摘要:

AbstractSolvation is the main driving force of electrophilic bromination since it is impossible to obtain a bromonium ion from bromine and alkene in the gas phase, whereas it is a very fast reaction in solution. The role of a protic solvent in this addition was investigated experimentally by extended Winstein–Grunwald relationships, kinetic solvent isotope effects andR, the rate ratios in two solvents of similar ionizing powers but different nucleophilicities. It is shown that electrostatic medium effects and electrophilic assistance to bromide ion departure are the main rate‐determining factors of the reaction. These two contributions are roughly independent of the double bond substituents. Nucleophilic solvent assistance to positive charge development is also found; however, it provides only a small acceleration, the magnitude of which depends on alkene structure. This nucleophilic solvent involvement is annulled when crowded substituents inhibit approach of the solvent to the cationic part of the transition state or when positive charge is delocalized by conjugated electron‐donating groups. These several solvent roles are identical in nature and in magnitude with those observed in heterolytic solvolysis. In halogenated solvents, the driving force of bromination arises from catalysis by a second bromine molecule which assists heterolysis of the bromine‐bromine bond, leading to the bromonium‐tribromide ion pair. Similar halogen catalysis occurs also in some solvolyses. Finally, return is also found in both reactions; reversible formation of bromonium ions is observed when their nucleophilic attack, the productforming last step, is made energetically difficult either by steric inhibition or by poor nucleophilicity of the trapping nucleophiles. Similarities and differences between electrophilic bromine addition and limiting solvolysis are discussed in terms of respective intermediate stabilities and heats of

ISSN:0894-3230    

DOI:10.1002/poc.610040902

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractEmpirical linear relationships between standard Gibbs free energy, ΔG298°, and parachor and between standard entropy,S298°, and parachor are presented for the aliphatic series H(CH2)nX with X = CH3, CHCH2, CCH, Cl, OH, SH, SCH3and SC2H5. In the series with X = OH and SH significant deviations of points forn= 4 and 5 are observed. INDO quantum‐chemical analysis points to the formation of cyclic conformers of butan‐1‐ol, pentan‐1‐ol and their corresponding thio analogues. The formation energy of the cyclic conformers (higher for thiols than for alcohols) is proportional to the sum of deviations from the linearity of the points

ISSN:0894-3230    

DOI:10.1002/poc.610040903

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractDirect irradiations of 2‐chloro‐6,7 : 8,9‐dibenzotricyclo [3.2.2.02,4] nona‐6,8‐diene (5‐Cl) were conducted in cyclohexane, acetic acid and wet acetonitrile. The products are 1‐methylfluoranthene (8) in all three solvents, the allylic chlorides 8‐chloro‐7‐methylene‐2,3 : 5,6‐dibenzobicyclo [2.2.2]octa‐2,5‐diene (3‐Cl) and 7‐chloromethyl‐2,3 : 5,6‐dibenzobicyclo [2.2.2] octa‐2,5,7‐triene (2‐Cl) in acetic acid and acetonitrile and solvolysis product amides in wet acetonitrile. Compound 5‐Cl had previously been shown to be the product of triplet sensitization of 2‐Cl and 3‐Cl, so that the singlet reaction reverses that of the triplet. The formation of 8 from 5‐Cl was quenched with piperylene, whereas that of the allylic chlorides was not. Quantum yields of products and singlet lifetimes in the three solvents were measured. The solvent effects are discussed. Deuterium‐labeling results on the formatio

ISSN:0894-3230    

DOI:10.1002/poc.610040904

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe reaction rates of the cleavage of phthalimide (PTH) were studied at 30°C in buffer solutions of trimethylamine, triethylamine, triethanolamine, carbonate, piperidine, dimethylamine, morpholine, piperazine andN‐methylpiperazine at different pH values. The reactivity of carbonate, triethylamine and triethanolamine toward PTH could not be detected under the experimental conditions applied. However, triethylamine revealed nucleophilic reactivity toward non‐ionized PTH (SH). The steric requirements of triethylamine and triethanolamine are ascribed to the absence of nucleophilic reactivities of these amines toward SH. Secondary amines show nucleophilic reactivity toward both SH and ionized PTH (S−). The absence of nucleophilic reactivity of trimethylamine and the presence of nucleophilic reactivity of secondary amines toward S−are attributed to the occurrence of intramolecular general acid‐base catalysis due to the presence of a mobile proton at the nucleophilic site of the nucleophile. Intermolecular general base catalysis was observed in the reactions of SH with all secondary amines except piperidine and such catalysis could be detected in the reactions of S−with morpholine, piperazine andN‐methylpiperazine. Nucleophilic second‐order rate constants for the reactions of secondary amines with SH (k1) and S−(k2) reveal the Brønsted plots of slopes ofca0.8 and 0.3, respectively. Similarly, the general base‐catalysed third‐order rate constants (k4) for the reactions of secondary amines with SH yield a Brønsted plot of slope 1.2. These observations suggest the occurrence of diffusion‐controlled trapping stepwise and pre‐association stepwise mechanisms in the aminolysis of non‐ionized and ion

