Values Of Activation Entropy Research Articles

WITHDRAWN: Kinetics and mechanism of the reactions of alkanediamine-linked dinuclear platinum(II) complexes with some biorelevant-nucleophiles

The kinetics and mechanism of substitition reactions of dinuclear platinum(II) complexes of the general formula [{Pt(H2O)}2(N,N,N′,N′-tetrakis(2-pyridylmethyl)-N(CH2)n-N](ClO4)4,viz., n = 2 (PtEn); 3 (PtProp); 6 (PtHex); 8 (PtOct) and 10 (PtDec) were investigated in 0.10 M aqueous perchloric acid medium as a function of concentration of l-glutathione (Glu) and dl-penicilliamine (Pen) nucleophiles and temperature under pseudo first-order conditions using UV–visible spectrophotometry. The reactivity data of the dinuclear Pt(II) complexes were compared with those of their mononuclear analogues of the form [Pt(H2O)(N,N-bis(2-pyridylmethyl)-N(CH2)n-CH3](ClO4)2, n = 1 (Ptbpea); 2 (Ptbppa); 5 (Ptbpha) and 9 (Ptbpda) under the same experimental conditions. The observed rate constants for the substitution of the aqua ligands follow the simple rate law: kobs = k2[Nu]. The reactivity of the complexes increases as the chain length of the linker or tail increases except Prop which showed a higher reactivity towards the nucleophiles than the rest of the complexes. The overall trend of reactivity for the dinuclear platinum(II) complexes is PtEn < PtHex < PtOct < PtDec < PtProp. The reactivity of the mononuclear complexes follows the same trend as the dinuclears and increases in the order Ptbpea < Ptbppa < Ptbpha < Ptbpda. In all cases, the mononuclear complexes are more reactive than their dinuclear analogues, which is an indication of the steric retardation on the rate of substitution due to the conformations of the dinuclear platinum(II) complexes. Glu was found to be a better nucleophile than Pen. The relatively small and positive values for the enthalpy of activation, negative values for entropy of activation and the dependence of the second-order rate constants on the concentration of entering nucleophiles supports an associatively activated mechanism.

Shape memory effect in metallic glasses

Shape memory effect in metallic glasses

Efficient synergistic chemical fixation of CO2 by simple metal-free organocatalysts: Mechanistic and kinetic insights

BackgroundCarbon dioxide is a versatile, non-toxic, and renewable C1 building block source to produce valuable chemicals such as cyclic carbonates. The purpose of this research is development of efficient, metal-free and affordable catalysts for cycloaddition reaction of CO2 with epoxides at mild conditions. MethodsIn the present work, a group of novel bifunctional organocatalysts consisting of different hydrogen-bond donors (HBDs) and halogen ion is simply synthesized. Under the optimized conditions, the reaction is kinetically and thermodynamically studied. Additionally, the catalytic active species are determined, and a mechanism is proposed with the help of electrospray‐ionization time‐of‐flight mass spectrometry (ESI-TOF MS). Significant findingsThe most active catalytic system is obtained by chemically interaction of 3-bromopropionic acid (BPA) with DMF at 80 °C (BPA-DMF). An excellent product yield of 98% and a high TOF value are obtained with 0.99 mol% of BPA-DMF, which is attributed to the synergistic effects of carboxyl functional groups and halide ions. The activation energy was found to be 50.75 kJ/mol. The positive value of Gibbs free activation energy (ΔG‡) and negative value of activation entropy (ΔS‡) confirm a non-spontaneous, endergonic and kinetically controlled reaction.

Degradation kinetic of anthocyanins from rose (Rosa rugosa) as prepared by microencapsulation in freeze-drying and spray-drying

