Conjugate Acid-base Research Articles

Rhodium-Catalyzed Asymmetric Conjugate Additions of Boronic Acids Using Monodentate Phosphoramidite Ligands

ADVERTISEMENT RETURN TO ISSUEPREVAdditions & Corr...Additions & CorrectionsORIGINAL ARTICLEThis notice is a correctionRhodium-Catalyzed Asymmetric Conjugate Additions of Boronic Acids Using Monodentate Phosphoramidite LigandsJean-Guy Boiteau, Rosalinde Imbos, Adriaan J. Minnaard, and Ben L. FeringaCite this: Org. Lett. 2003, 5, 8, 1385Publication Date (Web):March 20, 2003Publication History Published online20 March 2003Published inissue 1 April 2003https://doi.org/10.1021/ol031982qCopyright © 2003 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views659Altmetric-Citations14LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (29 KB) Get e-AlertsSUBJECTS:Conjugate acid-base pairs,Ligands Get e-Alerts

PH and pK determinations by high-resolution solid-state 13C NMR: acid-base and tautomeric equilibria of lyophilized L-histidine.

Acid-base properties of lyophilized powders of L-histidine have been systematically investigated using parent solutions at pH varying from 1.8 to 10. For the first time, high-resolution solid-state 13C NMR was shown to allow separate observation of all three acid-base pairs in the successive deprotonations of the carboxylic end, the imidazolium cation, and the terminal ammonio group of histidine. 1H CRAMPS NMR spectra directly visualize the absence of the N3-H(pi) tautomer in neutral and anionic species. Solid-state titration shifts are enlarged by approximately 1-4 ppm with respect to those measured in solution, permitting unambiguous observation of conjugate acid-base pairs. Calculated pK's from solid-state acid-to-base ratios r are found equal to those classically measured in solution at 0 degrees C with a similar ionic strength of 0.1 mol x dm(-3). This proves that natural-abundance 13C solid-state determinations of r can be used to measure pK's in parent solutions without recourse to full titration curves and subsequent curve-fitting procedures. Such an approach also leads to noninvasive characterizations of the acidity of lyophilized powders, i.e., to the prediction of in situ pH of products obtained after rehydration and solubilization of powders. These results show the possibility of measuring the pK of nonvolatile acidic substrates dissolved in any sublimable solvent through lyophilization of the investigated solutions; this leads the way to pH and pK determinations when electrochemical or spectrophotometric measurements are impossible or ambiguous, e.g., for concentrated solutions, polyacids, or mixtures of acidic solutes, and possibly to the establishment of pK scales in nonaqueous solvents and in melts.

Neutralization of 1-(Aminoethyl)phosphonic Acid and Protonation of Its Salt Anion. Processing Dilatometric Data with the CPESSP Program

Volume changes resulting from neutralization of 1-(aminoethyl)phosphonic acid and protonation of the anion of its bis(tetraethylammonium) salt were determined by the dilatometric method. Protonation constants, apparent molar volumes of the acid and its ionized forms, and volume changes on neutralization of the acid and protonation of its salt were calculated. The presence of the betaine form of the acid in solutions was detected. The rule of the constancy of the sum of the volume effects of the protonation and neutralization reactions was shown to be valid for conjugate acid-base pairs of dibasic acids.

IMPROVED PEPTIDE MAPPING BY CAPILLARY ZONE ELECTROPHORESIS USING TRIETHYLENETETRAMINE PHOSPHATE BUFFER AS THE ELECTROLYTE SOLUTION

