Steric Effects Of Alkyl Groups Research Articles

TTABr 미셀 용액속에서 2-알킬벤즈이미다졸 음이온에 의해 추진되는 디페닐-4-니트로페닐 포스페네이트(DPNPIN)의 탈인산화반응

This study is mainly focused on micellar effect of tetradecyltrimethyl ammonium bromide(TTABr) solution including alkylbenzimidazole(R-BI) on dephosphorylation of diphenyl-4-nitrophenylphosphinate(DPNPIN) in carbonate buffer(pH 10.7). Dephosphorylation of DPNPIN is accelerated by <TEX>$BI^{\Theta}$</TEX> ion in <TEX>$10^2$</TEX> M Carbonate buffer(pH 10.7) of <TEX>$4{\times}10^{-4}$</TEX> M TTABr solution up to 80 times as compared with the reaction in Carbonate buffer by no benzimidazole(BI) solution of TTABr. The value of pseudo first order rate constant(<TEX>$k_{\psi}$</TEX>) of the reaction in TTABr solution reached a maximum rate constant increasing micelle concentration. The reaction mediated by <TEX>$R-BI^{\Theta}$</TEX> in micellar solutions are obviously slower than those by <TEX>$BI^{\Theta}$</TEX>, and the reaction rate were decreased with increase of lengths of alkyl groups. It seems due to steric effect of alkyl groups of <TEX>$R-BI^{\Theta}$</TEX> in Stern layer of micellar solution. The surfactant reagent, TTABr, strongly catalyzes the reaction of DPNPIN with R-BI and its anion(<TEX>$R-BI^{\Theta}$</TEX>) in Carbonate buffer(pH 10.7). For example, <TEX>$4{\times}10^{-4}$</TEX> M TTABr in <TEX>$1{\times}10^{-4}$</TEX> M BI solution increase the rate constant(<TEX>$k_{\psi}=99.7{\times}10^{-4}1/sec$</TEX>) of the dephosphorylation by a factor ca. 28, when compared with reaction(<TEX>$k_{\psi}=3.5{\times}10^{-4}1/sec$</TEX>) in BI solution(without TTABr). And no TTABr solution, in BI solution increase the rate constant(<TEX>$k_{\psi}=3.5{\times}10^{-4}1/sec$</TEX>) of the dephosphorylation by a factor ca. 39, when compared with reaction (<TEX>$k_{\psi}=1.0{\times}10^{-5}1/sec$</TEX>) in water solution(without BI).

CTABr 미셀 용액속에서 2-Alkylbenzimidazole 음이온에 의해 추진되는 Isopropyl phenyl-4-nitrophenyl phosphinate(IPNPIN)의 탈인산화반응

