Slow step

The measurement of a from the experimental slope of the Tafel equation may help to decide between rate-determining steps in an electrode process. Thus in the reduction water to evolve H2 gas, if the slow step is the reaction of with the metal M to form surface hydrogen atoms, M—H, a is expected to be about If, on the other hand, the slow step is the surface combination of two hydrogen atoms to form H2, a second-order process, then a should be 2 (see Ref. 150). 

If reaction XVIII-42 is the slow step, the Langmuir-Hinshelwood rate law is... 

Nitration in acetic acid, in sulpholan and in carbon tetrachloride showed kinetic phenomena similar to those shown in nitromethane this is significant for it denies nitromethane a chemical involvement in the slow step. (Originally the rate of isomerization of nitromethane to its aci-form was believed to be a factor in the reaction. )... 

In pursuing the point about the differences between the case where the electrophile is preformed and where it is formed in a slow step the authors remark... 

The observation of nitration at a rate independent of the concentration and the nature of the aromatic means only that the effective nitrating species is formed slowly in a step which does not involve the aromatic. The fact that the rates of zeroth-order nitration under comparable conditions in solutions of nitric acid in acetic acid, sulpholan and nitromethane differed by at most a factor of 50 indicated that the slow step in these three cases was the same, and that the solvents had no chemical involvement in this step. The dissimilarity in the rate between these three cases and nitration with acetyl nitrate in acetic anhydride argues against a common mechanism, and indeed it is not required from evidence about zeroth-order rates alone that in the latter solutions the slow step should involve the formation of the nitronium ion. 

Because the carbon-halogen bond breaks m the slow step the rate of the reaction depends on the leaving group Alkyl iodides have the weakest carbon-halogen bond and are the most reactive alkyl fluorides have the strongest carbon-halogen bond and are the least reactive... 

The slow step m base catalyzed enolization is formation of the enolate ion The second step proton transfer from water to the enolate oxygen is very fast as are almost all proton transfers from one oxygen atom to another... 

The kinetics of reactions cataly2ed by very strong acids are often compHcated. The exact nature of the proton donor species is often not known, and typically the rate of the catalytic reaction does not have a simple dependence on the total concentration of the acid. However, sometimes there is a simple dependence of the catalytic reaction rate on some empirical measure of the acid strength of the solution, such as the Hammett acidity function Hq, which is a measure of the tendency of the solution to donate a proton to a neutral base. Sometimes the rate is proportional to (—log/ig)- Such a dependence may be expected when the slow step in the catalytic cycle is the donation of a proton by the solution to a neutral reactant, ie, base but it is not easy to predict when such a dependence may be found. 

It has been suggested that the first step, weak coadsorption of NO and O2 on a reduced vanadium site, may represent the slow step in the mechanism. Subsequent formation of a N202-like intermediate could be a fast step, because it is known that in the gas phase the equiUbrium of NO—NO2 and N2O2 is estabflshed within microseconds (35). 

Scheme 10. Mechanislic possibililies for PF condensalion. Mechanism a involves an SN2-like attack of a phenolic ring on a methylol. This attack would be face-on. Such a mechanism is necessarily second-order. Mechanism b involves formation of a quinone methide intermediate and should be Hrst-order. The quinone methide should react with any nucleophile and should show ethers through both the phenolic and hydroxymethyl oxygens. Reaction c would not be likely in an alkaline solution and is probably illustrative of the mechanism for novolac condensation. The slow step should be formation of the benzyl carbocation. Therefore, this should be a first-order reaction also. Though carbocation formation responds to proton concentration, the effects of acidity will not usually be seen in the reaction kinetics in a given experiment because proton concentration will not vary.

Because of thetr electron deficient nature, fluoroolefms are often nucleophihcally attacked by alcohols and alkoxides Ethers are commonly produced by these addition and addition-elimination reactions The wide availability of alcohols and fliioroolefins has established the generality of the nucleophilic addition reactions The mechanism of the addition reaction is generally believed to proceed by attack at a vinylic carbon to produce an intermediate fluorocarbanion as the rate-determining slow step The intermediate carbanion may react with a proton source to yield the saturated addition product Alternatively, the intermediate carbanion may, by elimination of P-halogen, lead to an unsaturated ether, often an enol or vinylic ether These addition and addition-elimination reactions have been previously reviewed [1, 2] The intermediate carbanions resulting from nucleophilic attack on fluoroolefins have also been trapped in situ with carbon dioxide, carbonates, and esters of fluorinated acids [3, 4, 5] (equations 1 and 2)... 

It, therefore, appears that the equilibrium approximation is a special case of the steady-state approximation, namely, the case i > 2- This may be, but it is possible for the equilibrium approximation to be valid when the steady-state approximation is not. Consider the extreme but real example of an acid-base preequilibrium, which on the time scale of the following slow step is practically instantaneous. Suppose some kind of forcing function were to be applied to c, causing it to undergo large and sudden variations then Cb would follow Ca almost immediately, according to Eq. (3-153). The equilibrium description would be veiy accurate, but the wide variations in Cb would vitiate the steady-state description. There appear to be three classes of practical behavior, as defined by these conditions ... 

In the first step the hydrated ion and ligand form a solvent-separated complex this step is believed to be relatively fast. The second, slow, step involves the readjustment of the hydration sphere about the complex. The measured rate constants can be approximately related to the constants in Scheme IX by applying the fast preequilibrium assumption the result is k = Koko and k = k Q. However, the situation can be more complicated than this. - °... 

This equation can be obtained in another way which may be more instructive. Assume that the slow step in the oxidation is the transport of cation vacancies. The positive holes may then be considered to take up their equilibrium distribution, defined by Boltzmann s equation... 

The slow step is the transport of material through the oxide layer. 

A striking example of the interaction of solution velocity and concentration is given by Zembura who found that for copper in aerated 0-1 N H2SO4, the controlling process was the oxygen reduction reaction and that up to 50°C, the slow step is the activation process for that reaction. At 75 C the process is now controlled by diffiision, and increasing solution velocity has a large effect on the corrosion rate (Fig. 2.5), but little effect at temperatures below 50 C. This study shows how unwise it is to separate these various... 

Transport of a species in solution to and from an electrode/solution interface may occur by migration, diffusion and convection although in any specific system they will not necessarily be of equal importance. However, at the steady state all steps involved in the electrode reaction must proceed at the same rate, irrespective of whether the rate is controlled by a slow step in the charge transfer process or by the rate of transport to or from the electrode surface. It follows that the rate of transport must equal the rate of charge transfer ... 

Often, one step in a mechanism is much slower than any other. If this is the case, the slow step is rate-determining. That is, the rate of the overall reaction can be taken to be that of the slow step. Consider, for example, a three-step reaction ... 

The rate at which A is converted to D (the overall reaction) is approximately equal to the rate of conversion of B to C (the slow step). 

Reaction mechanism A sequence of steps by which a reaction occurs at the molecular level, 307,318-319q elementary steps, 307 intermediates, elimination of, 309-311 rate expression for, deducing, 308-309 slow steps, 307... 

Slow step, 308 Smalley, Richard E., 250 Smog, 310,565 Smoke alarms, 516,517f Soap, 595 Sodium... 

The form of experimental dependences shown in Fig. 6 indicates that slow steps are arranged consecutively in the coupled system and that desorption of the intermediate product, toluene, is fast compared with its further transformation to benzene. Otherwise, the concentrations of these two compounds (at least at the beginning of the reaction) would have shown a parallel increase [compare with (16) and with Fig. 5]. Similarly,... 

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