• Bulletin of the Korean Chemical Society
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• v.35 no.2
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• pp.471-476
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• 2014

A kinetic study on aminolysis of 2-chloro-4-nitrophenyl X-substituted-benzoates (2a-k) in 80 mol % $H_2O/_20mol%$ DMSO at $25.0^{\circ}C$ is reported. The Br${\emptyset}$nsted-type plot for the reactions of 2-chloro-4-nitrophenyl benzoate (2g) with a series of cyclic secondary amines curves downward (e.g., ${\beta}_1=0.25$, ${\beta}_2=0.85$ and $pK_a^o=10.3$), which is typical of reactions reported to proceed through a stepwise mechanism with a change in ratedetermining step (RDS). The Hammett plot for the reactions of 2a-k with piperidine consists of two intersecting straight lines, while the corresponding Yukawa-Tsuno plot exhibits an excellent linear correlation with ${\rho}_X=1.15$ and r = 0.59. Thus, it has been concluded that the nonlinear Hammett plot is not due to a change in RDS but is caused by stabilization of substrates through resonance interactions between the electron-donating substituent and the C=O bond. Substrates possessing a substituent at the 2-position of the leaving aryloxide deviate negatively from the curved Br${\emptyset}$nsted-type plot for the reactions of Y-substituted-phenyl benzoates (3a-i), implying that the steric hindrance exerted by the substituent at the 2-position is an important factor which governs the reactivity of Y-substituted-phenyl benzoates.

• Bulletin of the Korean Chemical Society
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• v.34 no.4
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• pp.1115-1119
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• 2013

A kinetic study is reported for nucleophilic substitution reactions of benzyl 2-pyridyl thionocarbonate (5b) and t-butyl 2-pyridyl thionocarbonate (6b) with a series of alicyclic secondary amines in $H_2O$ at $25.0^{\circ}C$. General-base catalysis, which has often been reported to occur for aminolysis of esters possessing a C=S electrophilic center, is absent for the reactions of 5b and 6b. The Br${\o}$nsted-type plots for the reactions of 5b and 6b are linear with ${\beta}_{nuc}$ = 0.29 and 0.43, respectively, indicating that the reactions of 5b proceed through a stepwise mechanism with formation of a zwitterionic tetrahedral intermediate ($T^{\pm}$) being the rate-determining step while those of 6b proceed through a concerted mechanism. The reactivity of 5b and 6b is similar to that of their oxygen analogues (i.e., benzyl 2-pyridyl carbonate 5a and t-butyl 2-pyridyl carbonate 6a, respectively), indicating that the effect of modification of the electrophilic center from C=O to C=S (i.e., from 5a to 5b and from 6a to 6b) on reactivity is insignificant. In contrast, 6b is much less reactive than 5b, indicating that the replacement of the $PhCH_2$ in 5b by the t-Bu in 6b results in a significant decrease in reactivity as well as a change in the reaction mechanism (i.e., from a stepwise mechanism to a concerted pathway). It has been concluded that the contrasting reactivity and reaction mechanism for the reactions of 5b and 6b are not due to the electronic effects of $PhCH_2$ and t-Bu but are caused by the large steric hindrance exerted by the bulky t-Bu in 6b.

• Macromolecular Research
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• v.11 no.5
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• pp.375-381
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• 2003

Silver salt complex membranes with glassy poly(methyl methacrylate) (PMMA) unexpectedly showed higher propylene permeance than those with rubbery poly(butyl methacrylate) (PBMA) where as neat PMMA is much less permeable to propylene than that of neat PBMA. Such unusual facilitated olefin transport has been systematically investigated by changing the side chain length of polymethacrylates (PMAs) from methyl, ethyl to butyl. The ab initio calculation showed almost the same electron densities of the carbonyl oxygens in the three PMAs, expecting very similar intensity of the interaction between carbonyl oxygen and silver ion. However, the interaction intensity decreases with the length of the alkyl side chain: PMMA > PEMA > PBMA according to wide angle X-ray scattering and FT-Raman spectroscopy. The difference in the interaction intensity may arise from the difference in the hydrophilicity of the three PMAs, as confirmed by the contact angle of water, which determines the concentrations of the ionic constituents of silver salts: free ion, contact ion pair and higher order ionic aggregate. However, propylene solubilities and facilitated propylene transport vary with the side chain length significantly even at the same concentration of the free ion, the most active olefin carrier, suggesting possible difference in the prohibition of the molecular access of propylene to silver ion by the side chains: the steric hindrance. Therefore, it may be concluded that both the hydrophilicity and the steric hindrance associated with the side chain length in the three PMAs are of pivotal importance in determining facilitated olefin transport through polymer/silver salt complex membranes.