ABSTRACTIn this work, anthocyanin of rose residue was microencapsulated, using spray-drying (1:1 (w/w) of GA to MD, the inlet air temperature was170°C, the drying airflow rate was 5.0 m3/min, feed rate was 0.36 L/h) and freeze-drying (processing 48 h at −52°C, vacuum degree was 0.45 mbar) with gum arabic and maltodextrin as external materials. The total phenolic content (TPC), anthocyanin content (ACN), antioxidant activity, moisture content, water activity (Aw), solubility, hygroscopicity, colorimetric property, particle micromorphology and size distribution of the microencapsulated anthocyanin were measured. Thermal degradation kinetic and thermodynamic parameters of ACN extracts and microcapsules in the accelerated model were determined. The spray-dried powder (SDP) presented a spherical shape while the freeze-dried powders (FDP) were in indefinite and laminated structures with more homogeneous distribution. After microencapsulation, the retention rate of TPC and ACN was 86.00% and 75.85% for SDP, 91.44% and 95.12% for FDP, respectively. Both of spray-dried and freeze-dried strategy provided efficient preservation for rose residues, enabling them to have lower moisture content, water activity, significantly higher solubility (P < .05) compared with the rose anthocyanin extracts (RAE), and have high-fidelity color that was similar to RAE. Meanwhile, the antioxidant activity of the microcapsules decreased with the deteriorating of bioactive compounds. In the degradation kinetic study of anthocyanin at the temperature 70, 80, 90°C, both FDP and SDP had longer half-life duration, lower absolute value of activation entropy, and better effect of freeze-drying embedding method compared to RAE. Therefore, microencapsulation, especially by freeze-drying method, could efficiently enhance the stability of anthocyanin in rose residue during thermal processing and storage, and thus greatly facilitate the utilization of by-product of rose essential oil.Abbreviations: RAE: rose anthocyanin extracts; GA: gum arabic; MD: maltodextrin; SDP: spray-dried powders; FDP: freeze-dried powders; TPC: total phenolic content; ACN: anthocyanin content

Highly selective, sustainable synthesis of limonene cyclic carbonate from bio-based limonene oxide and CO2: A kinetic study

Bio-derived cyclic carbonates are of significant research interest as building blocks for non-isocyanate polyurethanes (NIPUs). Cyclic carbonates from limonene are bio-renewable monomers for the production of fully bio-based polymers from citrus waste; however, there are currently very few reports on their synthesis. This work reports the synthesis of five-membered cyclic carbonates from bio-based limonene oxide (LO) and CO2 catalysed by commercially available inexpensive, tetrabutylammonium halides (TBAX). The cycloaddition of CO2 with commercial LO mixture of cis/trans-isomers (40:60) is highly stereoselective and the trans-isomer exhibits considerably higher conversion than the cis-isomer. Therefore, a stereoselective method of (R)-(+)-limonene epoxidation was performed to achieve a significantly higher yield of the trans-isomer (87 ± 2%) than cis-isomer, which leads to high conversion and yield to the corresponding cyclic carbonates. The catalytic effect of halide anions (X¯) and the influence of operational reaction parameters such as temperature, pressure, and catalyst amount were studied. High conversion (87%) was obtained after 20 h at 120 °C, 40 bar CO2 using 6 mol% tetrabutylammonium chloride (TBAC) catalyst. A detailed study of the reaction kinetics revealed the reaction to be first-order in epoxide (LO), CO2 and catalyst (TBAC) concentrations. Moreover, the temperature dependence of the reaction was studied using Arrhenius and Eyring equations. The activation energy (Ea) of the reaction was calculated to be 64 kJ mol–1. The high positive value of Gibbs free energy (ΔG‡ = 102.6 kJ mol–1) and negative value of activation entropy (ΔS‡ = –103.6 J mol–1) obtained as result of the thermodynamic study, indicate that the reaction was endergonic and kinetically controlled in nature.

Metal-catalyzed alkyne oxidation/CH functionalization: Effects of oxidant, temperature, and metal catalyst on chemoselectivity.

Gold-catalyzed intermolecular alkyne oxidation has attracted much synthetic attention, but mostly suffering undesired over-oxidation. Recent experiments demonstrated that over-oxidation could be dramatically suppressed in zinc(II)-catalyzed intermolecular alkyne oxidation/CH functionalization. By means of first-principle density functional theory calculations, we explored the mechanism of the M-catalyzed intermolecular alkyne oxidations (M = Zn(OTf)2 and Au+ PR3 ) as well as the effects of oxidants, temperature, and metal catalysts on chemoselectivity, in an effort to disclose the origin of the extraordinary chemoselectivity pertaining to zinc catalysis. Our calculations indicate that the Zn-catalyzed intermolecular alkyne oxidation/CH functionalization proceeds by a Friedel-Crafts alkylation mechanism rather than metal carbene insertion mechanism. The chemoselectivity of CH functionalization against over-oxidation in Zn catalysis, in comparison with gold catalysis, can be jointly controlled by four factors: (1) the use of less nucleophilic N-oxide, (2) the enhanced electrophilicity and carbocationic nature of the carbenic site in the α-oxo metal carbenoid intermediate, (3) enhanced steric repulsion to incoming oxidant exerted by bulky ancillary ligand in the close nearby of the carbenic site to disfavor intermolecular over-oxidation and (4) the large negative value of activation entropy in the intermolecular over-oxidation pathway, that jointly give rise to lower activation free energy for the intramolecular cyclization/CH functionalization pathway than for the intermolecular over-oxidation pathway. © 2018 Wiley Periodicals, Inc.