ISSN:0894-3230    

DOI:10.1002/poc.610040905

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractEmpirical expressions are obtained for bond dissociation energies (BDEs) of alkylsilanes and their derivatives. It is pointed out that the sensitivity of the BDEs of SiX bonds to methyl substitution depends on the electronegativity of the substituent X. When X is an atom or group with low or moderate electronegativity, the SiX BDE is insensitive to methyl substitution, but when X is a halogen atom or OH, the SiX BDEs increase with increasing methyl substit

ISSN:0894-3230    

DOI:10.1002/poc.610040906

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractAM1 and 3–21G computations, on the lowest singlet state of selected π‐ylides, support the earlier conclusion that π‐ylides are highly polar with highly reactive side‐chains. The computations also indicate that the corresponding triplet states are lower in energy and have substantially reduced polarity. Contrary to the expectations derived from Hückel calculations, AM1 and 3–21G calculations suggest that the best π‐ylide structures have non‐aromatic rings. The heats of isomerization contradict earlier conclusions reached using topological resonance energies. π‐Ylides are expected to afford different product structures depending on the electronic state of the π‐ylide involved in a given

ISSN:0894-3230    

DOI:10.1002/poc.610040907

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractP‐Nitrobenzyl cellulose xanthate (CelXNB) was obtained with a degree of substitution (DS) in the range 0.5–3. The first order rate constant for hydrolysis of CelXNB at 25°C, extrapolated to zero buffer concentration, is hydroxide ion catalysed, whereas the water‐catalyed path is about 2000 times faster than that observed forp‐nitrobenzyl ethylxanthate (EXNB), probably owing to the highly ordered cybotactic region of cellulose. Aminolysis of CelXNB produces the corresponding thioncarbamate ester; for simple alkylamines, the second‐order rate constants are similar to those obtained for EXNB. The secod‐order rate constants are similar to those obtained for EXNB. The second‐order rate constants at pH 11 for immobilization of bacterial α‐amylase and bovine serum albumin were 13.4 and 112 1 mol−1s−1, respectively, unexpectedly high values when compared with simple alkylamines, even considering the concentration of external reactive groups of the proteins. CelXNB with lowDSshould release 1 mol ofp‐nitro‐α‐toluenethiol for every mole of protein that becomes immobilized. The net weight increase of the cellulose matrix allows the calculation of the absolute molecular weight of the protein. Preliminary res

ISSN:0894-3230    

DOI:10.1002/poc.610040908

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe pyrolysis kinetics of RC6H4SStC4H9(RH,p‐NO2,p‐Cl,p‐F) were determined at 390–450°C and 7–15 Torr in a stirred‐flow reactor using toluene as carrier gas. The reaction products were 95% isobutene, 5% isobutane and the corresponding RC6H4SSH disulphanes. The first‐order rate constants,k(s−1), based on isobutene production, followed the Arrhenius equations: phenyltert‐butyl disulphide,k= 1013.49±0.31exp (– 182 ± 4 kJ mol−1)(RT)−1;pnitrophenyltert‐butyl disulphide,k= 1013.46 ± 0.32[exp(– 185 ± 5 kJ mol−1)(RT)−1];p‐chlorophenyltert‐butyl disulphide,k= 1014.44 ± 0.66[exp(– 196 ± 9 kJ mol−1)(RT)−1;p‐fluorophenyltert‐butyl disulphide,k= 1010.80 ± 0.16exp(– 144.5 ± 2 kJ mol−1)(RT)−1]; The observed reactivities, within the above temperature range, follow the orderpF>H>pCl>pNO2. A four‐centre, cuadrupolar cyclic transition state mechanism is proposed for the formation of the isobutene and aryldisulphane products. The optimized ground‐state molecula

ISSN:0894-3230    

DOI:10.1002/poc.610040909

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

ISSN:0894-3230    

DOI:10.1002/poc.610040901

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY