- Electronic and steric effects: How do they work in ionic liquids? the case of benzoic acid dissociation
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(Figure Presented) The need to have a measure of the strength of some substituted benzoic acids in ionic liquid solution led us to use the protonation equilibrium of sodium p-nitrophenolate as a probe reaction, which was studied by means of spectrophotometric titration at 298 K. In order to evaluate the importance of electronic effect of the substituents present on the aromatic ring, both electron-withdrawing and -donor substituents were taken into account. Furthermore, to have a measure of the importance of the steric effect of the substituents both para- and ortho-substituted benzoic acids were analyzed. The probe reaction was studied in two ionic liquids differing for the ability of the cation to give hydrogen bond and π-π interactions, namely [bm 2im][NTf2] and [bmpyrr][NTf2]. Data collected show that benzoic acids are less dissociated in ionic liquid than in water solution. Furthermore, the equilibrium constant values seem to be significantly affected by both the nature of ionic liquid cation and the structure of the acid. In particular, the ortho-steric effect seems to operate differently in water and in the aromatic ionic liquid, determining in this solvent medium a particular behavior for ortho-substituted benzoic acids.
- D'Anna, Francesca,Marullo, Salvatore,Vitale, Paola,Noto, Renato
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experimental part
p. 4828 - 4834
(2010/10/04)
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- Quantitative mass determination of conjugated polymers for single molecule conformation analysis: Enhancing rigidity with macrocycles
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A combination of advanced synthetic and single molecule spectroscopic techniques allowed us to demonstrate that macrocycles, covalently bound to a conjugated polymer backbone, raise the effective chain persistence length by at least a factor of 5.
- Becker, Klaus,Gaefke, Gerald,Rolffs, Jasmin,Hoeger, Sigurd,Lupton, John M.
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supporting information; experimental part
p. 4686 - 4688
(2010/09/05)
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- CLEAVAGE OF THE PHENACYL ESTERS OF CARBOXYLIC AND THIOCARBOXYLIC ACIDS BY METAL ALKOXIDES
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During the cleavage of phenacyl esters with general formula XC6H4COCH2OCOR by metal alkoxides in alcohols and in ether-alcohol solutions the acids, esters, and ethers are formed as a result of cleavage of the C-C bond.The solvent participates in the formation of the ether and gives rise to transesterification of RCOOCH3 with catalytic participation of the metal alkoxide.The reactivity of the phenacyl esters of the thio acids is much higher.Before dissociation the esters of substituted benzoins C6H5COCH(OCOCH3).C6H4X-4 undergo irreversible rearrangement with the formation of isomeric products.
- Morozov, A. A.
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p. 1138 - 1143
(2007/10/02)
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- Mechanistic Studies on the Basic Hydrolysis of 2,2,2-Trichloro-1-arylethanones
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The basic hydrolysis of 2,2,2-trichloro-1-phenylethanone (1a), 2,2,2-trichloro-1-(p-methoxyphenyl)ethanone (1b), and 2,2,2-trichloro-1-(p-chlorophenyl)ethanone (1c) has been studied in the pH range 5.5-13.2.In all cases the reaction products were chloroform and the corresponding benzoate.The reaction is first order toward both the ethanone and hydroxide ion and proceeds via an addition-elimination-type mechanism.The initial addition step forms the corresponding ethanone hydrates, which, depending on the pH, will form the mono- and dianionic intermediates, the elimination of CCl3 from the mono- and/or dianionic species being the rate-limiting step of the reaction.
- Zucco, Cesar,Lima, Claudio F.,Rezende, Marcos C.,Vianna, Jose F.,Nome, Faruk
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p. 5356 - 5359
(2007/10/02)
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- Effect of Distortion on the Hydrolytic Reactivity of Amides. 2. N-Pyramidalization: Decomposition of N-Benzoylaziridines in Aqueous Media
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The decomposition of para-substituted N-benzoylaziridines (H, OCH3, NO2, Br) in buffered aqueous media is studied at 25 deg C as a function of pH in order to assess the effect of N-pyramidalization on the hydrolytic reactivity of the amide bond.Overall, the reaction shows three dominant terms: OH- and H2O attack on the neutral form and H2O attack on the protonated form of the amide.In base, the exclusive reaction is rate-limiting and irreversible attack of OH- on the C=O unit leading to normal hydrolytic products.This is shown by the first-order dependence on -> from pH 8 to 14 of the hydrolysis rate and by the fact that ca. 50percent 18O-enriched amide recovered from the hydrolysis medium as a function of time shows no 18O loss.Relative to N,N-dimethylbenzamide (kOH-25 deg C = 6.0 * 10-6 M-1 s-1), N-benzoylaziridine is ca. 200 000-fold more susceptible to OH- attack (kOH-25 deg C = 1.1 M-1 s-1).The kOH- terms follow a ?ρ relationship with ρ = 1.68.In acid, the products are not the expected hydrolytic ones of benzoic acid and aziridine.Rather, exclusive ring opening occurs to give p-X-C6H4C(=O)NHCH2CH2OX.In acetate buffers, product analysis by 1H NMR indicates that the ring-opened material consists of alcohol and acetate (X = H and C(=O)CH3).