An acidic separation buffer for peptide mapping by capillary zone electrophoresis (CZE) in both bare fused-silica and acrylamide-coated capillaries is evaluated. The buffer system consists of a mixture of the aliphatic polyamine triethylenetetramine and phosphoric acid in aqueous media. The study has been performed at pH 3.0, which is within the buffering capacity range in the acidic domain determined by the pKa1 values of both the tetramine (3.2) and the polyprotic inorganic acid (2.1). Consequently, the solution resulting by combining phosphoric acid and triethylenetetramine at pH 3.0 is a buffering system exhibiting two conjugate acid-base pairs, both effective at controlling the protonic equilibra at this pH value. The suitability of the triethylenetetramine phosphate buffer at performing efficient peptide mapping by CZE has been evaluated on a tryptic digest of the protein cytochrome c from horse heart. The study is based on the comparison of the tryptic peptide separation performed by CZE with either triethylentetramine phosphate or sodium phosphate buffers, both containing the same concentration of phosphate ions. The results show that both separation performance and electroosmotic flow are greatly influenced by the incorporation of triethylenetetramine in the buffer system. With the triethylenetetramine phosphate buffer, the electroosmotic flow results in being either suppressed or reversed from cathodic to anodic in bare fused-silica and in acrylamide-coated capillaries, respectively. This effect is indicative of the specific adsorption of positively charged molecules of triethylenetetramine into the immobilized region of the electric double layer at the interface between the capillary wall and the electrolyte solution. The improvement of peak shape observed with the triethylenetetramine phosphate buffer is also related to the adsorption of the tetramine evidenced by the variation of the electroosmotic flow. The specific adsorption of cationic triethylenetetramine results in masking the silanol groups and other active adsorption sites on the capillary wall, that are believed to be responsible for the poorer performance obtained with sodium phosphate buffer. Thus, the results demonstrate that besides being an effective component of the buffering system, triethylenetetramine acts as a dynamic coating reagent influencing, significantly, efficiency and resolution of the tryptic peptides.

Conjugate addition of amines to alpha,beta-enones promoted by CeCl(3).7H(2)O-NaI system supported in silica gel.

ADVERTISEMENT RETURN TO ISSUEPREVNoteNEXTConjugate Addition of Amines to α,β-Enones Promoted by CeCl3·7H2O−NaI System Supported in Silica GelGiuseppe Bartoli, Marcella Bosco, Enrico Marcantoni, Marino Petrini, Letizia Sambri, and Elisabetta TorregianiView Author Information Department of Organic Chemistry “A. Mangini”, University of Bologna, v.le Risorgimento 4, I-40136 Bologna, Italy, and Department of Chemical Sciences, University of Camerino, via S. Agostino 1, I-62032 Camerino (MC), Italy [email protected]Cite this: J. Org. Chem. 2001, 66, 26, 9052–9055Publication Date (Web):November 22, 2001Publication History Received27 August 2001Published online22 November 2001Published inissue 1 December 2001https://doi.org/10.1021/jo0108764Copyright © 2001 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views2534Altmetric-Citations147LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit Read OnlinePDF (83 KB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information SUBJECTS:Addition reactions,Amines,Conjugate acid-base pairs,Ketones,Silica Get e-Alerts

Photoinduced reaction of arene singlets with carbon tetrachloride in zeolite Y

Picosecond fluorescence methodologies combined with nanosecond, laser flash photolysis have been applied to examine the reaction of arene excited singlets with CCl 4 in dehydrated, zeolite NaY. Fluorescence quenching by CCl 4 was shown to originate mainly from a static process, with a much greater drop in fluorescence intensity than could be accounted for by a random distribution of CCl 4 among the anthracene-occupied supercages. A similar effect was observed in a previous study of arene fluorescence quenching by nitroalkanes in NaY, and was explained by a mechanism involving coadsorption of the arene and nitroalkane to conjugate acid–base sites. This places the reactants in close proximity – a requirement for the static quenching. The results of the present study support this quenching mechanism by showing that a different electrophilic species (i.e., CCl 4) also behaves like nitroalkanes, while reductive quenching by N, N-dimethylaniline and triethylamine does not. Quenching of pyrene fluorescence by CCl 4 produced pyrene radical cations through electron transfer to CCl 4, with the cation yield proportional to the level of fluorescence quenching. One-third of these radical cations reacted with the fragmentation products of CCl 4 anions to form stable products; the remaining fraction was stable, but reacted with added ferrocene. These results demonstrate that the addition of small quantities of CCl 4 to the zeolite leads to efficient charge separation by a single-photon mechanism.

Hydroxorhodium complex-catalyzed carbon-carbon bond-forming reactions of silanediols with alpha,beta-unsaturated carbonyl compounds. Mizoroki-Heck-type reaction vs conjugate addition.