• Proceedings of the Membrane Society of Korea Conference
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• 2004.05a
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• pp.155-160
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• 2004

Permeation experiments of a commercial nanofiltration membrane (nominated as ESNA 1) were carried out with aqueous solutions of various single salts, that is, five chlorides (NH$_4$Cl, NaCl, KCl, MgCl$_2$ and $CaCl_2$), three nitrates $(NaNo_3,\;Mg(No_3)_2\;and\;Ca(NO_3)_2)\;and\;three\;sulfates\;((NH_4)_2SO_4,\;Na_2SO_4\;and\;MgSO_4)$. The experimental results showed that (1) the permeate volume flux of the ESNA 1 membrane increased and decreased with the growth of the applied pressure and the feed concentration of salts, respectively. The real rejection of ESNA 1 membrane to most single salts increased with the growth of the permeate volume flux. (2) The reflection coefficients of ESNA 1 membrane to chlorides, nitrates and sulfates are 0.97, 0.96 and 0.99, respectively. The solute permeability of most salts except for magnesium and calcium salts increased with the growth of feed concentration. (3) The sequence of the rejections of ESNA 1 membrane to anions is $R({SO_4}^{2-})>R(CI)>R(NO_3)$ at the same feed concentration. While the sequence of the rejections to cations is cataloged into two cases: $R(Na^+)>R(K^+)>R(Mg^{2+})>R(Ca^{2+})$ at the concentration of 10 mol/$m^3$ and $R(Mg^{2+})>R(Ca^{2+})>R(Na^+)>R(K^+)$ at the concentration of 100 mol/$m^3$. The separation capability of a NF membrane is usually affected by the electrostatic effect and the steric-hindrance effect. In this case, the electrostatic effect is the major factor at low concentration and the steric-hindrance effect is the major factor at high concentration. Both the specific sorption and the hydration also reasonably influenced the separation performance of NF membrane to salts.

• Bulletin of the Korean Chemical Society
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• v.11 no.6
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• pp.521-527
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• 1990

The binding characteristics and analytical applications of anilinium ion complexes with 18-crown-6 were studied by polarography and NMR. First, the electrochemical reduction of the 10 species of mono and dimethylsubstituted anilinium ion complexes with 18-crown-6 as host in methanol are examined. The addition of 18-crown-6 to anilinium guest solution the polarographic waves remain well defined but shifted toward more negative potentials, indicating the complex formation. The values of formation constants, log Κ for 10 species of methylsubstituted anilinium ion complexes with 18-crown-6 varies from 2.7 to 4.8 in methanol at $25^{\circ}C$. The stability order of complexes for 18-crown-6 is anilinilum > 4-methyl > 3,4-dimethyl > 3-methyl > 3,5-dimethyl > 2,4-dimethyl > 2,5-dimethyl > 2,3-dimethyl > 2-methyl > 2,6-dimethylanilinium ion. The steric hindrance shows significant effect. Second, Proton NMR was used to elucidate their interaction characteristics. From the results of so called NMR titration techniques, the behaviors of binding sites on complexation, and the stoichiometry and stability order of complex were obtained. And the later results show the satisfactory agreement with the quantitative values obtained by polarography. Finally, the individual determinations of anilinium ion mixtures were also accomplished by addition of 18-crown-6. In some mixtures of methyl or dimethylanilinium ions the reduction peaks of differential pulse method appeared into one unresolved wave attributed to the small difference of half-wave potential, ${\Delta}E_{1/2}$. In the presence of 18-crown-6, the polarographic waves were resolved into individual maxima because of the shift toward more negative direction by the difference of selectivity of anilinium ions with 18-crown-6. It may be concluded that quantitative analysis of methylanilinium ion mixture make possible because the half-wave potential shift by the selectivity difference due to the steric hindrance between methyl group and 18-crown-6 on complexation.

• Applied Chemistry for Engineering
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• v.30 no.4
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• pp.509-511
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• 2019

Hydrazine derivatives used in fine chemicals, pharmaceuticals and cosmetics can be synthesized by reduction reaction from diazonium derivatives. The reduction method using $SnCl_2$ facilitates the reaction conversion, but the use of $SnCl_2$ is limited when residual heavy metals are issued in the final product. For the conversion of protected (2-hydroxyphenyl)diazonium derivatives into its hydrazine derivatives in Ramalin preparation process, the reduction method was developed by using $NaHSO_3$. In this study, the effect of steric hindrance according to the protected 2-hydroxygroupinphenyldiazonium derivatives was found, and linear C1~C5 alkyl groups for the hydroxy protection were preferable during the diazonium reduction reaction. Considering the economical efficiency and industrial production for the preparation of Ramalin, a variety of protecting groups were investigated. As a result, 2-(allyloxyphenyl)hydrazine was obtained with 85% yield and 99.7% purity when the hydroxy protecting group was used as an allyl group that could be easily deprotected.

