Polar Substituent Effects Research Articles

Effects of solvents and substituents controlling the adiabatic or diabatic modes of the cis → trans isomerization of styrylpyrenes in the excited singlet state

The effects of polar solvents and polar substituents on the potential energy surfaces of the cis → trans isomerization of 1-styrylpyrenes in the excited singlet state are investigated by fluorescence spectroscopy. In less polar solvents, cis-1-styrylpyrene undergoes an adiabatic isomerization from the cis excited singlet state ( 1c ∗) to the trans one ( 1t ∗), overcoming an energy barrier around the perpendicular conformation ( 1p ∗). Increase of solvent polarity and/or the introduction of polar p-substituents on the phenyl ring can change the reaction course and induce the deactivation from 1p ∗ resulting in a diabatic isomerization.

Radical Stabilization and Ground State Polar Substituent Effects in the Thermal Decomposition of Azoalkanes

Radical Stabilization and Ground State Polar Substituent Effects in the Thermal Decomposition of Azoalkanes

Dissociation of proton-bound complexes and proton affinity of benzamides

Proton-bound heterodimers of substituted benzamides 1–15 and N,N-dimethyl benzamides 16–30, respectively, with a series of reference bases were generated under chemical ionization conditions. Their dissociation into the protonated amide AH + and protonated reference base BH + was studied by metastable ion techniques and by collision-induced dissociation (CID) to examine substituent effects on the proton affinity (PA) of the benzamides and to elucidate some aspects of the dissociation dynamics of proton-bound clusters. The PAs of the substituted benzamides were determined by bracketing the amide by a pair of reference bases to give rise to more and less abundant signals of the protonated base in the mass-analyzed ion kinetic energy (MIKE) spectra of the proton-bound heterodimers. The substituent effects observed agree with O-protonation in both the primary and the tertiary benzamides. However, the susceptibility of the benzamide to polar substituent effects is remarkably small, which indicates a “resonance saturation” of the amide group. The relative abundances of AH + and BH + in the MIKE and collisional activation (CA) mass spectra depend strongly on the pressure of the collision gas during CID, and in certain cases a reversal of the relative abundances with increasing pressure that favors the formation of BH + from a less basic reference base is observed. Although this effect underlines the limited possibilities of the “kinetic method” for PA determination by CID of proton-bound heterodimers, it uncovers important kinetic effects during the dissociation of proton-bound heterodimers and of proton transfer reactions in the gas phase. In the case of the protonated amide clusters, the observed intensity effects in the CA mass spectra are explained by a double-well potential energy surface caused by solvation of the protonated base by the polar amide in the protonated heterodimer.

Further Investigations on Polar Substituent Effects on the NMR Chemical Shifts ofN-Acylanilines and Related Compounds. Correlation Analytical Approach and Solvent Effects

The 1H, 13C, and 15N chemical shifts of various substituted anilides of the carboxylic and sulfonic acids ZNHC6H4Y (Z = C6H5CO, CH3CO, CF3CO, and CH3SO2) and related compounds were correlated with σ and related substituent constants. The ρ-values from the Hammett type plots of the chemical shifts were shown to provide some important information concerning the polarization of the amide chromophore. The solvent effect on ρ showed the fact that the local π-polarization of an amide is strengthened in polar or hydrogen-donating solvents.

Anomalous polar substituent effect on 15N NMR chemical shifts of Aniline derivatives. Acylanilides and related compounds

AbstractThe 15N chemical shifts of various m‐ and p‐substituted anilides and related derivatives, ZNHC6H4Y [Z = C6H5CO, CH3CO, CF3CO, CH3SO2, (C6H5O)2PO and H], were correlated with σ− constants. With all series of anilides investigated, the plotted points for the m‐ and p‐substituted derivatives tend to be separated from each other and to lie on two different regression lines. Reinvestigation from this point of view revealed that similar separations occur generally with substituted aniline derivatives. The anomalous polar substituent effect was ascribed to the very large contribution of a resonance effect. In addition, the local π‐polarization effect causes a negative slope for the m‐substituted series in the case of carboxylic anilides.