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTHydroxorhodium Complex-Catalyzed Carbon−Carbon Bond-Forming Reactions of Silanediols with α,β-Unsaturated Carbonyl Compounds. Mizoroki-Heck-Type Reaction vs Conjugate AdditionAtsunori Mori, Yasuaki Danda, Toshinari Fujii, Kazunori Hirabayashi, and Kohtaro OsakadaView Author Information Chemical Resources Laboratory, Tokyo Institute of Technology 4259 Nagatsuta, Yokohama 226-8503, Japan Cite this: J. Am. Chem. Soc. 2001, 123, 43, 10774–10775Publication Date (Web):October 5, 2001Publication History Received1 April 2001Revised10 July 2001Published online5 October 2001Published inissue 31 October 2001https://doi.org/10.1021/ja015928lCopyright © 2001 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views1541Altmetric-Citations116LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit Read OnlinePDF (33 KB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information SUBJECTS:Addition reactions,Conjugate acid-base pairs,Reagents,Rhodium,Solvents Get e-Alerts

Nanomolar E-selectin inhibitors: 700-fold potentiation of affinity by multivalent ligand presentation.

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTNanomolar E-Selectin Inhibitors: 700-Fold Potentiation of Affinity by Multivalent Ligand PresentationGebhard Thoma, Rudolf O. Duthaler, John L. Magnani, and John T. PattonView Author Information Novartis Pharma AG CH-4002 Basel, Switzerland GlycoTech Corporation, 14915 Broschart Road Rockville, Maryland 20850 Cite this: J. Am. Chem. Soc. 2001, 123, 41, 10113–10114Publication Date (Web):September 22, 2001Publication History Received19 June 2001Published online22 September 2001Published inissue 1 October 2001https://doi.org/10.1021/ja0164430Copyright © 2001 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views404Altmetric-Citations39LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit Read OnlinePDF (41 KB) Get e-AlertsSupporting Info (2)»Supporting Information Supporting Information SUBJECTS:Carbohydrates,Conjugate acid-base pairs,Inhibition,Inhibitors,Ligands Get e-Alerts

Stabilization of a tyrosine O-sulfate residue by a cationic functional group: formation of a conjugate acid-base pair.

Sulfated tyrosine [Tyr(SO3H)]-containing peptides showed characteristic peak patterns in their liquid secondary-ion mass spectrometry (LSIMS) spectra. Protonated molecules were desulfated more easily than their deprotonated counterparts. Therefore, the stabilities of the Tyr(SO3H) residues were well-reflected by peak patterns in their positive-ion spectra. These intrinsic peak patterns were investigated by comparing the behavior of each Tyr(SO3H) residue in acidic solution. As the peptide chain was lengthened and the number of cationic functional groups increased, the peak representing the [MH]+ of a Tyr(SO3H)-containing peptide became more prominent than that representing the desulfated [MH-SO3]+. These alterations in peptide structure also increased the stability of the Tyr(SO3H) residue in acidic solution. Based on the desulfation mechanism of an aryl monosulfate, we predicted that intramolecular cationic functional groups would stabilize Tyr(SO3H) residues by forming conjugate acid-base pairs (or salt bridges) both in the gaseous phase and in acidic solution. In accordance with this theory, Arg residues would take primary responsibility for this self-stabilization within Tyr(SO3H)-containing peptides. Moreover, a long peptide backbone was expected to have a weak protective effect against desulfation of the [MH]+ in the gaseous phase. Tyr(SO3H) residues were also stabilized by adding an external basic peptide containing multiple Arg residues. Formation of such intermolecular acid-base pairs was demonstrated by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) which detected conjugated peptide ions. The energetically favorable formation of conjugate acid-base pairs prompted by Tyr(SO3H) residues might be a driving force for protein folding and protein-protein interaction.