- Slebocka-Tilk, H.,Brown, R. S.
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p. 805 - 808
(2007/10/02)
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- Substituent Constants of the Pyrazol, 1,2,3-Triazol, Benzotriazol, and Naphthotriazol Group
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The synthesis of benzoic acids with the title substituents in p- and m-position (15, 13, 11, 9) and their ethyl and methyl esters is described.The ?p and ?m values of the substituents obtained by alkaline hydrolysis of the esters in ethanol/water and methylcellosolve/water demonstrate an inductive electron withdrawing and a mesomer electron donating effect of these groups which are important for dye chemistry.
- Sachweh, Volker,Langhals, Heinz
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p. 1627 - 1639
(2007/10/02)
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- Studies of the Borderline between Concerted and Stepwise Mechanisms of Elimination : E1cB Elimination of Fluoren-9-ylmethyl Carboxylate Esters
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Rates of β-elimination of carboxylate leaving groups from fluoren-9-ylmethyl carboxylate esters in methanolic sodium methoxide at 25 deg C are reported.An E1cB mechanism with rate-determining formation of a carbanion intermediate is assigned on the basis of near identity of measured elimination rates and rates of carbanion formation predicted from a Taft correlation, and the similarity with elimination of 1-(1-acetoxy-1-methylethyl)indene for which the mechanism has been established by Ahlberg and Thibblin.Values of ρ=0.42 and βlg=0.27 measured for substituted benzoate leaving groups are a little larger than expected (ca. 0.24 and 0.18, respectively) and the discrepancy is tentatively ascribed to conformational enhancement of remote substituent effects, rather than to a contribution of E2 elimination.The effects of alkyl and aryl substitution α to the leaving group are discussed, especially in relation to the borderline between concerted and stepwise mechanisms.The measurements fail to confirm an earlier inference that the borderline shows a discontinuity in transition-state structure at the point of mechanistic change.
- O'Ferrall, Rory A. More,Larkin, Finbar,Walsh, Peter
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p. 1573 - 1580
(2007/10/02)
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- Kinetics and Mechanism of Ru(III)-catalysed Oxidation of Aromatic Aldehydes by Sodium Metaperiodate in Alkaline Medium: A Change in Mechanism from Hydride Loss in Acid Medium to Proton Loss in Alkaline Medium
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The title reactions are first order in the substrate and first order in the catalyst, the order with respect to periodate being zero.The dependence on alkali is unity.The reactivity order is p-nitro > m-nitro > m-bromo > m-methoxy > p-bromo > p-chloro- > H ca. p-methyl > p-methoxy.The pH-rate profile attains a minimim at neutral pH and the rate increases on either regions of this pH.The Hammett plot shows a fair linearity with a ρ-value of + 1.66 pointing to a rate-limiting proton loss.A mechanism involving the complexation of the monoanion of the aldehydes with the Ru(III) followed by rate-limiting proton loss is postulated.Activation parameters have been computed.
- Radhakrishnamurti, P. S.,Misra, P. C.
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p. 427 - 430
(2007/10/02)
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- METAL-CATALYZED SIMULTANEOUS PRODUCTION OF HYDROGEN AND ALKALINE SALTS OF ORGANIC ACIDS
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Metal catalysis of the dehydrogenation of primary alcohols to alkaline salts of the corresponding organic acids is described.Conditions (120-170 deg C) are much milder than those required by the uncatalyzed reactions (220-260 deg C).Hydrogen is evolved in the ratio of two molecules per molecule of alcohol.
- Chiusoli, Gian Paolo,Giroldini, William,Salerno, Giuseppe
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p. 371 - 374
(2007/10/02)
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