• Journal of Life Science
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• v.16 no.2 s.75
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• pp.206-212
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• 2006

In order to synthesize novel anticonvulsants, we researched that the reactions of diamines with 2,5-dimethoxytetrahydrofuran and 1,3-acetonedicarboxylic acid. The reaction of ethylenediamine with 2,5-dimethoxytetrahydrofuran and 1,3-acetonedicarboxylic acid afforded 8-(2-pyrrol-1-yl-ethyl)-8-aza-bicyclo[3,2,1]octan-3-one (yield; 5.0%) and 1,2-di-(8-aza-bicyclo[3,2,1]octan3-onyl)ethane (yield; 17.0%). In case of 1,3-diaminopropane, 8-(3-pyrrol-1-yl-propyl)-8-aza-bicyclo[3,2,1]octan-3-one(yield; 6.0%) and 1,3-di-(8-aza-bicyclo[3,2,1]octan-3-onyl)propane (yield; 21.0%) were obtained. In case of 1,8-diaminooctane, 8-(8-pyrrol-1-yl-octyl)-8-aza-bicyclo-[3,2,1]octan-3-one (yield; 2.6 %) and 1,8-di-(8-aza-bicyclo[3,2,1]octan-3-onyl)octane (yield; 24.9%) were obtained. In diaminobenzene reactions, synthetic yields of 8-aza-bicyclo-[3,2,1]octan-3-one derivatives were higher than those of pyrrole derivatives because re actions were done under room temperature. The longer the carbon chain of diaminoalkane is, the more reactive N atom is due to more electron donating effect, and the less steric hindrance around the carbon gave the higher chemical yields. The reaction of p-phenylenediamine as a diaminobenzene with 2,5-dimethoxyte-trahydrofuran and 1,3-acetonedicarboxylic acid produced p-dipyrrolylbenzene (yield; 4.0%), 8-(4-pyrrol-1-yl-phenyl)-8-aza-bicyclo[3,2,1]octan-3-one (yield; 12.0%), and 1,4-di-(8-aza-bicyclo[3,2,1]octan-3-onyl)benzene (yield; 59.0%). In case of m-phenylenediamine, 8-(3-pyrrol-1-yl-phenyl)-8-aza-bicyclo[3,2,1]octan-3-one(yield; 2.0%) and 1,3-di-(8-aza-bicyclo[3,2,1]octan-3-onyl)benzene (yield ; 28.0%) were obtained. But, synthesis of 1,2-di-(8-aza-bicyclo[3,2,l]octan-3-onyl)benzene by treatment of o-phenylenediamine was not successful, presumably due to the steric hindrance of 8-aza-bicyclo-[3,2,1]octan-3-one rings.

• Korea-Australia Rheology Journal
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• v.11 no.3
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• pp.241-246
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• 1999

In this study, we consider the effect of nonionic surfactants on the rheological responses of emulsion systems under the action of a uniform do electric field. The model emulsions consist of a less conducting dispersed phase and a more conducting continuous phase. When the shear flow is weak, the positive viscosity effect is produced due to the formation of chain-like morphology. The nonionic surfactants used here generate two distinctively different effects. Specifically, first, the steric hindrance induced by the surfactant molecules renders the structure unstable, and thereby reduces the degree of positive viscosity effect. Secondly, the presence of surfactant molecules also prevents the rotation of the dispersed droplets by anchoring across the interface or by decreasing the size of dispersed phase. The second effect suppresses the negative viscosity effect.

• Journal of the Korean Society of Industry Convergence
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• v.5 no.2
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• pp.147-152
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• 2002

Because of the difference of the relative reactivity between two hydroxyl groups of the hydroquinone due to the steric hindrance of the substituent, many combinations of the substituent direction in the polyesters derived from asymmetrical diphenols such as monosubstituted hydroquinones was expected. It was studied how the mode of the direction affected the properties of the resulting polyesters in terms of the transition temperatures of the thermotropic polyesters prepared from terephthalic acid, 2,4-dichloroterephthalic acid, and phenylhydroquinone by the reaction using p-Toluenesulfonylchloride in pyridine. The direction was tried to control the relative reactivity by changing the reaction temperature and addition time of the hydroquinone, and by modifying it through an association of the hydroquinones with DMF.

• Journal of Powder Materials
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• v.5 no.2
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• pp.133-138
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• 1998

The effect of $\beta$-$Si_3N_4$ seeding on microstructural development of silicon nitride based materials has been investigated. In particular, to observe more distinctly the abnormal grain growth in pressureless sintering, fine $\alpha$-$Si_3N_4$(mean particle size: 0.26 ${\mu}m$) powder classified by sedimentation method was used. It was possible to prepare silicon nitride with abnormally grown grains under low nitrogen pressure of 1 atm thanks to the heterogeneous nucleation on $Si_3N_4$ seed particles. The size and morphology of silicon nitride grains were strongly influenced by the presence of $\beta$-$Si_3N_4$ seed and overall chemical composition. For specimens with initially low $\beta$-content, the large grains grew without a significant impingement by other large grains. On the contrary, for specimens with initially high $\beta$-content, steric hindrance was effective. The resulting microstructure was less inhomogeneous and characterized by unimodal grain size distribution.