An introduction to the kinetics of the triethylamine-mediated selenocyclofunctionalization of 4-pentenoic acid

The aim of this work is exploration of kinetic, thermodynamic and mechanistic aspects of triethylamine-mediated selenocyclofunctionalization of 4-pentenoic acid by means of phenylselenyl halides (PhSeCl and PhSeBr). The kinetics and mechanism for these reactions have been investigated by the UV–Vis spectrophotometry. The rate constants, as well as thermodynamic parameters (ΔH≠, ΔS≠) have been determined using UV–Vis method at three different temperatures (288, 298 and 308 K) in THF as a solvent. All studied reactions gave negative values for entropy of activation, which is in agreement with the SN2 mechanism of substitution. In addition, obtained values for the rate constants have indicated the higher reactivity of PhSeCl as reagent for cyclization.

Homolytic Decomposition of Sulfonyl Chlorides

The products, kinetic parameters, and activation entropy of thermal decomposition of sulfonyl chloride groups in chlorosulfonated polyethylene (CSPE) have been studied for the first time, and it has been demonstrated that the decomposition mechanism involves simultaneous cleavage of two bonds (carbon–sulfur and sulfur–chlorine) to give an SO2 molecule and two free radicals, which can be useful in the process of polymer structuring. The radical mechanism has been confirmed by the formation of 2,3-diphenyl-2,3- dimethylbutane on heating CSPE in isopropylbenzene. The simultaneous cleavage of two bonds is supported by the low activation energy and pre-exponential factor in the Arrhenius equation, as well as by negative activation entropy values.

Reversible release of nitric oxide from an iron(II) nitrosyl complex containing a biomimetic S4N chelate. A facile release of nitric oxide

The release of NO by [Fe(NO)(Et2NpyS4)], where (Et2NpyS4)2− = 2,6-bis(2-mercaptophenylthiomethyl)-4-substituted pyridine(2-), has been studied in the absence and presence of a trapping agent. The results show that [Fe(NO)(Et2NpyS4)] releases NO spontaneously in solution with a slow rate, k-NO = 1.7 × 10−4 s−1 at 23 °C, in a reversible reaction. NO release becomes faster when the reaction intermediate [Fe(Et2 NpyS4)] was trapped by CO, thereby preventing the back reaction. The release of NO was studied as a function of CO concentration and temperature. The reported activation parameters, especially the positive activation entropy values for the release of NO, favor the operation of a dissociative interchange (Id) mechanism. Thus, [Fe(NO)(Et2NpyS4)] can serve as a NO deliverer.

Kinetics and mechanism of the substitution reactions of some monofunctional Pt(II) complexes with heterocyclic nitrogen donor molecules. Crystal structure of [Pt(bpma)(pzBr)]Cl2·2H2O

Substitution reactions of [Pt(terpy)Cl]+ (terpy = 2,2′;6′,2′′-terpyridine), [Pt(bpma)Cl]+ (bpma = bis(2-pyridylmethyl)amine), [Pt(dien)Cl]+ (dien = diethylenetriamine or 1,5-diamino-3-azapentane) and [Pt(tpdm)Cl]+ (tpdm = tripyridinedimethane) with nitrogen donor heterocyclic molecules, such as 3-amino-4-iodo-pyrazole (pzI), 5-amino-4-bromo-3-methyl-pyrazole (pzBr) and imidazole (Im), were studied in aqueous 0.10 M NaClO4 in the presence of 10 mM NaCl using variable-temperature UV–vis spectrophotometry. The second-order rate constants k2 indicate decrease in reactivity in the order [Pt(terpy)Cl]+ > [Pt(bpma)Cl]+ > [Pt(tpdm)Cl]+ > [Pt(dien)Cl]+. The most reactive nucleophile among the heterocyclic compounds is imidazole, while pzI shows slightly higher reactivity than pzBr. Activation parameters were also determined and the negative values for entropies of activation, ΔS≠, support an associative mode of substitution for all substitution processes. Crystal structure of [Pt(bpma)(pzBr)]Cl2·2H2O was determined by single-crystal X-ray analysis. The coordination geometry of the complex is distorted square-planar while the bond distance Pt–N2(pzBr) is longer than the other three Pt–N distances.