Substituent effects on the rate of dehydrogenation of primary and secondary alcohols

The kinetics of catalytic dehydrogenation of eight secondary alcohols to ketones on a ZnOCr 2O 3 catalyst has been studied in a flow reactor at 360°C and atmospheric pressure. The experimental data were fitted to several kinetic models using a nonlinear least-squares method. The most suitable model was found to be a Langmuir-Hinshelwood-type equation, assuming single-site adsorption of alcohol and surface reaction as the rate-determining step. Reaction rate constants and adsorption coefficients of dehydrogenation of secondary alcohols were correlated by the Taft equation with structural parameters characterizing polar or steric effects of substituents in R 1CH(OH)R 2. Similar data, obtained previously, on nine primary alcohols were included in this correlation. The reaction mechanism is discussed.

Amidines. Part 32. Influence of substitution at the amino nitrogen atom on the sensitivity to substitution at the imino nitrogen atom. pKaValues of N1-methyl-N1-phenylformamidines in water–ethanol solutions

A series of new N1-methyl-N1-phenylformamidines (RNCH–NMeC6H5, 13 compounds) containing variable substituents at the imino nitrogen atom has been synthesized. The pKa values of the compounds have been measured in four binary ethanol–water solvents and correlated with Hammett-type substituent constants, and with pKa values of the corresponding primary amines determined in the same solvents. It is shown that substitution at the amino nitrogen atom has considerable influence on the sensitivity of the amidino group to the polar effects of substituents at the imino nitrogen atom. The parameters for both types of regression depend on the solvent.

The effect of polar substituents on the conformation and stereochemistry of enolate radicals

ESR measurements and AM1 calculations show that ester substituted radicals 2 and 6 prefer conformation A as a result of allylic strain effects. But dipolar repulsion between substituents X and CO 2Et In 2 and 6 has a pronounced effect on the conformation and the stereoselectivity of radicals 2 and 6.

Studies on organophosphorus compounds 61 substituent effects in organophosphorus esters

AbstractExamination of several model organophosphorus compounds, bicyclic phosphates (1), p‐substituted phenylphosphonates (2), and acidic alkylphosphonates (3) reveals that the polar substituent effect in organophosphorus esters parallels that of carbon compounds. No significant variation in polar effects of different alkyl groups with up to 10 carbon atoms is observed. As demonstrated by us, the commonly used Kabachnik polar constant σΦ for alkyl groups of organophosphorus compounds reflects the steric effect associated with steric hindrance to solvation of acidic anions. Therefore, a set of solvation steric parameters Eps has been calculated from pKa values of organophosphorus acids. This parameter can be applied to describe the structural effect on dissociation of organic acids and, after suitable modification, may reflect the steric effect of alkyl groups in hydrolytic reactions of organophosphorus esters.

Double‐layer capacitance measurements of self‐assembled layers on gold electrodes

AbstractThe double‐layer capacitance of self‐assembled unsubstituted and ω‐substituted n‐alkanethiol layers on gold electrodes was studied using a potential step technique. A four‐element equivalent circuit model was used for the evaluation of the current response. The double‐layer capacitance and the conductivity were both found to decrease with increasing chain length for alkanethiol homologues. Substitution with COOH, OH, and NH2 in the ω‐position caused the double‐layer capacitance to rise dramatically, whereas the general pattern of decreasing capacitance with increasing chain length was confirmed for a series of homologue ω‐mercaptoalcohols. The effect of polar substituents is explained by the introduction of permanent dipoles into the self‐assembled layers. The capacitance was independent of the applied potential for dodecanethiol but increased with voltage for butanethiol when tested in the range of −200 to 500 mV versus Ag/AgCl. Buffer strength had little influence on the double‐layer capacitance above 5 mM (phosphate buffer, pH 7.0). The self‐assembly of mercaptoethanol was completed within 3 minutes from an aqueous solution and that of alkanethiols within 10 minutes using acetone solutions. The electrodes were stable for at least several hours.

Transmission of polar substituent effects through the bicyclo[1.1.1]pentane ring system as monitored by 19F NMR shifts

19F substituent chemical shifts (SCS) for a series of 3-substituted (X) bicyclo[1.1.1]pent-1-yl fluorides are reported which indicate strong electronegativity (σ x ) dependent “through-space” effects.

