Exchange reactions, homogeneous

Comparison between Experimental Reorganization Parameters for some Electrochemical and Homogeneous Exchange Reactions... 

Exchange reactions have the advantage that long routes of synthesis with radionuclides or radioactive compounds are avoided. This is of particular interest if the chemical yields of the syntheses are low. Homogeneous exchange reactions may be applied for labelling of compounds with halogens, e.g. 

A reaction that has been studied in some detail is the homogeneous exchange reaction with isotopic hydrogen in benzene which occurs in the presence of catalytic amounts of Pt(II) and Pt(IV) salts 154). The exchange rate was found to follow the kinetics k (exchange) a [PtCl4 ] [C6Hg][Cl ] . The proposed mechanism is shown in Fig. 1 and... 

Second, it can be shown that the conventional relationships [Eq. (i), 12.3.7.2] between the rates of electrochemical and homogeneous exchange reactions can be extended to chemically irreversible processes. Therefore, if the assumptions contained in this equation are valid, the electrochemical rate constant, k, at the intersection of the cathodic and anodic log rate-potential plots will be related approximately to the rate constant, k. ., for the homogeneous cross reaction between the same pair of redox couples - ... 

Homogeneous reactions. Homogeneous noncatalytic reactions are normally carried out in a fluidized bed to achieve mixing of the gases and temperature control. The sohds of the bed act as a heat sink or source and facihtate heat transfer from or to the gas or from or to heat-exchange surfaces. Reaclious of this type include chlorination of hydrocarbons or oxidation of gaseous fuels. 

The pre.sent account follows a Journey in this arena from solution calorimetric studies dealing with nucleophilic carbene ligands in an organometallic system to the use of these thermodynamic data in predicting the feasibility of exchange reactions to applications in homogeneous catalysis. 

This chapter attempts to survey the studies which have been made on the various electron transfer reactions, occurring between metal ions (of the same element) in homogeneous solution. These reactions include the types known as exchange reactions... 

Ion-exchange processes are strictly heterogeneous reactions that involve two phases. They, however, mostly comply with homogeneous, second-order reaction kinetics. An exchange reaction represented by... 

Esr spectroscopy has also been used to study pure solvent dynamics in electron self-exchange reactions (Grampp et al., 1990a Grampp and Jaenicke, 1984a,b). When the systems are not linked by a spacer (i.e. TCNQ- /TCNQ (TCNQ = tetracyanoquinodimethane), the homogeneous bimolecular rate constants /chom are given by (10), with fcA the association constant and kET... 

The significance of these quantities is analogous to that for the activation parameters for homogeneous self-exchange reactions. Thus, AH equals the activation enthalpy for conditions... 

Relationships having the same form as eq 14 can also be written for the enthalpic and entropic contributions to the intrinsic free energy barriers (10). Provided that the reactions are adiabatic and the conventional collision model applies, eq 14 can be written in the familiar form relating the rate constants of electrochemical exchange and homogeneous self-exchange reactions (13) ... 

In the following sections, we shall explore the applicability of such relationships to experimental data for some simple outer-sphere reactions involving transition-metal complexes. In keeping with the distinction between intrinsic and thermodynamic barriers [eq 7], exchange reactions will be considered first, followed by a comparison of driving force effects for related electrochemical and homogeneous reactions. 

Also, the observed rates probably refer to outer-sphere pathways, and the rate constants for the corresponding homogeneous self-exchange reactions are available or can be estimated from rate data for closely related cross reactions (15). These h ex... 

Rate Constants and Thermodynamic Parameters for Selected Electrochemical Exchange and Homogeneous Self-Exchange Reactions at 25°C. 

In the homogeneous case, the derivation of an activation-driving force relationship and of an expression of the intrinsic barrier is less straightforward. It is interesting in this connection to relate the kinetics of crossexchange reactions (31) to those of the two self-exchange reactions (also called identity or isotopic reactions) [(32) and (33)]. 

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