Time-dependent strength of colloidal gels

Colloidal silica gels are shown to stiffen with time, as demonstrated by both dynamic light scattering and bulk rheological measurements. Their elastic moduli increase as a power law with time, independent of particle volume fraction; however, static light scattering indicates that there are no large-scale structural changes. We propose that increases in local elasticity arising from bonding between neighboring colloidal particles can account for the strengthening of the network, while preserving network structure.

The kinetics of isomerization of allylbenzene homogeneously catalysed by palladium(II) in acetic acid

The changes in the kinetics observed during the isomerization of allylbenzene catalysed by palladium(II) are interpreted in terms of the slow formation of a hydrido complex of palladium(II) which subsequently attains a constant concentration in an equilibrium system. The kinetics during these phases are shown to be consistent with first-order dependence on the concentration of an active catalyst formed in a bimolecular reaction from a mononuclear palladium(II) complex and with a regenerative hydrido-π-alkene-σ-alkyl mechanism of isomerization. The hypothesis that a further stage in the kinetics reflects a change in the rate determining step to one involving alkene displacement from the catalyst is supported by the experimental evidence. The concentration of active catalyst is shown not to fall appreciably until all the allylbenzene has undergone isomerization, but thereafter there is a slow reduction of catalytic activity which is not completely restored by the addition of further allylbenzene. It is suggested that the slow formation of a π-allylic complex is responsible.

Isotope studies of the isomerization of olefins homogeneously catalysed by palladium, platinum, and rhodium complexes

No entry of deuterium into olefin could be detected as a result of isomerization of allylbenzene in the presence of palladium(II) catalysts in CH3CO2D or CH3OD. The deuterium isotope effect on the rate of isomerization was found to be high in the case of CH3CO2D, but slight in the case of CH3OD. In the case of catalysis by rhodium complexes, entry of deuterium into olefin occurs in CH3OD but not in CH3CO2D. Isotope effects parallel those observed with palladium(II). The isomerization of PhCD2CH2CH=CH2 catalysed by palladium(II) results in a near-to-random distribution of deuterium throughout the side chain. The results are consistent with a hydrido-π-olefin complex mechanism of isomerization. Differences between catalysis by palladium(II) and rhodium(III) are discussed in terms of possible differences in the mechanism of formation of the active catalyst. In the case of palladium it is concluded that a relatively rapid rate of σ-π interchange and hence of double bond shift occurs within the equilibrium system of catalyst and olefin, and that exchange of olefin with the substrate constitutes the slower, rate-determining step. The behaviour of platinum(II) is generally similar to that of palladium(II) but is only effective at higher temperatures.

The homogeneous catalysed isomerization of 2-allylphenol and of (Z)-2-propenylphenol in the presence of a palladium catalyst

The isomerization of 2-allylphenol (ap) in the presence of bis(benzonitrile)-dichloropalladium(II) displays kinetics which differ in certain respects from the normal isomerization of olefins catalysed homogeneously by transition metal complexes. The most unusual feature is that the rate increases continuously so long as a detectable concentration of ap remains. The behaviour is believed to be due to a concentration of active catalyst which increases with time and with concentration of the product, 2-propenylphenol (pp). The participation of the phenolic hydroxyl group, resulting in chelation of pp to palladium is considered to be responsible for the difference in behaviour between ap and, for example, allylbenzene. The isomerization of (Z)-pp also displays unusual characteristics which are explained within the framework of this hypothesis.

Equilibria involving allylbenzene, chloride ion, and palladium(II) in acetic acid solution

The addition of allylbenzene to sodium di-μ-chloro-tetrachlorodipalladate(11) in acetic acid solution produces changes in the absorption spectrum which are compared with those produced by chloride ion and by pyridine. The occurrence and disappearance of certain isosbestic points over various ranges of ligand to palladium ratios together with evidence from plots of absorbance against mole ratio indicates that complex formation occurs in discrete steps. Evidence is presented which supports the view that under the conditions normally employed for the homogeneous catalytic isomerization of alkenes, the palladium is present as a monomeric palladium-alkene complex.