Aluminum Tris(2,6-diphenylphenoxide)-ArCOCl Complex for Nucleophilic Dearomatic Functionalization

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTAluminum Tris(2,6-diphenylphenoxide)-ArCOCl Complex for Nucleophilic Dearomatic FunctionalizationSusumu Saito, Toshihiko Sone, Masaaki Murase, and Hisashi YamamotoView Author Information Graduate School of Engineering, Nagoya University CREST, Japan Science and Technology Corporation (JST) Chikusa, Nagoya 464-8603, Japan Cite this: J. Am. Chem. Soc. 2000, 122, 41, 10216–10217Publication Date (Web):September 27, 2000Publication History Received25 April 2000Published online27 September 2000Published inissue 1 October 2000https://doi.org/10.1021/ja0014382Copyright © 2000 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views1058Altmetric-Citations21LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit Read OnlinePDF (77 KB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information SUBJECTS:Anions,Conjugate acid-base pairs,Crystal structure,Organic compounds,Substitution reactions Get e-Alerts

The Combination of Anionic and Radical Reactions to Oxidative Tandem Processes Exemplified by the Synthesis of Functionalized Pyrrolidines

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTThe Combination of Anionic and Radical Reactions to Oxidative Tandem Processes Exemplified by the Synthesis of Functionalized PyrrolidinesUllrich Jahn, Markus Müller, and Susanne AussiekerView Author Information Institut für Organische Chemie Technische Universität Braunschweig Hagenring 30, D-38106 Braunschweig, Germany Cite this: J. Am. Chem. Soc. 2000, 122, 21, 5212–5213Publication Date (Web):May 13, 2000Publication History Received16 February 2000Published online13 May 2000Published inissue 1 May 2000https://doi.org/10.1021/ja000565vCopyright © 2000 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views1512Altmetric-Citations83LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit Read OnlinePDF (61 KB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information SUBJECTS:Conjugate acid-base pairs,Cyclization,Enolates,Lithium,Pyrrolidines Get e-Alerts

Highly Enantioselective Catalytic Conjugate Additions to Cyclohexadienones

ADVERTISEMENT RETURN TO ISSUEPREVAdditions & Corr...Additions & CorrectionsNEXTORIGINAL ARTICLEThis notice is a correctionHighly Enantioselective Catalytic Conjugate Additions to CyclohexadienonesRosalinde Imbos, Mirjan H. G. Brilman, Mauro Pineschi, and Ben L. FeringaCite this: Org. Lett. 1999, 1, 11, 1873Publication Date (Web):November 10, 1999Publication History Published online10 November 1999Published inissue 1 December 1999https://doi.org/10.1021/ol9902778Copyright © 1999 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views507Altmetric-Citations8LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (9 KB) Get e-AlertsSUBJECTS:Conjugate acid-base pairs,Stereoselectivity Get e-Alerts

Novel Synthesis of Functionalized Cyclobutane Derivatives via Intramolecular Conjugate Addition of Alkenyltrimethylstannane Functions to α,β-Alkynic Esters Mediated by Copper(I) Chloride

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTNovel Synthesis of Functionalized Cyclobutane Derivatives via Intramolecular Conjugate Addition of Alkenyltrimethylstannane Functions to α,β-Alkynic Esters Mediated by Copper(I) ChlorideEdward Piers, Eva M. Boehringer, and James G. K. YeeView Author Information Department of Chemistry, University of British Columbia, 2036 Main Mall, University Campus, Vancouver, British Columbia V6T 1Z1, Canada Cite this: J. Org. Chem. 1998, 63, 24, 8642–8643Publication Date (Web):November 1, 1998Publication History Received4 September 1998Published online1 November 1998Published inissue 1 November 1998https://doi.org/10.1021/jo981803wCopyright © 1998 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views303Altmetric-Citations22LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit Read OnlinePDF (54 KB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information SUBJECTS:Chemical reactions,Conjugate acid-base pairs,Functionalization,Organic compounds,Reaction products Get e-Alerts

New Method for Introduction of a Silyl Group into α,β-Enones Using a Disilane Catalyzed by a Copper(I) Salt1