This study is mainly focused on micellar effect of cetyltrimethyl ammonium bromide(CTABr) solution including alkylbenzimidazole(R-BI) on dephosphorylation of isopropyl-4-nitrophenylphosphinate(IPNPIN) in carbonate buffer(pH 10.7). The reactions of IPNPIN with R-<TEX>$BI^{\ominus}$</TEX> are strongly catalyzed by the micelles of CTABr. Dephosphorylation of IPNPIN is accelerated by <TEX>$BI^{\ominus}$</TEX> ion in <TEX>$10^{-2}$</TEX> M carbonate buffer(pH 10.7) of <TEX>$4{\times}10^{-3}$</TEX> M CTABr solution up to 89 times as compared with the reaction in carbonate buffer by no benzimidazole(BI) solution of <TEX>$4{\times}10^{-3}$</TEX> M CTABr. The value of pseudo first order rate constant(<TEX>$k_{\Psi}$</TEX>) of the reaction in CTABr solution reached a maximum rate constant increasing micelle concentration. Such rate maxima are typical of micellar catalyzed bimolecular reactions. The reaction mediated by R-<TEX>$BI^{\ominus}$</TEX> in micellar solutions are obviously slower than those by <TEX>$BI^{\ominus}$</TEX>, and the reaction rate were decreased with increase of lengths of alkyl groups. It seems due to steric effect of alkyl groups of R-<TEX>$BI^{\ominus}$</TEX> in Stern layer of micellar solution. The surfactant reagent, CTABr, strongly catalyzes the reaction of IPNPIN with R-BI and its anion(R-<TEX>$BI^{\ominus}$</TEX>) in carbonate buffer(pH 10.7). For example, <TEX>$4{\times}10^{-3}$</TEX> M CTABr in <TEX>$1{\times}10^{-4}$</TEX> M BI solution increase the rate constant(<TEX>$k_{\Psi}=98.5{\times}10^{-3}\;sec^{-1}$</TEX>) of the dephosphorylation by a factor ca.25, when compared with reaction(<TEX>$k_{\Psi}=3.9{\times}10^{-4}\;sec^{-1}$</TEX>) in <TEX>$1{\times}10^{-4}$</TEX> M BI solution(without CTABr). And no CTABr solution, in <TEX>$1{\times}10^{-4}$</TEX> M BI solution increase the rate constant(<TEX>$k_{\Psi}=3.9{\times}10^{-4}\;sec^{-1}$</TEX>) of the dephosphorylation by a factor ca.39, when compared with reaction (<TEX>$k_{\Psi}=1.0{\times}10^{-5}\;sec^{-1}$</TEX>) in water solution(without BI). This predicts that the reactivities of R-<TEX>$BI^{\ominus}$</TEX> in the micellar pseudophase are much smaller than that of <TEX>$BI^{\ominus}$</TEX>. Due to the hydrophobicity and steric effect of alkyl group substituents, these groups would penetrate into the core of the micelle for stabilization by van der Waals interaction with long alkyl groups of CTABr.

ChemInform Abstract: Kinetics and Mechanism of Acid Catalyzed Decomposition of 1-Phenyl-3,3- dialkyltriazenes.

Five model 1-phenyl-3,3-dialkyltriazenes (methyl, ethyl, 2-propyl, butyl, cyclohexyl) have been synthesized and their acid-catalyzed decomposition kinetics have been investigated spectrophotometrically in aqueous ethanol (40 vol.%) with pivalic acid as the catalyst. The results show that the rate-determining step is catalyzed by the proton. The decrease in the observed rate constant at higher concentrations of pivalic acid is explained by the formation of an unreactive complex of the nondissociated acid and respective triazene. The steric effect of alkyl groups on the catalytic rate constants is discussed.

Diphenyl-4-nitrophenylphosphinate(DPNPIN)의 탈인산화반응에 미치는 Alkylbenzimidazole의 친핵적 및 Cetylpyridinium chloride(CPyCl) 미셀 촉매효과

Abstract This study is mainly focused on micellar effect of cetylpyridinium chloride(CPyCl) solution including alkyl- benzimidazole(R-BI) on dephosphorylation of diphenyl-4-nitrophenylphosphinate(DPNPIN) in carbonate buffer(pH 10.7). The reactions of DPNPIN with R-BI ⊖ are strongly catalyzed by the micelles of CPyCl. Dephosphorylation of DPNPIN is accelerated by BI ⊖ ion in 10 -2 M carbonate buffer(pH 10.7) of 4×10 -3 M CPyCl solution up to 100 times as compared with the reaction in carbonate buffer by no BI solution of 4×10 -3 M CPyCl. The value of pseudo first order rate constant(k mBI ) of the reaction in CPyCl solution reached a maximum rate constant increasing micelle concentration. Such rate maxima are typical of micellar catalyzed bimolecular reactions. The reaction mediated by R-BI ⊖ in micellar solutions are obviously slower than those by BI ⊖ , and the reaction rate were decreased with increase of lengths of alkyl groups. It seems due to steric effect of alkyl groups of R-BI

A kinetic investigation, supported by theoretical calculations, of steric and ring strain effects on the oxidation of sulfides and sulfoxides by dimethyldioxirane in acetone

