Structure-reactivity correlations in the reaction of ethyl bromoacetate with aliphatic carboxylate ions

The second-order rate constants at three different temperatures have been determined for nucleophilic substitution reactions of ethyl bromoacetate with various substituted aliphatic carboxylate ions (XCH₂COO⁻) in 90% acetone-10% water (v/v) mixture and the activation parameters have been evaluated. The rate data have been analysed in terms of electronic and steric effects by using various single and multi-parameterequations. Analysis of the results shows that localized and steric effects are more important than the delocalized effect. The regression coefficient of the inductive component is negative indicating that inductively electron-releasing substituents accelerate the reaction and electron-withdrawing substituents retard it. The steric component corresponds to the reaction being subject to steric acceleration by the substituents. The per cent steric effect has been calculated. The Br?nsted type correlation is fair, and theβ _N value has been determined.     

A quantum-mechanical study of the inductive and steric effects for the example of <Emphasis Type="Italic">tert</Emphasis>-butylalkanes

A relation between the inductive and steric effects (concepts of the classical theory of molecular structure) and electron density distribution parameters was revealed by quantum chemistry methods. The propagation of the inductive and steric influence of the (CH₃)₃C and CH₃ groups along carbon chains in tert-butylalkanes was analyzed. The steric effect of the (CH₃)₃C group was shown to extend to two CH₂ groups, and the inductive effect, to one CH₂ group. Definitions of the standard (CH₃)₃CCH₂ and (CH₃)₃C(CH₂)₂ groups were given. Configurations of transferable groups decreasing additive approach errors were suggested. Additive procedures for calculating the properties of molecules containing (CH₃)₃C were refined.     

Separation of polar and steric effects in the oxidation of ortho-substituted benzaldehydes by N-bromobenzamide

The kinetics of the oxidation of twelve ortho-substituted benzaldehydes by N-bromobenzamide (NBB) to the corresponding benzoic acids have been studied. The reaction is first order with respect to NBB, the aldehyde and hydrogen ions. The addition of benzamide has no effect on the reaction rate. (PhCONH₂Br)⁺ has been postulated as the reactive oxidising species. The correlation of rates with the single substituent-parameter equations is poor. The correlation with Charton’s equation of inductive, resonance and steric parameters is satisfactory. However, excellent correlations were obtained, when Charton’s steric parameter was used along with Taft’s σ₁; andσ _R ⁺ substituent constants. The polar reaction constants have negative values. The reaction is subject to steric hindrance by the ortho-substituents.     

29Si-NMR-chemische Verschiebung von Verbindungen mit vier elektronegativen Substituenten am Silicium

²⁹Si-NMR Chemical Shift of Compounds with Four Electronegative Substituents at Silicon The influence of the d-orbitals of IIIrd row elements and also the inductive and steric effects of the organoxy groups on the ²⁹Si-NMR chemical shift of compounds with four electronegative substituents attached to silicon wer evaluated. A bridging effect on the transmission of the inductive effects over the oxygen atom which is bonded with silicon was stated.     

Gas-phase reactions of divalent Ni complex ions with acetonitrile: Chelate ring size,inductive, and steric effects

Ni(II) complexes of a series of pentadentate polyamine ligands have been reacted with CH₃CN in the gas phase using a modified quadrupole ion trap mass spectrometer. The ligands have structural features such that upon complexation, chelate ring size, sterics, and inductive effects can be evaluated in the gas phase. Rate and equilibrium constants for CH₃CN addition to the metal complexes show that there is a general decrease in the gas-phase reactivity as the chelate ring size is increased. Density functional theory calculations at the B3LYP/LANL2DZ level of theory have been used to obtain minimum energy structures and Mulliken charges for the complexes. The decreased reactivity observed as the chelate ring size is increased correlates with a decrease in the atomic charge on the metal. A larger chelate ring size enhances ligand flexibility and improves the overlap of the ligand’s donor atoms with the metal center. Adding methyl groups adjacent to or on the nitrogen donor groups of a ligand also decreases the rate and equilibrium constants for the reactions of a given complex with CH₃CN. Analysis of Mulliken charges for these complexes indicates that both inductive and steric effects are responsible for lower complex reactivity. These results suggest that while the gas-phase reactivity of a metal complex with CH₃CN is very dependent on the functional groups directly bound to the metal, in some cases steric effects can conceal the correlation between reactivity and coordination structure.     