Kinetics and mechanism of the substitution reactions of some bifunctional palladium(II) complexes with different nitrogen-donor heterocycles

Substitution reactions of three Pd(II) complexes, [Pd(cbdca)Cl2]2− (cbdca = cyclobutane-1,1-dicarboxylato), [Pd(ox)Cl2]2− (ox = oxalato) and [Pd(mal)Cl2]2− (mal = malonato), with different five- and six-membered N-heterocycles, such as pyrazole, 3-amino-4-iodo-pyrazole (pzI), 5-amino-4-bromo-3-methyl-pyrazole (pzBr), 1,2,4-triazole, pyrazine, imidazole, pyridazine and pyrimidine, were investigated in aqueous 0.10 M NaClO4 with the presence of 20 mM NaCl using variable-temperature stopped-flow spectrophotometry. Substitution of these complexes occurs in two consecutive reversible steps, and their reactivity decreases in order [Pd(cbdca)Cl2]2− > [Pd(mal)Cl2]2− > [Pd(ox)Cl2]2−. The most reactive nucleophile among the five-membered heterocycles is imidazole, while pyridazine is the most reactive among the six-membered heterocycles. Activation parameters were determined for all reactions, and negative values for entropy of activation, ΔS≠, support an associative mode of substitution. The reaction between [Pd(mal)Cl2]2− complex and pzBr was investigated by 1H NMR, and the obtained results confirm that substitution with nitrogen-donor nucleophile does not lead to decomposition of the complex.

Solution Structures, Stabilities, Kinetics, and Dynamics of DO3A and DO3A–Sulphonamide Complexes

The Gd(3+)-DO3A-arylsulphonamide (DO3A-SA) complex is a promising pH-sensitive MRI agent. The stability constants of the DO3A-SA and DO3A complexes formed with Mg(2+), Ca(2+), Mn(2+), Zn(2+), and Cu(2+) ions are similar, whereas the logKLnL values of Ln(DO3A-SA) complexes are 2 orders of magnitude higher than those of the Ln(DO3A) complexes. The protonation constant (log KMHL) of the sulphonamide nitrogen in the Mg(2+), Ca(2+), Mn(2+), Zn(2+), and Cu(2+) complexes is very similar to that of the free ligand, whereas the logKLnHL values of the Ln(DO3A-SA) complexes are lower by about 4 logK units, indicating a strong interaction between the Ln(3+) ions and the sulphonamide N atom. The Ln(HDO3A-SA) complexes are formed via triprotonated *Ln(H3DO3A-SA) intermediates which rearrange to the final complex in an OH(-)-assisted deprotonation process. The transmetalation reaction of Gd(HDO3A-SA) with Cu(2+) is very slow (t1/2 = 5.6 × 10(3) h at pH = 7.4), and it mainly occurs through proton-assisted dissociation of the complex. The (1)H and (13)C NMR spectra of the La-, Eu-, Y-, and Lu(DO3A-SA) complexes have been assigned using 2D correlation spectroscopy (COSY, EXSY, HSQC). Two sets of signals are observed for Eu-, Y-, and Lu(DO3A-SA), showing two coordination isomers in solution, that is, square antiprismatic (SAP) and twisted square antiprismatic (TSAP) geometries with ratios of 86-14, 93-7, and 94-6%, respectively. Line shape analysis of the (13)C NMR spectra of La-, Y- , and Lu(DO3A-SA) gives higher rates and lower activation entropy values compared to Ln(DOTA) for the arm rotation, which indicates that the Ln(DO3A-SA) complexes are less rigid due to the larger flexibility of the ethylene group in the sulphonamide pendant arm. The fast isomerization and the lower activation parameters of Ln(DO3A-SA) have been confirmed by theoretical calculations in vacuo and by using the polarizable continuum model. The solid state X-ray structure of Cu(H2DO3A-SA) shows distorted octahedral coordination. The coordination sites of Cu(2+) are occupied by two ring N- and two carboxylate O-atoms in equatorial position. The other two ring N-atoms complete the coordination sphere in axial positions. The solid state structure also indicates that a carboxylate O atom and the sulphonamide nitrogen are protonated and noncoordinated.