Chemical modification of crosslinked phenyl acrylate copolymers with monoethanolamine

AbstractCopolymerisation of phenyl methacrylate, 4‐chlorophenyl acrylate, 4‐nitrophenyl acrylate and 2,4,6‐tribromophenyl acrylate with divinyl benzene was carried out in aqueous suspension medium at 80°C using benzoyl peroxide as radical initiator. The resulting beaded copolymers were characterised by FTIR. optical and scanning electron microscopy. Polymer analogue reactions of these particulate copolymers with monoethanolamine were carried out at 60.80 and 100°C in dimethyl sulphoxide as solvent. The progress of the reactions was followed by observing the disappearance of phenyl ester carbonyl absorption at 1750 cm−1 and the appearance of 1650cm−1 vibration corresponding to the characteristic amide carbonyl. The percentage conversions were calculated, and the effect of polar substituents in the phenyl acrylates as well as temperature on the reaction are discussed.

SYNTHESIS AND SUBSTITUENT EFFECT ON31P NMR CHEMICAL SHIFT OF ORTHO HYDROXYARYL DIALKYL PHOSPHINE OXIDES

Abstract In this paper, the synthetic methods for the preparation of o-hydroxyaryl dialkyl phosphine oxides (I) are introduced. The title compounds with low bulky substituent are synthesized successfully by the reactions of the Grignard reagents with diethyl o-hydroxyarylphosphonates derived from the rearrange- ment of aryl dialkyl phosphates in good yields. The title compounds with high steric hindered alkyl groups can be prepared by the reaction of dialkyl o-chlorophenylphosphinate with sodium. Whether the dialkyl aryl phosphates on treatment with LDA undergo a rearrangement reaction is mainly determined by the nature of the substituents on the phenyl ring. If the substituent is en electron-donating group, the rearrangement reaction is predominant. If it is an electron-withdrawing one, the nucleophilic substitution reaction takes place. The steric and polar effects of substituents on 31P NMR chemical shifts for the title compounds are well described by the intramolecular local van der Waals interaction energy (EVDW-r) and local dipole interaction energy (Edip-p), which were calculated by MM2(85) program.

Terminal Polar Substituent Effect on the Liquid Crystalline Properties of a Series of 4-Octyloxy-N-(4-substituted benzylidene)anilines

Abstract Mesomorphic properties are studied on the title compounds where substituents are F, Cl, Br, I, NO2, and CN. Besides the known mesophases, nematic and smectic Ad, in CN and NO2 homologues, stable smectic A and B phases were observed in either of the Cl and Br derivatives. Metastable nematic and smectic A phases were found in F derivative. The effects of terminal polar substituents, stabilization of the smectic A phase by moderately polar substituents, and of the nematic phase by strongly polar substituents, were confirmed.

Transmission of polar substituent effects in the adamantane ring system as monitored by 19F NMR: hyperconjugation as a stereoinductive factor

A limited series of (E)- and (Z)-5-substituted adamant-2-yl fluorides 8 and 9, respectively, has been synthesized and characterized, and the 19 F chemical shifts have been measured in several solvents. A wide range of 19 F substituent chemical shifts (SCS, ppm) are obtained for the former system (ca. 9.1 (c-C 6 H 12 ), 9.6 (CDCl 3 ), and 11.8 (HFIP)) compared to the latter (ca. 0.6 (c-C 6 H 12 ), 0.7 (CDCl 3 ), and 2.3 (HFIP)). Factorization of the 19 F SCS into polar field (ρ F σ F ) and residual contributions ( 19 F SCS-ρ F σ F ) reveals the predominance of the latter solvent-independent component for the E fluorides (8). Comparison of 8 with a similar dissection of the 19 F SCS of 4-substituted bicyclo[2.2.]oct-1-fluorides 2 strongly suggests that the origin of the large residual contributions for 8 is «through-three-bond] electron delocalization or double hyperconjugation. The importance of this long-range electronic mechanism as a factor governing π-facial diastereoselection in 2,5-disubstituted adamantanes is discussed

Through-bond transmission of polar substituent effects in the bicyclo [2.2.2]octane ring system as monitored by119Sn NMR chemical shifts: A119Sn NMR study of 10-substituted 9-trimethylstannyltriptycenes

