200 times less basic than the corresponding saturated tertiary enamines. The baseweakening effect has been attributed to the electron-withdrawing inductive effect of the double bond and the overlap of the electron pair on the nitrogen atom with the tt electrons of the double bond. It was pointed out that the kinetic protonation in the hydrolysis of these enamines occurs at the nitrogen atom, whereas the protonation under thermodynamic control takes place at the -carbon atom, which is, however, dependent upon the pH of the solution (84,85). The measurement of base strengths of enamines in chloroform solution show that they are 10-30 times weaker bases than the secondary amines from which they are derived (4,86). 

In addition to pyridine, the six-membered diamine pyrimidine is also found commonly in biological molecules, particularly as a constituent of nucleic acids. With a pKa of 1.3, pyrimidine is substantially less basic than pyridine because of the inductive effect of the second nitrogen. 

Mg/Me (Me=Al, Fe) mixed oxides prepared from hydrotalcite precursors were compared in the gas-phase m-cresol methylation in order to find out a relationship between catalytic activity and physico-chemical properties. It was found that the regio-selectivity in the methylation is considerably affected by the surface acid-basic properties of the catalysts. The co-existence of Lewis acid sites and basic sites leads to an enhancement of the selectivity to the product of ortho-C-alkylation with respect to the sole presence of basic sites. This derives from the combination of two effects, (i) The H+-abstraction properties of the basic site lead to the generation of the phenolate anion, (ii) The coordinative properties of Lewis acid sites, through their interaction with the aromatic ring, make the mesomeric effect less efficient, with predominance of the inductive effect of the -O species in directing the regio-selectivity of the C-methylation into the ortho position. 

The effect of introducing electron-withdrawing groups, e.g. Cl, N02, close to a basic centre is to decrease the basicity, due to their electron-withdrawing inductive effect (cf. substituted anilines below, p. 70) thus the amine... 

Product distributions obtained on esterification of nucleosides and nucleotides under basic conditions throw further light on factors affecting selective reactivity. p-Toluenesulfonylation of adenosine 5 -monophosphate in aqueous alkali yielded exclusively (in 54-61% yield) the 2 -p-toluenesulfonate.107 Lack of reaction at HO-3 was attributed either to formation of a phosphoric p-toluenesulfonic anhydride, which sterically protected this hydroxyl group, or to the higher acidity of HO-2. It has been shown that the acidic site (with pKa 12.5) in adenosine is associated with the presence of both HO-2 and HO-3, as replacement of either of these by hydrogen, or of HO-2 by methoxyl, results in loss of this acidity.108 Inductive effects, or the sta-... 

The true gas phase basicity order is observed in the montmorillonite (43), whereas in solution the well known amine anomaly exists, i.e. the expected inductive effects of the organic carbon chain are screened off by the solvent. For example, identical AG values of protonation are found in solution for methyl- and butylammonium. 

Also other Type B and C series from Table II are consistent with the above elimination mechanisms. The dehydration rate of the alcohols ROH on an acid clay (series 16) increased with the calculated inductive effect of the group R. For the dehydrochlorination of polychloroethanes on basic catalysts (series 20), the rate could be correlated with a quantum-chemical reactivity index, namely the delocalizability of the hydrogen atoms by a nucleophilic attack similar indices for a radical or electrophilic attack on the chlorine atoms did not fit the data. The rates of alkylbenzene cracking on silica-alumina catalysts have been correlated with the enthalpies of formation of the corresponding alkylcarbonium ions (series 24). Similar correlations have been obtained for the dehydrosulfidation of alkanethiols and dialkyl sulfides on silica-alumina (series 36 and 37) in these cases, correlation by the Taft equation is also possible. The rate of cracking of 1,1-diarylethanes increased with the increasing basicity of the reactants (series 33). 

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