Induktive und sterische Effekte bei der BECKMANN-CHAPMAN-Umlagerung von Alkyl-ketoxim-pikryläthern

The rates of the BECKMANN -CHAPMAN rearrangement of the ketoximpicryl-ethers 1a–1k in 1, 4-dichlorobutane are correlated with parameters describing the bulk of the substituted groups R and R′. From regression analyses it is concluded that the relative rate constants are mainly controlled by steric contributions in the ground state, such as geminal interactions of R with R′ and vicinal interactions of R′ with O? X, and less by inductive effects. Conformational equilibria between (Z)- and (E)-isomers of N-alkyl-N-picryl-amids have been studied by NMR. in several solvents.     

Why the addition of neutral oxygen donor groups promotes selectivity for larger metal ions

Abstract

The origin of selectivity enhancement for large metal ions that occurs on the addition of neutral oxygen donors to existing ligands in such a way as to form additional five-membered chelate rings is analyzed in terms of inductive and steric effects. Molecular mechanics calculations are used to examine the degree of strain that develops in five-membered, aliphatic chelate rings of ethers and amines as a function of the size and charge of the metal ion. Although five-membered chelate rings that contain saturated neutral oxygen donors are found to exhibit an inherent steric preference for large metal ions, experimental evidence suggests that for the majority of cases where enhanced selectivity for larger metal ions has been observed after the addition of neutral oxygen donors, the selectivity enhancement is largely the result of steric and inductive changes to other donor groups in the ligand, e.g. amines, rather than the result of increasing the denticity of the ligand.     

Investigation of 2,3-polymethylenequinolines

The rate of the reaction of 4-chloro-6-R-2,3-polymethylenequinolines with sodium methoxide decreases as the number of methylene groups increases; this is explained by the inductive and steric effects of the polymethylene chain. The rate constants of the reaction are in conformity with the Arrhenius equation and are found to exist in a correlation relationship with the σ_cat substituent constants for quinoline.     

A multinuclear 1H, 13C, and 15N magnetic resonance study of ten 4-nitropyridine N-oxides

The ¹H, ¹³C, and ¹⁵N NMR chemical shifts of ten 4-nitropyridine N-oxide derivatives are assigned. The shielding of the pyridine ring nitrogen is sensitive to ring substitution through inductive effects, steric effects by ortho-substituents, and the possibility for electron delocalisation (resonance energy). In solution, 3-ethylnitrosoamino-4-nitropyridine N-oxide has two tautomers. The proposed reason is the steric crowding between vicinal 4-nitro and 3-ethylnitrosoamino groups, causing a disturbance to amino nitrogen that can delocalize its lone pair to the oxygen atom of the nitroso group.     

Phosphonium salts and phosphoranes—VI: 31P NMR spectra of acylmethylenephosphoranes

The ³¹P NMR spectra of 36 acylmethylenephosphoranes have been measured and the chemical shifts have been interpreted in terms of steric interactions and the resonance stabilization of the ylides. It is proposed that the observed solvent shift effects are best accounted for by a change in the resonance stabilization and not, as previously suggested, by a change in the conformational equilibrium position. The inductive electron-withdrawing interaction of α-aryl groups leads to a deshielding of the phosphorus nucleus, whilst the deshielding effect produced by α-acyl substituents can be rationalised in terms of a combination of the mesomeric and inductive interactions. Syntheses of 21 new phosphoranes are described.     

Cautions regarding the physical interpretation of statistically based separation of the ortho substituent effect into inductive,mesomeric, and steric components—III : Some remarks concerning the significance of steric effects in acidic and alkaline ester hydrolyses

It is demonstrated that acidic and alkaline hydrolyses of o-substituted benzoates and acidic esterification of o-substituted benzoic acids depend upon inductive, “mesomeric”, and steric effects. The “mesomeric” contribution is essentially connected with the nitro and the methoxy groups exerting special effects. The rate data of some acidic and alkaline reactions which may be regarded as being comparable yielded regression equations indicating that steric effects are the same in both reactions. The Taft-Ingold assumption seems to be correct for these reactions, whereas the averaging procedure applied by Taft is questionable. The results taken as a whole lend strong support to the hypothesis that the so-called ortho effect is essentially connected with the size of the substituents and not strictly electrical in nature.     

