Reaction rate rate-determining step

Rate constant The proportionality constant in the rate equation for a reaction, 288 Rate-determining step The slowest step in a multistep mechanism, 308 Rate expression A mathematical relationship describing the dependence of reaction rate upon the concentra-tion(s) of reactant(s), 288,308-309 Rayleigh, Lord, 190... 

For most phase-transfer catalysed reactions, the rate-determining step is the interaction of the reactive substrate with the anionic species in the organic phase and, compared with the corresponding interfacial reaction in the absence of the catalyst, rate enhancements of 107 are not uncommon. The virtual absence of water from the organic phase under strongly basic liquiddiquid or soliddiquid two-phase conditions allows for the formation of water-sensitive anions, such as carbanions (Chapter 6), and obviates the need for strictly anhydrous conditions and the use of bases such as sodium hydride or sodamide, etc. The phase-transfer catalytic process consequently has lower safety risks and is environmentally more friendly. 

In a first-order reaction, the rate-determining step involves a transformation where one reactant reacts to give one product, that is, A — B. In first-order reactions, there is an exponential decrease in the reactant concentration, so that at any given time, the transformation rate is dependent on the corresponding concentration of the reactant at the same time. This can be expressed in the following way ... 

Thermal ring opening to form the merocyanine form is less dependent upon the type of substituent keeping a AG of near to 101 kJmol for BIPS, 6-nitro-BIPS and 6,8-dinitro-BIPS. [37] The ring opening reaction s rate-determining step is probably a rotation cis to trans about the central p-methine bond and this is also consistent with the polar CCC or CCT transition state which have a rather double p-methine bond. 

There are several places in electrochemical reactions where rate-determining steps can occur. First, if a cathode potential is sufficiently negative, transport of reactants to the electrode will not be able to keep pace with the events that transfer charge as the electrode demands. Then, transport in solution and the electrode events have to be satisfied with what the transport rate can bring to the interface. Transport is the rds. 

The interpretation of the kinetics is based on the mechanisms proposed by Schechter et al.55) and Smith 57), for the reaction of secondary amines with epoxides, extended to include the primary amine reaction. The rate-determining step is assumed to be the reaction of amine, epoxide and hydroxyl or other proton-donor species, HX, to form a termolecular complex, Eq. (3-13). The proposed reaction scheme is ... 

Shafizadeh5 has suggested that a unimolecular (SNO reaction is operative. In this type of reaction, the rate-determining step would be the formation of a carbonium ion, with the removal of the ethylthio group subsequent attack on this ion by the nucleophile would be rapid. In an SNj reaction, the removal of the ethylthio group and the attack by the nucleophile would be simultaneous. The SNi reaction seems more probable here. 

Cyanohydrin formation was reported for the range of reactions for which k and K have been reported in water.81 This was the first application of NBT to a carbon-carbon bond-forming reaction. The rate-determining step was taken to be the attack of cyanide on the carbonyl compound to give the cyanohydrin anion. The results are summarized in Table 9. 

Van Ho and Harriott have postulated a different mechanism involving intermediate adsorbed carbon atoms.188 This is based on their observation that a larger fraction of the surface of the catalyst was covered by adsorbed carbon monoxide during reaction. The rate determining step was assumed to... 

In chemical reactions, the rate-determining step works in much the same way. From a conceptual standpoint, consider a reaction with two elementary steps where the first step is the slow one (the rate-determining step). The concentration of the reactants in the first step is far more important than the concentration of the additional reactants in the second step. That is because the second step can t start until the first step has been completed. In our fast-food example, it s the equivalent of saying that you can t speed up the process by adding more cashiers. You can have fifty cashiers, but if there are no french fries for them to pick up, they can t do anything. 

The correct answer is (D). In this reaction, the rate-determining step is the second step. The rate law should be written from this step. However, when writing the rate law ... 

In the second type of bimolecular surface reaction, the rate-determining step takes place between adsorbed reactant A and absorbed reactant B sitting on adjacent sites... 

It has been argued that insight into catalytic mechanisms can most readily be obtained by choosing reactions whose rate-determining step is unimolecular. The reactions best known to be unimolecular in homogeneous media are undoubtedly the solvolyses of tertiary butyl halides [161-163]. Their SN1 mechanisms are typified by the solvolysis of f-butyl bromide in an ethanol + water medium... 

In some reactions, the rate-determining step appears to be attack on the d-electron density of the metal atom. Thus the hexaammine [Ru(NH3)6]3+ undergoes aquation only very slowly at room temperature but reacts rapidly with NO, as noted previously. Substitution in reactions such as ... 

As shown in Equation 19, the substituent effects on the ovicidal activity can be separated into hydrophobic and electronic factors. There is an optimal value for hydrophobicity for the ovicidal action of this set of compounds. That the a value in Equation 19 is similar to those obtained for in vitro reactions would suggest that a common step is critical in biological as well as in vitro reactions. For the in vitro reactions, the rate-determining step is considered to be the formation of cyclic sufonium ions (40, 41). 

In this reaction, the rate-determining step is Step [1]. 

In the Sj-jl reaction, the rate-determining step is the formation of the carbocation, an endothermic reaction. According to the Hammond postulate, the stability of the carbocation detennines the rate of its formation. 

Reagent Reaction sequence Rate-determining step... 

In the case of a multistep reaction, the measured jEX may not be an activation energy of an elementary reaction. For example, if in a two-step reaction the rate-determining step is preceded by a rapid equilibrium-controlled reaction, the AH of the first reaction contributes to the measured activation energy (Stumm and Morgan, 1981). The activation energy determined by the temperature dependence of the reaction is then only an apparent activation energy. 

When a carbocation is formed by loss of a nucleophile, as in the SN1 reaction, elimination of a proton will always compete with substitution, provided that there is a (3-C-H bond in the cation. These eliminations are described as unimolecular eliminations (El)because, as with the competing SN1 reaction, the rate-determining step is the unimolecular reaction of the original halide to give the carbocation. For example, /-butyl chloride in ethanol gives the /-butyl cation, which can either react with a solvent molecule to give the ethyl ether 36 (SN1, solvolysis, reaction 5.25) or a proton can be removed by a solvent molecule to give 2-methylpropene 37 (El, reaction 5.26). If more than one type of J3-C-H... 

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