Substitution reactions of some bis(2,2'-bipyridine) and mixed 2,2'-bipyridine, 2,2',2'-terpyridine complexes of ruthenium(II). II. The kinetics of substitution of nitrite and other nucleophiles

The rates of substitution of the complex ions [Ru11(H2O)bipytrpy]2+ and [Ru11(H2O)2 bipy2]2+, where bipy = 2,2'-bipyridine and trpy = 2,2',2"-terpyridine, by a number of nucleophilic reagents in aqueous solution, have been determined as a function of temperature. The reactions were observed to be second order and rates fall in the sequence Ni > NO2 > SCN-> pyridine. The range of Arrhenius parameters is very large with some unusually low frequency factors. The diaquo complex appears to undergo substitution leading to disubstituted derivatives in a single step. In the case of substitution by nitrite, there is evidence that the final products are nitro complexes formed by rapid isomerization of intermediate nitrito complexes.

The isomerization of n-phenylbutenes catalysed by palladium complexes

4-Phenylbut-1-ene isomerizes to a mixture of cis- and trans-1-phenylbut-2-ene and trans-1-phenylbut-1-ene in the presence of bis(benzonitrile)dichloropalladium(11). The products are not formed entirely in consecutive reactions but direct conversion of 4-phenylbut-1-ene into trans-1-phenylbut-1-ene occurs extensively. Similarities are observed in the kinetics with those observed previously for allylbenzene. The isomerization of 1-phenylbut-2-ene to 1-phenylbut-1-ene is comparatively slow in spite of conjugation between the aromatic ring and the double bond in the side-chain within the latter compound. The rate-controlling step in the isomerization of 1-phenylbut-2-ene is believed to be an olefin displacement reaction, but an induction period renders the conclusions with regard to the kinetics of the initial isomerization of 4-phenylbut-1-ene less certain.

Substitution reactions of some Bis(2,2'-bipyridine) and mixed 2,2'-bipyridine, 2,2',2'-terpyridine complexes of ruthenium(II)

Substitution reactions of the complex ions [RuIIX bipy trpy]+ and [RuIIX2 bipy2]0, where X = Cl, Br, I; bipy = 2,2?-bipyridine; trpy = 2,2?2?- terpyridine, take place in aqueous solution via extremely rapid formation of the aquoammines. The rates of substitution of the aquo groups by NO-2 and N-3 occur by measurably slow processes at room temperature and are not base catalysed. The aquo complexes are immediately converted by OH- into hydroxo complexes. The pKa values of the aquo complexes are shown to be 10.0 for [RuII(H2O) bipy trpy]2+ and about 9-10 for [RuII(H2O)2 bipy2]2+, the latter being an estimate since oxidation to ruthenium(III) in alkaline solution could not be avoided. The visible and ultraviolet spectra are reported.

The catalytic isomerization of allylbenzene in the presence of palladium(II) chloride

Allylbenzene has been shown to isomerize rapidly in the presence of palladmm(11) chloride in acetic acid, mainly to trans-β-methylstyrene but to a small extent to the cis isomer. The reaction is not simple. The initial formation of an olefin complex is succeeded by a further slow reaction involving the palladium complex before isomerization takes place. The isomerization reaction itself is zero order with respect to allylbenzene except nearing completion when it becomes first order in allylbenzene concentration. The reaction is first order with respect to catalyst. Rates have been measured by a technique involving N.M.R. spectrometry. Substitution in allylbenzene inhibits isomerization in the case of 2-bromoallylbenzene but not entirely in the case of the corresponding methyl derivative. The mechanism is discussed in the light of an intermediate hydrido complex.

Palladium-catalysed olefin isomerization

The mechanism whereby olefins undergo isomerization in the presence of sodium chloropalladate(II) has been shown to involve the stepwise movement of hydrogen atoms from the one carbon atom to its nearest neighbour. Evidence leading to this conclusion is furnished by an examination of the products of isomerization both by vapour phase chromatography which gave the product distribution, and by N.M.R. which, in conjunction with selective labelling of the olefin using deuterium, enable a picture to be drawn of the pattern of hydrogen migration.