Kinetics and Mechanism of the Pyridinolysis of Aryl Furan-2-carbodithioates in Acetonitrile
摘要:
Kinetic studies on the pyridinolysis of aryl furan-2-carbodithioates 1 are carried out at 60.0 degreesC in acetonitrile. The biphasic rate dependence on the pyridine basicity with a breakpoint at pK(a)degrees = 5.2 is interpreted to indicate a change of the rate-limiting step from breakdown (betax = 0.7-0.8) to formation (betax = 0.2) of the tetrahedral intermediate, T+/-:, at the breakpoint as the basicity of the pyridine nucleophile is increased. Observation of the breakpoint is possible with pyridines since the expulsion rate of the pyridine (k-(a)) from T+/- is sufficiently low, with the low k(-a)/k(b) ratio leading to a low breakpoint, pK(a)degrees. The clear-cut change in the cross-interaction constants, pxz, from a positive (rhoxz = +0.86) to a small negative (rhoxz = -0.11) value at the breakpoint supports the mechanistic change proposed. The magnitudes of rhoz and activation parameters are also consistent with the proposed mechanism.
Non-toxic corrosion-protection pigments based on manganese
申请人:——
公开号:US20040011252A1
公开(公告)日:2004-01-22
Corrosion-inhibiting pigments based on manganese are described that contain a trivalent or tetravalent manganese/valence stabilizer complex. An inorganic or organic material is used to stabilize the trivalent or tetravalent manganese ion to form a compound that is sparingly soluble, exhibits low solubility, or is insoluble in water, depending upon the intended usage. Specific stabilizers are chosen to control the release rate of trivalent or tetravalent manganese during exposure to water and to tailor the compatibility of the powder when used as a pigment in a chosen binder system. Stabilizers may also modify the processing and handling characteristics of the formed powders. Manganese/valence stabilizer combinations are chosen based on the well-founded principles of manganese coordination chemistry. Many manganese-valence stabilizer combinations are presented that can equal the performance of conventional hexavalent chromium or tetravalent lead systems. It is emphasized that this abstract is provided to comply with the rules requiring an abstract which will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.
Non-toxic corrosion-protection pigments based on rare earth elements
申请人:——
公开号:US20040104377A1
公开(公告)日:2004-06-03
A corrosion-inhibiting pigment comprising a rare earth element and a valence stabilizer combinded to form a rare earth/valence stabilizer complex. The rare earth element is selected from cerium, terbium, praseodymium, or a combination thereof, and at least one rare earth element is in the tetravalent oxidation state. An inorganic or organic material is used to stabilize the tetravalent rare earth ion to form a compound that is sparingly soluble in water. Specific stabilizers are chosen to control the release rate of tetravalent cerium, terbium, or praseodymium during exposure to water and to tailor the compatibility of the powder when used as a pigment in a chosen binder system. Stabilizers may also modify the processing and handling characteristics of the formed powders. Many rare earth-valence stabilizer combinations are presented that can equal the performance of conventional hexavalent chromium systems.
The aminolyses of the title substrates with anilines and benzylamines are investigated in acetonitrile. A clean second-order kinetics is obtained with a first-order rate law in the amine concentration, which is uncomplicated by the fast proton transfer step. The large magnitude of rho(z) (rho(1g)) as well as rho(x) (rho(nuc)) together with relatively large positive p(xz) values is consistent with a stepwise mechanism in which thiophenolate ion expulsion from the intermediate is rate limiting. For the reactions of aryl dithio-2-thiophenates with benzylamines the magnitude of rho(x) and rho(z) values is relatively smaller suggesting that both the addition and expulsion of thiophenolate are partially rate determining. Relatively large secondary kinetic isotope effects, k(H)/k(D) greater than or equal to 1.7, with deuterated nucleophiles, support involvement a concurrent proton transfer to the departing thiophenolate ion in the transition state. (C) 1998 John Wiley & Sons, Inc.