摘要:

Rate and equilibrium constants for reactions of carbocations, and a few other electrophiles, with anionic and neutral nucleophiles in several solvents are summarized. Equilibrium constants for such reactions are a measure of the carbon basicities of the nucleophiles, and are discussed in the context of differences in proton and carbon basicities. The factors affecting the kinetic reactivities of nucleophiles are not well understood, even though the reactivity patterns are frequently the same toward widely different electrophiles.

ISSN:0008-4042    

DOI:10.1139/v86-370

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

The photochemical step in the functioning of visual and bacterial rhodopsins entailscis–transortrans–cisisomerization and changes in the state of protonation of the retinylidene Schiff base chromophore. In this review our present knowledge on these two events is discussed as well as the role of interactions between the chromophore and the surrounding protein, opsin. The relation between protonation and hydrogen bonding at the Schiff base nitrogen, the problems of stabilization of the proton bridge and charge separation are also discussed. A new proposal is made which implies Schiff base to counter-ion proton translocation with concomitant isomerization and reprotonation of the chromophore.

ISSN:0008-4042    

DOI:10.1139/v86-371

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

The mechanism and the scope of the synthesis of bis(4-dialkylaminoaryl)squaraines from di-n-butyl squarate andN,N-dialkylanilines have been studied. Results show that water and acid are key factors of the synthesis, and these two factors have been optimized. Yields of squaraine are also found to be sensitive to the steric effect provided by the alkyl chain in dialkyl squarates as well as to the concentration of the aniline reagent used in the synthesis. Mechanistic results suggest that alkyl squarate is the precursor of the synthesis, and that squaraine is formed by diarylation of alkyl squarate with anN,N-dialkylaniline derivative. Under optimized conditions, a number of squaraines have been synthesized. Yields, which are comparable to those synthesized from squaric acid, are obtained. Product analysis with good material balance has been achieved, and results show that arylation and diarylation of the starting di-n-butyl squarate are major side reactions in the squaraine synthesis.

ISSN:0008-4042    

DOI:10.1139/v86-372

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

The reaction of primary and secondary amines with 2,2-di(4-nitrophenyl)-1,1-difluoroethene (1) in acetonitrile solvent gives first 2,2-di(4-nitrophenyl)-1-fluoro-1-aminoethene (2) and then 2,2-di(4-nitrophenyl)-1,1-difluoro-1-aminoethane (3). With excess amine, pseudo-first-order rate constants for the production of2were measured, which showed a second-order reaction, together with a catalysed third-order reaction. In addition to the reagent amines, the reaction is also catalysed by tertiary amines and bases such as oxalate and acetate, but not by chloride and perchlorate, nor by ammonium ions. The enthalpy of activation for the reaction of piperidine with1in acetonitrile is 3.7 kcalmol−1, but for the catalysed reaction an apparent value of −2.2 kcal mol−1was obtained. It is concluded that the reaction proceeds via a pre-equilibrium to a zwitterion, followed by another equilibrium giving a carbanion that yields the product (2) by a rate-determining cleavage of the carbon–fluorine bond.

ISSN:0008-4042    

DOI:10.1139/v86-373

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

The interfacial behaviour of vinyltriphenyl and propargyltriphenylphosphonium cations at the mercury electrode – methanolic solution interface is studied by phase selective ac polarography using dropping Hg electrodes of short (≤5 s) and longdrop times and the hanging Hg drop electrode.The various adsorption parameters are determined on the basis of non-ionic adsorption isotherms involving capacitance data independent as well dependent on time.