The oxidations of alkyl 4-nitrophenyl, and dialkyl, sulfides and sulfoxides by dimethyldioxirane in acetone occur by concerted mechanisms but the sulfides respond differently from the sulfoxides to variation in the alkyl group. The reactions of the sulfides are inhibited by the steric effects of alkyl groups and these predominate over their inductive effects. By contrast, the reactions of these limited sets of sulfoxides are insensitive to alkyl steric effects but there is an indication of steric acceleration when a broader set of sulfoxides is considered. This behaviour is rationalised in terms of the differences in dipolar charge and its solvation between the ground state and transition state for the two types of substrate. The oxidations of cyclic sulfides and sulfoxides also exhibit contrasting behaviour. The reactivity of the sulfides is insensitive to ring strain but is explicable in frontier orbital terms whereas that of the sulfoxides is partly dependent upon the change in ring strain between reactant and product on oxidation, a difference rationalised in terms of the relative positions of the transition states in the reaction coordinates of the two oxidations. The reactivity of 4-, 5- and 6-membered cyclic sulfoxides is also dependent on a ring-size related property of the transition state. Calculations at the B3-LYP/6-31G* level of density functional theory on both ground states and transition states, including simulation of solvation by acetone, strongly support the mechanistic conclusions reached in this and earlier work.

Steric effects on forming different by-products in the formylation reaction of 2,4-dialkylphenol (dialkyl = t-Bu/ t-Bu, t-Bu/Me and Me/Me) proved by their structural and spectral characterizations

In the formylation reaction of 2,4-dialkylphenol (2,4-di- tert-butylphenol, 2- tert-butyl-4-methylphenol and 2,4-dimethylphenol) in the presence of hexamethylenetetramine, steric effects of alkyl groups play important roles in forming different types of by-products, namely 2,4-di- tert-butyl-6-[(6,8-di- tert-butyl-2 H-1,3-benzoxazin-3(4 H)-yl)methyl]phenol ( 1), 2- tert-butyl-4-methyl-6-[(6- tert-butyl-8-methyl-2 H-1,3-benzoxazin-3(4 H)-yl)methyl]phenol ( 2) and tris(2-hydroxy-3,5-dimethylbenzyl)amine hydrochlorate ( 3). These three compounds are fully characterized and single-crystal structures of 1 and 3 are further elucidated.

ChemInform Abstract: On the Steric Effect of Alkyl Groups at the Nitrogen Atom on the Regiochemistry of Dehydrobromination of Ammonium Salts Containing a 2,3‐Dibromoalkyl Group

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

The effect of alkyl groups on carbanion stability part II. Analysis of α‐ and β‐alkyl substitution on experimental and theoretical acidities

AbstractMultivariational analyses were performed to separate the contribution of electronic and steric effects of alkyl groups on the experimental hydrocarbon acidities of four different sets of compounds, consisting of hydrocarbons (SET‐I), acetophenone derivatives (SET‐II), and acetone CH3COCHR1R2 derivatives, where the acidity of the protons on the alkyl substituted site, (SET‐III), and the acidity of the methyl protons are both investigated (SET‐IV). Significant regression was obtained upon backward elimination for the first and last set. Due to the colinearity of the parameters in SETs‐II and ‐III the significance of the correlation is not so high. As expected, the gas phase acidity of the hydrocarbons in SET‐I is very sensitive to the polarizability and electronegativity of the alkyl substituents. The substituent have similar effects on the solution acidities of the ketone derivatives in SETs‐II, ‐III, though suppressed due to solvation, unlike the cases in SET‐II and SET‐III, the effects of the substituents in SET‐IV, where the substituents are relatively far away from the carbanion center generated are strongly reduced and reversed. This different behavior may be attributed to the domination of the solvent effects. Theoretical acidities of these compounds were calculated using the semiempirical methods such as MNDO, AM1 and PM3. The results are, quantitatively and qualitatively, unsatisfactory for the gas phase, as well as for the solution acidities. Multivariational analysis of substituent effects shows the insufficiency of the present parametrization to reflect the substituent effect properly. It seems that all these semiempirical methods are useless in prediction of carbon acidities, without reparametrization using carefully selected sets of compounds.