Dehydrative Esterification of Acetic Acid by Ethanol as a Model Reaction for Polyester Synthesis Catalyzed by Scandium Perfluoroalkanesulfonates and Bis(perfluoroalkanesulfonyl)imides

Abstract Scandium perfluoroalkanesulfonates and perfluoroalkanesulfonimides act as catalysts for the polyesterification of dicarboxylic acids by diols. We performed the model esterification between ethanol and acetic acid to measure the extent of the conversions and equilibrium constants for each scandium catalyst. According to the Arrhenius and Eyring constants, the activity of each scandium catalyst depends on its fluorine content and type of ligand, as reflected in the activation entropy values.

Proximity effect on the general base catalysed hydrolysis of amide linkage: The role of cationic surfactant, CTABr

The bis phenoxide forms of (1,2)bis(2-hydroxybenzamido)ethane(I), (1,5)bis(2-hydroxybenzamido)3-azapentane(II), (1,3)bis(2-hydroxybenzamido)propane(III), and (1,8)bis(2-hydroxybenzamido)3,6-diazaoctane(IV) undergo facile hydrolysis of one of the amide groups (0.02 ≤ [OH − ]T (mol dm − 3) ≤ 0.5, 10% MeOH (v/v) + H2O medium) without exhibiting [OH − ] dependence. The reactivity trend follows I ~ II > > III ~ IV with low activation enthalpy {25.7 ± 2.8 ≤ ΔH≠ (kJ mol − 1) ≤ 64.8 ± 7.0}. The high negative and comparable values of activation entropy {− 234 ± 8 ≤ ΔS≠ (J K − 1 mol − 1) ≤ −127 ± 20} are consistent with closely similar, and ordered transition states which can be assembled by favourably oriented phenoxide groups. The solvent kinetic isotope effect for I, k H2O/k D2O + H2O ~1 (20 and 50 volume% D2O), indicates that proton transfer is not involved as a part of the rate controlling process. The observed slowing down of the rate of this reaction for I in the micellar pseudo phase of CTABr also supports the proposed mechanism. Under pre-micellar conditions, however, rate acceleration is observed, a consequence believed to be associated with the capping effect of the hydrophobic tail of the surfactant cation forming the reactive ion-pair, CTA + , (I-2H)2 − exclusively in the aqueous pseudo phase.

Allenylidene to Indenylidene Rearrangement in Cationicp-Cymene Ruthenium(II) Complexes: Solvent, Counteranion, and Substituent Effects in the Key Step toward Catalytic Olefin Metathesis

