Effect of strain on gas‐phase basicity of (E)‐1‐methyl‐2‐(1‐methyl‐2‐adamantylidene)adamantane

Steric strain, caused by forced deformation of double bond geometry, has significant impact on alkene reactivity, as was shown by a study of the gas‐phase basicity of (E)‐1‐methyl‐2‐(1‐methyl‐2‐adamantylidene)adamantane. The Gibbs free energy of the strain effect in this compound was 42 kJ/mol. This analysis was made based on adamantylideneadamantane as a congeneric reference compound with a planar double bond. It was concluded that gas‐phase basicity could serve as a valuable alternative indicator for quantification of steric strain in alkenes. Copyright © 2015 John Wiley & Sons, Ltd.     

Quantifying electronic effects of common carbohydrate protecting groups in a piperidine model system

A study of the substituent effects of protecting groups in hydroxypiperidines was carried out and compared with the electronic effects in glycosylation chemistry. 1‐Deoxynojirimycin, the aza‐sugar analogue of 1‐deoxy‐D ‐glucose, was used as a carbohydrate model, and protected with the most common carbohydrate protecting groups. The different stabilization of positive charge on the ring heteroatom was determined by pK_a measurements. The protecting groups could be ranked in the following way after their destabilization of the piperidinium ion: benzoyl≥acetyl?4,6‐O‐benzylidine>benzyl≈methyl>H>3,6‐anhydro>tert‐butyldimethylsilyl. The observed effects of having protecting groups with different electronic characteristics were found to be in agreement with the “armed–disarmed” concept. Comparison of the pK_a of benzylated and benzoylated epimers of 3‐hydroxy‐6‐hydroxymethyl piperidines showed increased stabilization of the piperidinium ion in the axial epimer. The difference between axial and equatorial epimers was larger in the benzylated than in the benzoylated piperidines.     

Evaluation of the protonation thermochemistry obtained by the extended kinetic method

An evaluation of the results obtained by the extended kinetic method for a series of representative bases is presented here. Analysis of the original experimental data is conducted using the orthogonal distance regression (ODR) statistical treatment. A comparison with the proton affinities and protonation entropies obtained from variable temperature equilibrium constant measurements demonstrate deviations, which may be ascribed to random and systematic errors. Considerable random errors are associated with the extended kinetic method if the number of reference bases and the range of effective temperatures are too low. It is also confirmed that large systematic errors on proton affinities and protonation entropies are obtained when large protonation entropy is associated with the considered system. It is, however, encouraging to note that the gas phase basicities obtained by the extended kinetic method are generally comparable to that obtained by other methods within a few kJ/mol.     

Effect of polymer microstructure on solubility characteristics and interactions

The effect of polymer microstructure on the solubility characteristics and propensity for interactions of polybutadiene (PBD) has been studied by means of the general solvation equation using data reported by Alessi et al. An increase in the vinyl content from 8% to 69% did not affect the propensity of PBD to interact with and n electron pairs and with dipolar — polarisable probes. In contrast, the hydrogen-bond basicity decreased due to steric hindrance from the alkene side group.     

Protolytic Equilibrium of the Isomeric Phenyl-1,2,4-triazines

The protonation constants for the first and second stages (pK_BH+, pK_BH2+) of a series of 1,2,4-triazines with a phenyl substituent at various positions in the ring were determined in aqueous solution by a spectrophotometric method. The values of the basicity constants characterizing the first protonation of the heterocycles investigated was in the range of acidity of the medium of pH 3.5 to H ₀ -2, and the second from H ₀ -7.3 to H ₀ -8.7. The position of the phenyl substituent proved to have a significant effect on the size of pK_BH+. According to the results of ab initio calculations using HF/6-31G** for the heterocycles investigated the 1H⁺ form is thermodynamically most stable among the monocations, with the exception of 6-phenyl-1,2,4-triazine for which the existence of the monocation in the 1H⁺ and 2H⁺ forms are equally probable. In the case of the dications of all the triazines the 2,4-H,H²⁺ tautomer is the most preferred. The aromaticity of the 1,2,4-triazine ring is changed insignificantly on mono- and diprotonation.     

Phosphazenyl Phosphines: The Most Electron‐Rich Uncharged Phosphorus Brønsted and Lewis Bases

It was discovered that phosphazenyl phosphines (PAPs) can be stronger P‐superbases than the corresponding Schwesinger type phosphazene N‐superbases. A simple synthetic access to this class of PR₃ derivatives including their homologization is described. XRD structures, proton affinities (PA), and gas‐phase basicities (GB) as well as calculated and experimental pK values in THF are presented. In contrast to their N‐basic counterparts, PAPs are also privileged ligands in transition metal chemistry. In fact, they are currently the strongest uncharged P‐donors known, exceeding classical and more recently discovered ligands such as PtBu₃ and imidazolin‐2‐ylidenaminophosphines (IAPs) with respect to their low Tolman electronic parameters (TEPs) and large cone angles.     

Acid-Base properties of MgCuAl mixed oxides

MgCuAl layered double hydroxides (LDHs) with a hydrotalcite like structure containing different proportions of Mg²⁺ and Cu²⁺ cations have been prepared. Thermogravimetry and X-ray diffraction data indicated that the transformation of LDH into mixed oxides is effective after calcination at 723 K, irrespective of the composition. The acid-base properties of these mixed oxides have been investigated using adsorption microcalorimetry and X-ray photoelectron spectroscopy with NH₃ (for acidity) and SO₂ (for basicity) as probe molecules. Their catalytic behaviour for the conversion of cyclohexanol has been tested. The acid-base properties and the selectivity of catalysts has been related to their composition. This revised version was published online in July 2006 with corrections to the Cover Date.     

Solvent effect on catalytic properties of the HCI-DMF-1,1,2,2-tetrachloroethane system

Ionization of 2- and 3-nitroanilines was studied in HCl-DMF-1,1,2,2-tetrachloro-ethane (TCE) solutions at 25 °C. The ionization capability of the medium and basicity constants pK _i of indicators change depending on the ratio of the components. The numerical values of pK _i are found to depend on the analytical composition of the DMF-TCE solvent. The solvent effect on pK _i is associated with a change in the solvation of the nonionized form of the indicators.Translanted fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1446–1449, June, 1996.     

Calorimetric determination of activated carbons in aqueous solutions

The interactions among five samples of activated carbons, obtained from different lignocellulosic materials with different degrees of activation of approximately 20% and aqueous solutions of phenol and 4-nitro phenol are studied by means of the determination of immersion enthalpies. It is established that the obtained activated carbons are of a basic character and show values for the pH at the point of zero charge, pH_PZC, that range from 7.4 to 9.7 and, in all cases, higher total basicity contents than the values obtained for total acidity. The immersion heat of the activated carbons in CCl₄ and water is determined obtaining values which are higher for CCl₄ immersion and vary from 31.4 to 48.6 J g⁻¹. The hydrophobic factor, hf, it is calculated from the relation between of the immersion heat of the activated carbons in CCl₄ and the immersion heat in water, the obtained values were 2.98 and 6.75, which are greater than 1 due to the greater values obtained in CCl₄ when compared to the values obtained in water. Immersion enthalpies in phenol solution range from 7.6 to 13.9 J g⁻¹ and for the case of 4-nitro phenol such enthalpies range from 12.7 to 20.5 J g⁻¹; all the 5 samples studied showed a higher value for the heat of immersion in aqueous solutions of 4-nitro phenol.