ISSN:0008-4042    

DOI:10.1139/v86-374

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

The kinetics of disproportionation of elemental iodine to iodide and iodate ions has been studied in basic aqueous media using Raman and uv–visible spectroscopy. The IO stretching vibrations for IO−and I2OH−were observed at 430 ± 2 and 560 ± 2 cm−1, respectively. The totally symmetric stretching vibration forwas observed at 685 ± 2 cm−1. The Raman results indicate that I2OH−is a linear molecule with a stronger I—O bond than IO−. The rate expression at 25 °C in 1 mol dm−3NaOH was found to bewhereThe reaction is primarily a reaction of the iodine +1 oxidation-state species IO−and I2OH−. It proceeds through the +3 oxidation-state speciesThe following equilibrium and rate constants were determined:

ISSN:0008-4042    

DOI:10.1139/v86-375

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

Thymine in aqueous solution was decomposed with quantum yields of 0.3 and 0.4 under N2-saturated and aerated conditions by far-ultraviolet light (> 180 nm, far-uv), and quantum yields of 2 × 10−4and 3 × 10−4by near-ultraviolet light (> 220 nm, near-uv), respectively. The main photolytic products by far-uv were 5,6-dihydrothymine (DHT) (selectivity:S(DHT) = 0.2) and 5-hydroxymethyluracil (HMU) (S(HMU) = 0.1) under N2-saturated conditions,cis- andtrans-5,6-Dihydroxy-5,6-dihydrothymine (TG), 6(5)-hydroperoxy-5(6)-hydroxy-5,6-dihydrothymine (HTP) (S(TG) + S(HTP) = 0.2), andN1-formyl-N2-pyruvylurea (FPU) (S(FPU) = 0.4) were obtained under aerated conditions. These products were attributed to the reactions of thymine with the H and OH produced by photolysis of water.

ISSN:0008-4042    

DOI:10.1139/v86-376

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

When potassium hexacyanomanganate(III) dissolves in acidic solution it rapidly disproportionates into hexacyanomanganate(IV) and Mn(II). Hexacyanomanganate(IV) then slowly decomposes to yield Mn(II) and (CN)2. Kinetics of the latter reaction has been studied. The reaction is found to be first order with respect to, H+, and Mn(II) concentrations and the experimental rate law has the formv = kobs[Mn(IV)] = (ka + kb[H+] + kc[Mn(II)])[Mn(IV)]. At 40 °C and ionic strength 2.0,ka,kb, andkcvalues are (1.78 ± 0.01) × 10−4 s−1, (5.97 ± 0.05) × 10−5 s−1 M−1, and (3.40 ± 0.18) × 10−3 s−1 M−1, respectively. A mechanism with three parallel pathways is proposed, the deduced rate law being similar to the experimental one. Activation parameters, ΔH≠and ΔS≠for the rate constantska,kb, andkcare also reported.

ISSN:0008-4042    

DOI:10.1139/v86-377

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

The interaction between ethylN-(diphenylmethylene)glycinate (DPG) and hydroxy proton donors or pyrrole has been investigated by ir spectrometry. The equilibrium constants, enthalpies, and entropies of complex formation have been determined in carbon tetrachloride solution and compared with the data obtained for the complexes involvingN-benzylidene-methylamine and the same proton donors. The ir spectra studied mainly in the νOH, νC=N, νC=O, and νC—Oregions suggest that hydrogen bond formation occurs at the N atom of the imino group and at the O atom of the carbonyl group. The results are discussed in terms of the basicity at the two acceptor sites and of the accessibility of the lone pair of electrons. The ir spectra of the solid adduct of DPG with HCl show that protonation takes place on the N atom. The protonated structure is possibly stabilized by an intramolecular hydrogen bond.

ISSN:0008-4042    

DOI:10.1139/v86-378

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

The first dissociation constants pK1of four diprotonated α-aminothionamides, determined spectrophotometrically, are between −8.1 to −8.8; the second dissociation constants pK2, of three of these compounds, determined titrimetrically, are between 7.3 and 7.7. Both pK1and pK2values are close to those calculated from current electrostatic theory for equilibria in solutions of zero ionic strength, in spite of the fact that data for pK1values were obtained in solutions of high ionic strength. Results indicate a much larger effect of the charged ammonium group on the basicity of the thionamide group than on the basicity of the amide group. A possible explanation based on the greater hydration of the protonated amide group is advanced.

ISSN:0008-4042    

DOI:10.1139/v86-379

出版商:NRC Research Press    

年代:1986

数据来源: NRC