The reaction of [(η6-p-cymene)RuCl(PCy3)][OTf] with Ph2C(OH)C≡CH, at rt in benzene-d6 or in dichloromethane-d2, affords the allenylidene complex [(η6-p-cymene)RuCl(═C═C═CPh2)PCy3][OTf] ({4a}), which is used in situ as a catalyst for the ring-closing metathesis (RCM) reaction of diallyltosylamide. A kinetic study is performed by 1H NMR spectroscopy in benzene-d6 and dichloromethane-d2. The rates of the olefin metathesis process increase with the concentration of the ruthenium complex, but are independent of the olefin concentration, due to rate-determining transformation of the allenylidene complex. The TOF values are higher in benzene-d6 than in dichloromethane-d2. The spontaneous or thermally induced transformations of the allenylidene complexes [(η6-p-cymene)RuCl(═C═C═CAr2)PR3][X] (Ar = Ph, X = CF3SO3−(OTf) ({4a}), BF4− ({4b}), PF6− ({4c}), SbF6− ({4d}) ; X = OTf, Ar = p-MeOC6H4 ({4e}), Ar = p-ClC6H4 ({4f}), Ar = p-FC6H4 ({4g})), prepared in situ from the cationic precursors [(η6-p-cymene)RuCl(PCy3)][X] (X = OTf (8a), BF4− (8b), PF6− (8c), SbF6− (8d) and the propargylic alcohols (4-YC6H4)2C(OH)C≡CH (Y = H, OMe, Cl, F), are studied by UV−visible spectroscopy by following the disappearance of the allenylidene moiety Ru═C═C═CAr2. The allenylidene to indenylidene rearrangement involves attack by the ortho-carbon of the aryl rings to the electrophilic allenylidene Cα, to form an intermediate arenium ion, and H-transfer from aryl to Cβ. Faster rates are observed in benzene than in dichloromethane in all cases. The first-order rate constants, kobs, for complexes 4a−d (Ar = Ph) depend on the counteranion nature and decrease in the order BF4− > OTf ≫ PF6− > SbF6− in benzene, while they are insensitive to the counteranion nature in dichloromethane. Measurements performed at different temperatures afford the activation parameters ΔH‡ = 22 ± 2 kcal mol−1 and ΔS‡ = −1 ± 5 cal mol−1 K−1 for complex {4a} at 18−50 °C and ΔH‡ = 24 ± 1 kcal mol−1 and ΔS‡ = 1 ± 1 cal mol−1 K−1 for complex {4e} at 25−55 °C. The values of activation entropy, which do not support a rate-determining phosphine dissociation step, and the rate effects of the aryl p-substituents are consistent with an early transition state in the C−C bond forming step of the ring-closing process. Rate differences between benzene and dichloromethane are discussed in terms of counteranion interactions in the less polar aromatic solvent. The rates of the allenylidene transformation affect directly the rates of the RCM reaction, as indicated by parallel solvent, proton, counteranion, and aryl substituent effects in the two processes, thus establishing the role of allenylidene−indenylidene rearrangement in the acid-free olefin metathesis.

Mechanistic studies of substitution reaction of trans-(diaqua)-(N,N' -ethylene-bis-salicylamide)chromium(III) ion

The kinetics of substitution of trans-[Cr(Salm)(OH 2 ) 2 ]+ with some biologically important ligands (Nu - ) viz. glycine (Gly), p-aminobenzoic acid (Paba), glycyl-glycine (Gly-gly), L -histidine (His), L -isoleucine (Ile) has been studied spectrophotometrically over the range: 25 ≤ 5 t ≤ 45 °C, 1.8 ≤ pH ≤ 5.5, 0.01 ≤ [Nucleophile] ≤ 0.30, I = 0.5 mol dm -3 (KNO 3 ). Although there are two replaceable aqua ligands in the complex, only mono substitution occurred, which is evident from Job's curve. The kinetic studies also showed one aqua ligand substitution. Unlike trans-[Cr(Salen)(OH 2 ) 2 ] + , the rates of aqua ligand substitution were found slower indicating moderate labilization of axial aqua ligand. The reaction takes place via an outersphere association between the Cr III complex and various nucleophiles followed by transformation of the outer into inner sphere complexes by slow interchange. The anation rate constants (10 4 k an /s -1 ) at 25 °C were found to be 14.98 (Gly), 8.45 (Ile), 10.52 (Paba), 14.23 (His) and 13.78 (Gly-gly) and the corresponding ΔH # (kJ mol -1 ) values are 57.7 ± 1.6, 38.3 ± 1.3, 43.0 ± 0.8, 34.2 ± 2.7 and 39.7 ± 3.1, the ΔS # (JK -1 mol -1 ) values are -105 ± 5, -175 ± 5, -157 ± 3, -65.8 ± 9 and -166 ± 11. The higher value of the k an in comparison to k ex and highly negative values of activation entropy and variation of k an by changing nucleophiles, support I a mechanism for the substitution reactions.