A series of 10-substituted(X)-9-trimethylstannyltriptycenes (2a) were synthesized and characterized and the 119 Sn chemical shifts measured. Comparisons were made of the 119 Sn substituent chemical shifts (SCS) of 2a and the corresponding values of 4-substituted(X)bicyclo [2.2.2] oct-1-yltrimethylstannanes (1a), which confirm the previous conclusion that «through-three-bond» effects or double-hyperconjugation strongly regulate the 119 Sn shifts of the latter system. A more restricted series of 10-substituted(X)-9-methyltripycenes (2c) were acquired and their 13 CH 3 SCS compared with those of the corresponding bicyclo [2.2.2] octanes (1c). Significant «through-bond» contributions are also indicated for the latter system

Effect of terminal polar substituents on the nature of smectic A liquid crystals of a series of 4-(4″-octyloxybenzoyloxy)benzylidene-4′-substituted anilines

Abstract The nature of smectic A liquid crystals and the phase transitions from smectic A to nematic phases are studied in the homologous series of 4-(4″-octyloxy-benzoyloxy)benzylidene-4′-substituted anilines, in which the substituents are H, F, Cl, Br, I, NO2 and CN. Measurements have been performed for the dipole moment of the molecule, the smectic A lattice period, the temperatures and the entropies of phase transitions, and the temperature dependence of the orientational order parameter, all of which provide information on the intermolecular pair potentials. It is shown that the introduction of a terminal polar substituent brings about counteracting contributions to the stability of the smectic A state. It is suggested that the reentrant nematic phase transition can appear in the strongly polar mesogens if the extent of antiparallel association of the molecules increases on lowering the temperature.

ChemInform Abstract: Electrophilic Additions of Positive Iodine to Alkynes Through an Iodonium Mechanism.

Alkynes react with bis(pyridine)iodonium(I) tetrafluoroborate(1) and nucleophiles (CH 3 COOH, HCOOH, Cl − , pyridine, Br − , I − ) to give 1,2-iodofunctionalized alkenes. The regiochemistry of the processes is in accordance with the polar effects of the triple bond substituents. With regard to stereochemistry, terminal alkynes yield anti addition products, but internal ones produce anti addition with weaker nucleophiles (CH 3 COOH, HCOOH, Cl − , pyridine), syn with stronger nucleophiles (I − ), and a mixture of anti and syn with borderline nucleophiles (Br − ) or when the internal alkyne bears a bulky group. An ionic mechanism through a vinyleneiodonium ion is proposed to explain the obtained results

Electrophilic additions of positive iodine to alkynes through an iodonium mechanism

Alkynes react with bis(pyridine)iodonium(I) tetrafluoroborate(1) and nucleophiles (CH 3 COOH, HCOOH, Cl − , pyridine, Br − , I − ) to give 1,2-iodofunctionalized alkenes. The regiochemistry of the processes is in accordance with the polar effects of the triple bond substituents. With regard to stereochemistry, terminal alkynes yield anti addition products, but internal ones produce anti addition with weaker nucleophiles (CH 3 COOH, HCOOH, Cl − , pyridine), syn with stronger nucleophiles (I − ), and a mixture of anti and syn with borderline nucleophiles (Br − ) or when the internal alkyne bears a bulky group. An ionic mechanism through a vinyleneiodonium ion is proposed to explain the obtained results

Amidines. Part 33. Full ab initio 3-21G optimization of the molecular structures of fluoro derivatives of formamidine and their protonation products. Quantitative structure–basicity relations of amidines

ab initio 3-21G geometry optimizations for eight derivatives of formamidine with up to three fluorine substituents, and all corresponding formamidinium cations protonated at the imino nitrogen, show strong conjugation in the amidine system between the CN double bond and the lone electron pair on the amino nitrogen atom. Substitution with fluorine at each atom of the NC–N group exerts considerable influence on this conjugation, and as a result the geometry of the molecule can be drastically changed. The introduction of the electron-withdrawing fluorine substituent at the amino nitrogen atom of formamidines leads to a tetrahedral bond arrangement at this atom. For amidines unsubstituted at the amino group and for all protonated systems, co-planarity is predicted by calculation, and all substituents lie in the plane of the NC–N group.Calculations of energies of protonations as well as proton transfer between amidines and ammonia show that changes in basicities of amidines caused by substitution at certain sites of the amidine group depend on polar effects of substituents at other sites. This provides an explanation for the observed changes of ρ values caused by substituents at different sites in correlations of experimental pka values of amidines with Hammett σ constants. The influence of substitution at both nitrogen atoms on tautomeric equilibria in amidines is discussed.