Kinetics and Mechanism of Acid Catalyzed Decomposition of 1-Phenyl-3,3-dialkyltriazenes

Five model 1-phenyl-3,3-dialkyltriazenes (methyl, ethyl, 2-propyl, butyl, cyclohexyl) have been synthesized and their acid-catalyzed decomposition kinetics have been investigated spectrophotometrically in aqueous ethanol (40 vol.%) with pivalic acid as the catalyst. The results show that the rate-determining step is catalyzed by the proton. The decrease in the observed rate constant at higher concentrations of pivalic acid is explained by the formation of an unreactive complex of the nondissociated acid and respective triazene. The steric effect of alkyl groups on the catalytic rate constants is discussed.

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.

Steric effects of alkyl groups: A ‘cone angle’ approach

AbstractA ‘cone angle’, θR, is defined for an alkyl group (R), which is proposed as a measure of the steric effect exerted by the group. The θR values for some 57 groups have been calculated mathematically by constructing the corresponding Corey—Pauling—Koltun molecular models. The variation in θR with the bulk of a group is satisfactory. However not all types of alkyl groups can be accommodated in this cone angle approach. In the case of the groups for which the cone angle can be measured, correspondences between θR and the two existing steric parameters (Taft's Es scale and Dubois' E′s scale) have been established. For some 23 alkyl groups θR has been used to explain the variation in the rate constant of the base‐catalysed hydrolysis of RCOOEt with the nature of R. Our studies independently substantiate the basic assumptions of the Taft—Ingold hypothesis that, while the base‐catalysed ester hydrolysis is mostly controlled by the steric and electronic effects of the R group, the acid‐catalysed one is controlled almost solely be the steric effect.

Stability Constants of Imidazole– and Alkylimidazole–Copper(II) Complexes

Abstract The stability constants of imidazole (HIm)– and alkylimidazoles–copper(II) acetate complexes were calculated from the potentiometric titration data with the aid of the SUPERQUAD computer program. The stability constants of ionic species of [CuALr]+ type (A : CH3COO, L : HIm a, 1-CH3·Im b, 2-CH3·HIm c, 4-CH3·HIm d, 2-C2H5·HIm e, 2-(CH3)2CH·HIm f, r=1, 2, 3, and 4) detected from the complexes were ascertained to be decreased in the order of b(1-substituted)≥d(4-substituted)&amp;gt;c(2-substituted) relating to the position of methyl group bonded to the imidazole ring, and c(2-CH3)&amp;gt;e(2-C2H5)&amp;gt;f(2-(CH3)2CH) relating to the kinds of alkyl group substituted on 2-position, respectively. The stability of these ionic species was discussed from the neutralization effect of the charge and the steric effect of alkyl group or acetate ion. The stability constants of the hydroxo species of [CuL2(OH)2] type indicated the extremely small values, and its order did not indicate a similar tendency with those of ionic species of [CuALr]+ types. The distribution of these complex species was investigated as a function of pH.

Relative importance of the inductive and steric effects of alkyl groups (R) in understanding the nature of the CoIII–C bond in alkyl cobaloximes, and comments on related alkyl cobalamins

We have shown that multiple regression analyses involving Taft's polar substituent constant, σ*, and Dubois' steric parameter, E′s, can satisfactorily (r2 0.9) explain the properties of alkyl cobaloximes (R = alkyl) and alkyl cobalamins related to the effects of the R groups. The properties studied are the redox potential of the CoIII–CoIV couple, Co→C charge transfer energy, trans influence and trans effect of R in alkyl cobaloximes, and trans influence of R in alkyl cobalamins. From these studies, it is concluded that the CoIII–R bond strength decreases with decrease in (σ*+λE′s) where λ is the mixing coefficient. As indicated by the values obtained, the steric demand of the corrin moiety is found to be an order of magnitude higher than that of dimethylglyoximate. Such semi-quantitative estimation is the first of its kind. The trans influence of R is found to be just opposite to the general observation for this static phenomenon and is designated as a special feature of the CoIII–R bond.

Electronic and Steric Effects of Alkyl Group on Denitrosation of 3-Alkyl-1-methyl-1-nitrosothioureas

Abstract A series of 3-alkyl-1-methyl-1-nitrosothioureas with R=CH3, C2H5, cyclo-C6H11 (3), (CH3)2CH (4), C2H5(CH3)CH, and (CH3)3C were synthesized and their rates of acid catalyzed (pH&amp;lt;6.5) denitrosation measured. According to rate measurements in CH3COOD–D2O, the kinetic isotope effect kH:kD is 1.25 for 4. Except 3, a linear plot of logkR⁄kMe for the denitrosation of RNHCSN(NO)CH3 vs. σ* provides ρ*=−0.98 (r=−0.997). The significant factor affecting the rate-determining step of the denitrosation of these N-nitrosothioureas at pH 4.6 is the electronic effect of the substituent at the N3 position.

Steric effects on the formation of alkyl radicals and alkyl carbenium ions

In order to determine the variation of the steric effect with branching on the formation of alkyl radicals and alkyl carbenium ions, logarithms of rate constants for formation were correlated with the branching equation. Significant correlations were obtained for all the sets studied. The results of the correlations are (1) the composition of the alkyl steric effect as a function of branching is highly variable both for alkyl radical and for alkyl carbenium ion formation. No single steric substituent constant could possibly account for all the data. (2) The relative importance of branching at C2 to that at C1 is a linear function of the amount of branching in the leaving group. This is true for both alkyl radical and alkyl carbenium ion formation. (3) Whereas branching at C1 and C2 causes steric acceleration in the formation of both alkyl radicals and alkyl carbenium ions, branching at C3 results in steric deceleration. The results demonstrate the utility of the branching equation in the study of the steric effects of alkyl groups. It is capable of representing the complete range of alkyl steric effects, and can detect a change from steric acceleration to steric deceleration due to branching at different carbon atoms of the same alkyl group. The expanded branching equation has been shown to be useful in the determination of the variation of alkyl steric effect with the order of branching at a given carbon atom. Radical formation is much more sensitive to a second branch at C1 than is carbenium ion formation.

Dynamic stereochemistry at sp2sp3 bonds part 1—the use of heterocyclic models for quantitative studies of steric effects of alkyl groups

AbstractThe barriers to rotation of the 3‐substituent in six 3‐RCH2‐4‐Me‐δ4‐thiazoline‐2‐thiones (R  Me, Et, Pr, Isobu, Isopr,t‐Bu) and in two analogous methiodides have been measured by the 1H DNMR technique. The barriers are discussed in relation to the possible conformations of the transition states, and comparisons are made with barriers to rotation of the same substituents in other frameworks. It is concluded that the relative steric effects are strongly dependent on the framework and that, in general, a conformational analysis of the individual cases is necessary to understand the steric effects of alkyl groups.

Steric effects—III : Composition of the E' s parameter. Variation of alkyl steric effects with substitution. Role of conformation in determining sterically active and inactive sites