Controlling sequential hydroboration and mechanistic pathway by means of Lewis base/solvent effects: A detailed kinetic and mechanistic study

11B NMR spectroscopy was utilized to study hydroboration reactions of 1-octene and 4-octene with H 2BCl·THF in THF. The dependence of k obs on the nucleophile concentration and temperature was investigated. The reactions exhibited simple second-order kinetics of the form k obs = k 2 ′ [ Nu ] . The activation parameters (Δ H ≠, Δ S ≠) for the disappearance of H 2BCl·THF were respectively found to be 51 ± 2 kJ mol −1, −115 ± 8 J K −1 mol −1 in the case of 1-octene and 44 ± 1 kJ mol −1, −146 ± 4 J K −1 mol −1 in the case of 4-octene. Similarly, the activation parameters (Δ H ≠, Δ S ≠) for the formation of R 2BCl·THF were found to be 80 ± 3 kJ mol −1, −36 ± 9 J K −1 mol −1 in the case of 1-octene and 71 ± 2 kJ mol −1, −63 ± 6 J K −1 mol −1 in the case of 4-octene. Based on the activation parameters, it was concluded that hydroboration reactions involving H 2BCl·THF in THF proceed through a direct attack mechanism, with the magnitude of the negative values of activation entropy signifying a compact transition state i.e. a limiting associative pathway ( A). The increase in the steric strain due to the compact nature of the transition state resulted in the rate of the second step of hydroboration of 4-octene being slightly higher than that of 1-octene, by a factor of 1.5. This could be accounted for by postulating a forced partial dissociation of THF from the complex due to steric crowding and reduction of electrophilicity of the boron atom because of attachment of the first alkyl chain. This could also account for small negative entropy values for this step. Alternatively, a parallel reaction most likely a rearrangement process through formation of a π-complex occurred as a result of steric strain that was created around the boron atom on attachment of two alkyl chains at the central position.

Kinetics and mechanism of the pyridine-catalyzed reaction of phenylselenenyl halides and some unsaturated alcohols

The kinetics and mechanism of the reaction of phenylselenenyl halides (PhSeX, X = Cl, Br) and some primary unsaturated alcohols (pent-4-en-1-ol and hex-5-en-1-ol) in tetrachloromethane media have been studied, under the pseudo-first order conditions, in the presence and absence of pyridine as catalyst by variable temperature UV–vis spectrophotometry. Under the kinetic conditions, both the slower uncatalyzed and faster catalyzed (by pyridine) paths give the change of absorbance of the reaction mixture at some wavelength. The obtained values for rate constants have shown that the reactions with phenylselenenyl bromide are slower. Also, the reactions of both phenylselenenyl halides with hex-5-en-1-ol are faster than those with pent-4-en-1-ol. The negative values for entropy of activation for all studied reactions confirm the S N2 mechanism of substitution.

Photochromic behaviour of Berry Red studied in solution and polymer films

In this work, the photokinetic study of Reversacol Berry Red (2,2-bis(4-methoxyphenyl)-5,6-dimethyl-2H-naphtho[1,2-b]pyran), a photochromic compound from James Robinson Ltd., has been carried out in solution and in polymeric films (Paraloid B-72 and Polyvinyl butyral) in order to quantitatively characterize its photochromic properties by means of objective parameters in different environments. In solution, Berry Red shows reversible and intense photocolouration at room temperature and significant solvatochromic effect in the spectrum of the coloured form. Embedded into polymeric films, Berry Red maintains its thermally reversible photochromic behaviour. However, the photocolouration quantum yield is appreciably reduced and the kinetic parameters of bleaching are significantly altered. The matrix has the effect of lowering both the activation energy (up to a half of the value in solution) and the pre-exponential factor of thermal bleaching (by about seven orders of magnitude). These compensating effects make the bleaching rates in films close to that in solution. The negative values of activation entropy determined in polymer films are related to the rearrangement of the molecule into the activated complex structure.

Thermal decomposition of natural dolomite

Thermal decomposition behaviour of dolomite sample has been studied by thermogravimetric (TG) measurements. Differential thermal analysis (DTA) curve of dolomite shows two peaks at 777·8°C and 834°C. The two endothermic peaks observed in dolomite are essentially due to decarbonation of dolomite and calcite, respectively. The TG data of the decomposition steps have also been analysed using various differential, difference-differential and integral methods, viz. Freeman-Carroll, Horowitz-Metzger, Coats-Redfern methods. Values of activation entropy, Arrhenius factor, and order of reaction have been approximated and compared. Measured activation energies vary between 97 and 147 kJ mol−1. The large fluctuation in activation energy is attributed to the presence of impurities such as SiO2, Al2O3, Fe2O3, Cl− etc in the samples. FTIR and XRD analyses confirm the decomposition reaction. SEM observation of the heat-treated samples at 950°C shows cluster of grains, indicating the structural transformation.