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTNew Method for Introduction of a Silyl Group into α,β-Enones Using a Disilane Catalyzed by a Copper(I) Salt1Hajime Ito, Tomoko Ishizuka, Jun-ichi Tateiwa, Motohiro Sonoda, and Akira HosomiView Author Information Department of Chemistry, Graduate School of Chemistry University of Tsukuba, Tsukuba Ibaraki 305-8571, Japan Cite this: J. Am. Chem. Soc. 1998, 120, 43, 11196–11197Publication Date (Web):October 16, 1998Publication History Received29 June 1998Published online16 October 1998Published inissue 1 November 1998https://doi.org/10.1021/ja9822557Copyright © 1998 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views2089Altmetric-Citations58LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit Read OnlinePDF (78 KB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information SUBJECTS:Chemical reactions,Conjugate acid-base pairs,Group 14 compounds,Salts,Silylation Get e-Alerts

The pH of buffers based on 2-amino-2-hydroxymethyl-1,3-propanediol (‘tris’) in synthetic sea water

E.m.f. measurements were made over the temperature range 273.15–318.15 K, and at five salinities from 20 to 40, using the cell Pt( s) H 2( g, p°) BH + and B in synthetic sea water AgCl( s) Ag( s) Pt( s) where p°=101.325 kPa, and BH + and B are the conjugate acid–base pair of 2-amino-2hydroxymethyl-1,3-propanediol (‘tris’) – each present at a concentration of 0.04 mol kg -1. The results were used to assign pH values based on the ‘total’ hydrogen ion scale to such buffer solutions based on this acid–base pair. Additional measurements were made over a range of buffer concentrations at a salinity of 35; these were used to calculate the stoichiometric (ionic medium) dissociation constant for ‘tris–H +’ in synthetic sea water.

Conformational Equilibria of β-Alanine and Related Compounds as Studied by NMR Spectroscopy

The vicinal proton−proton couplings of the dipolar form of β-alanine in water, alcohol−water, and dimethyl sulfoxide−water solutions indicate little conformational preference and are consistent with an essentially statistical equilibrium of the gauche or trans conformations. The position of the equilibrium is only slightly affected, over a temperature range of about 130°, by changes in dielectric constants ranging from 30 to 80 or by massive changes in ionic strength. Quantum-mechanical calculations at the HF/6-31G** and LMP2/cc-pVTZ levels were found to give rather good parallels with experiment, although suggesting the gauche conformation to be 2−3 kcal/mol more stable in water or methanol than actually observed. A number of related compounds, such as N,N,N-trimethyl-β-alanine and N,N-diethyl-β-alanine, as well as the conjugate acid and conjugate base of β-alanine, also show no significant conformational preference in water solution. In conformity with these results, the zwitterionic form of piperidine-3-carboxylic acid (nipecotic acid) has about the same preference for equatorial carboxylate as cyclohexanecarboxylic acid itself. Taurine shows no significant conformational preference except in basic solution, where the couplings indicate about 53% of the gauche conformation. In contrast, N,N,N-trimethyltaurine is predominantly trans in acidic or neutral solution. The conformational equilibria of the N,N,N-trimethyltaurine species are most likely governed by steric hindrance, because there are rather large tetrahedral groups at each end of the ethano chains. Yet, even here the energy difference between gauche and trans is only about 1.2 kcal.

Spectrophotometric study of the solubility and the protolytic properties of 1-(2-pyridylazo)-2-naphthol in different ethanol–water solutions

The solubility and the protolytic constants of 1-(2-pyridylazo)-2-naphthol (PAN) have been accurately determined at temperatures between 20.0 and 30.0°C in ethanol–water solutions with ethanol concentrations ranging from 10.0% to 95.0% (v/v). The measurement of the protolytic constants is based on the spectrophotometric determination of the concentrations of the corresponding conjugate acid–base pairs in ethanol–water solutions containing PAN and hydrochloric acid or sodium hydroxide. The traditionally used pH measurements in such studies in non-aqueous solutions are thus avoided. The solubility and the protolytic constants of PAN in pure water have been calculated by extrapolating the ethanol–water results to pure water where PAN is practically insoluble. The determination of the thermodynamic data mentioned above is necessary for elucidating the interactions of PAN with various metal ions in solutions and in cation-exchange membranes (e.g., Nafion ®) used as PAN-based optodes.