The steric effect of alkyl groups as parameterized by E' s covers a range of over seven powers of ten. In this article an attempt is made to portray in mechanism terms the manifold nature of the steric effect as a function of substitution, i.e. the composition of the steric effect. One most consider the contribution of the topological carbon sites as a function of the true coordinates of these atoms. To this end, we evaluate the geometry of these alkyl groups in carboxylic acid indirectly by a search for the probable conformations of these acids. The starting point in this study is the determination of the minimum energy conformations in carboxylic acids using the empirical force field method based on analogous alkylmethyl ketone models. This approach to the ground state behaviour of these acids leads directly to a division of the complete set of alkyl groups into two subsets within which a Conformational sequence or filitation is associated with the substitution process which generates the successive alkyl groups of that subset. It is shown that eclipsed conformations are preferred in the case of secondary and tertiary groups bearing Et and Me sub-groups, giving way to preferred bisected conformations when t-Bu and i-Pr sub-groups are present. For further elucidation of steric perturbation terms these two subsets must be considered independently. These important observations lead to an overall coherent interpretation of steric effects. Three regions of distinct behaviour as a function of substitution have been identified. In Region I (the so-called “mormal” region, where the contribution of the introduction of successive methyl groups to the overall E' s increases monotonically), besides excellent topological site correlations, an excellent hybrid correlation is proposed. For a conformationally homogeneous (i.e. eclipsed) set of alkyl groups the overall E' s of groups is well expressed by a linear combination of the weighted contribution of sub-groups and the Newman six-number. This correlation leads to an interpretation of the composition of the overall steric effect of an alkyl group, as measured by the E' s parameter, in terms of conformationally defined sites and stresses the steric importance of certain privileged sites. This result is consistent with the molecular mechanics viewpoint of preferred conformations but at variance with previous assumptions. The levelling effect (Region II), in the light of conformational information, is understood in terms of sterically active and inactive sites, while the inversion effect (Region III) is considered to be the result of molecular distortion brought about by relief of local internal steric strain within the alkyl groups. The X-ray structure of i-Pr 3CCO 2H illustrates this point.

Steric effects-II: Relationship between topology and the steric parameter. E's—topology as a tool for the correlation and prediction of steric effects

The steric effect of alkyl groups as characterized by the revised Taft E' s parameter is analysed using an approach based on the DARC topological system and its PELCO correlation method. This approach involves an analysis of the systemativ variation of E' s in a topological dequencing of alkyl groups and shows the existence of three regions of distinct behaviour: R I, a “normal” behaviour region ( ca 6 E' s units) in which the contribution of the introduction of successive Me groups to the overall steric effect increases monotonically (groups with 1 to 7 carbons); R II, a region, in which a “levelling” effect is observed, i.e. the contribution diminishes and becomes nil (groups with 8 and 9 carbons); and R III, where this contribution changes sign, “inversion” effect (groups with 10 carbons). Using a series of successive approximations, topological models are developed and tested. The conditions under which the topology may be used to represent the topography (i.e. the real 3-dimensional structure) are considered. The correlation of existing E' s values and the reliable prediction of experimentally unavailable steric effects are direct consequences of this treatment.

The Steric Effects of Alkyl Groups on the N–H Stretching Vibrations and the Rotational Isomerism of Alkylureas

Abstract The free N–H stretching vibrations of various trialkylureas, R2UR′, in dilute solutions were studied in order to resolve our questions concerning the rotational isomerism of dialkylureas. In the infrared spectrum of R2UR′, where R is isopropyl or cyclohexyl, an additional N–H band appears on the higher-frequency side of the trans N–H band. It was also observed that the larger the effective size of the R′ group on R2UR′ is, the stronger the intensity of the additional band becomes. It was concluded, in connection with the rotational isomerism of dialkylureas, that the additional band arises from a form in which the N–H group is out of the skeletal plane.

The γ-Radiation-induced Radical Addition Reaction of Alcohols

The -radiation-induced radical addition reactions of such alcohols as CH30H, C2H50H, n- C3H70H and i-C3H70H to such polyflorocyclobutenes as(CF2)2CH=CCI(1), (CF2)2CF=CCI (2)and(CF2)2CH=CF(3)were studied.In the case of (1)and(2), the attack of the -hydroxyalkyl radicals to the cyclobutenes occurred exclusively on the=CH or CF of the unsymmetrical double bond, yielding the corresponding 1:1 adducts. While, in the case of(3), the radicals attacked on either the =CHor =CF, yielding the two isomers, (CF2)2CH2CFCRR/OH(7)and(CF2)2CHFCHCRR' OH(8).The ratio of the two isomers varied markedly with the structure of the alcohols, and this was attributed to the steric effect of alkyl groups of the alcohols.