Cautions regarding the physical interpretation of statistically based separation of the ortho substituent effect into inductive,mesomeric, and steric components—II : Acid catalysed esterification of benzoic acids by methanol and cyclohexanol

In order to attain a better insight into the composition of Taft E_s^o, constants the rate data of hydrion catalysed esterification of both m,p-substituted and o-substiluted benzoic acids by cyclohexanol and methanol were submitted to a statistical analysis using inductive mesomeric and steric substituent constants and various dummy variables differently structured. Furthermore a principal component analysis with subsequent identification of the first principal component via multiple regression analysis was applied. It has been demonstrated that in the reactions of m,p substituted compounds some substituents capable of exerting strong mesomeric effects show peculiar characteristics deviating from the general trend. Since the same result was obtained in the correlations of ortho substituted benzoic acids this effect was taken into account using an appropriate dummy variable which in all cases improved the multiple coefficient of determination. It is concluded that the esterification rates of the ortho substituted compounds depend essentially upon inductive and steric effects (taken away OMe OEt and NO₂) as proposed by Taft. While generally the E_s^o values may be regarded as some measure of a steric effect, this is not true for the methoxy and ethoxy groups.     

Electronic properties of amino acid side chains: quantum mechanics calculation of substituent effects

Background  

Electronic properties of amino acid side chains such as inductive and field effects have not been characterized in any detail. Quantum mechanics (QM) calculations and fundamental equations that account for substituent effects may provide insight into these important properties. PM3 analysis of electron distribution and polarizability was used to derive quantitative scales that describe steric factors, inductive effects, resonance effects, and field effects of amino acid side chains.     

Steric effects of polar substituents evaluated in terms of energy by means of isodesmic reactions

Steric effects of various polar and some charged groups were estimated on sterically crowded cyclopropane cis-1,2-bis derivatives 2 or 3, in which the variable substituent is in the proximity of a t-butyl group or of a methyl group. The steric energy was evaluated with reference to the pertinent mono derivatives, that is as reaction energy of an isodesmic reaction, in which the crowded compound is formally synthesized from simple derivatives. Energies were calculated within the framework of the density functional theory at level B3LYP/6-311+G(d,p)//B3LYP/6-311+G(d,p) for 11 dipolar and 5 charged substituents. Interaction of charged substituents is not only steric (destabilizing) but also inductive (stabilizing). The steric effects evaluated in this way differ distinctly from the standard steric constants derived purely from the van der Waals radii of the substituents.     

Adducts of bis(8-quinolinate) oxovanadium(IV) with organic bases

We report the synthesis and study of bis(8-quinolinol) oxovanadium(IV) adducts with pyridine and substituted pyridines. These complexes were identified by IR and electronic spectra, magnetic susceptibility measurements and analytical data. The relation between the basicity of the pyridines and the VO stretching frequency has also been studied. The effects of steric hindrance and the inductive effects exercised by the functional groups, amino and methyl of the pyridine, in the formation of the adducts are discussed.     

Porphyrin acidity and metal ion coordination revisited: electronic substitution effects

Macrocycle acidity and Zn²⁺ ion coordination are reported for three porphyrin derivatives which differ in both steric and electronic substitution effects on the macrocycle π-conjugated system. The role of the electronic substitution effects in the macrocycle deprotonation and metal ion chelating was found to be dominating whereas the macrocycle nonplanar distortions contribute to the acidity and metal chelation rate of the studied porphyrins in less extent. The contributions of both resonance and inductive electronic substitution effects have been distinguished based on the relationship between the weighted sum of resonance and inductive Hammett constants and the acidity and metal ion chelation rate.     

Intermolecular hydrogen bonding between N-substituted caproamides and tetrahydrofuran

The results are reported of a study of hydrogen bonding between various N-substituted caproamides and tetrahydrofuran as an O-electron donor by means of FTIR spectroscopy. The spectroscopic characteristics for N-H...O hydrogen bonded complexes are given. The B3LYP functional with 6–31G** basis set has been used to calculate the structural parameters of the studied hydrogen bonded complexes. It can be assumed that both inductive and steric effects play an important role in the stability of these hydrogen bonded complexes.