Avoiding Interferences from Good's Buffers: A Contiguous Series of Noncomplexing Tertiary Amine Buffers Covering the Entire Range of pH 3–11

Of the 20 well-known buffers proposed by Good, all but 3 form metal ion complexes which can result in serious interferences, particularly in protein analyses. The structural features responsible for such complex formation have been identified. Based on a mechanistic analysis of the metal complexation process, it is proposed that tertiary amine compounds, having N-substituents which are ethyl or larger, are sterically inaccessible for initial bond formation with solvated metal ions in aqueous solution. Thus, in the absence of donor atoms on the α-, β-, or γ-carbons, metal complexation cannot proceed. The proposed noncomplexing compounds include Good's 3 noncomplexing buffers (Mes, Mops, Pipes) plus six related species. Mixed-mode acid dissociation constants (hydrogen ion in terms of activity, conjugate acid–base species in molar concentrations) have been determined for all compounds in their protonated form at 25°C, μ = 0.10m(NaNO3). The values for four compounds in this series are reported here for the first time: viz.,N,N′-diethylpiperazine for which logKa1m= 4.67 ± 0.03 and logKa2m= 8.83 ± 0.02;N,N,N′,N′-tetraethylmethylenediamine for which logKa1m< 1 and logKa2m= 11.01 ± 0.03;N,N′-di ethyl-N,N′-bis(3-sulfopropyl)ethylenediamine for which logKa1m= 5.75 ± 0.03 and logKa2m= 9.37 ± 0.02; and piperazine-N,N′-bis(2-ethanesulfonic acid) (Pipes) for which logKa1m= 2.81 ± 0.01 (the value of logKa2mhaving been previously reported).

Unsaturated Nitriles: Conjugate Addition−Silylation with Grignard Reagents

ADVERTISEMENT RETURN TO ISSUEPREVNoteNEXTUnsaturated Nitriles: Conjugate Addition−Silylation with Grignard ReagentsFraser F. Fleming, Yifang Pu, and Faith TercekView Author Information Department of Chemistry and Biochemistry, Duquesne University, Pittsburgh, Pennsylvania 15282 Cite this: J. Org. Chem. 1997, 62, 14, 4883–4885Publication Date (Web):July 11, 1997Publication History Received24 March 1997Published online11 July 1997Published inissue 1 July 1997https://doi.org/10.1021/jo9705430Copyright © 1997 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views506Altmetric-Citations22LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit Read OnlinePDF (129 KB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information SUBJECTS:Addition reactions,Chemical reactions,Conjugate acid-base pairs,Nitrogen compounds,Reagents Get e-Alerts

Students' misconceptions?Looking for a pattern

This article describes four chemical terms that students with well-considered reasons use in a way that is not accepted in chemistry. The studies identified students' misconceptions using multiple-choice tests. The distractors of the questions were keyed to the errors observed in a pilot study. From 4300–7500 German senior high school students completed the written tests. They were asked to select an option and give reasons for their answers. Furthermore, discussions were conducted with additional groups of students. It emerged that students limit the term isomerism to compounds of the same class. This idea, however, is not compatible with the modern definition of isomerism. Yet, in practice, only isomers belonging to the same class of compounds are dealt with. Students apparently conclude from the syllable “ox” in redox that oxygen is involved in all redox reactions. Oxygen only had a predominant position in the old oxidation concept. This concept has, however, been extended and oxygen has lost its special position. The idea that in any reaction between an acid and a base a neutral solution is formed has been found to be quite common among students. The term neutralization, coined 300 years ago, can no longer be used in this way. However, words like neutralization or neutral seem to keep the old idea alive. Many students hold the misconception that conjugate acid-base pairs consist of positively and negatively charged ions, which can somehow neutralize each other. The test questions developed can be easily used by teachers to find out whether the misconceptions described appear in a learning group. As it is known which misconceptions might occur teachers can reflect on how to respond to students' incorrect answers before entering the classroom. Discussions among the students have been found to be a good way of dealing with these problems. For further research the question is raised as to whether there are other concepts and terms that cause difficulties for students. © 1997 John Wiley & Sons, Inc. Sci Ed 81:123–135, 1997.