Rate determining step amines

In this solvent the reaction is catalyzed by small amounts of trimethyl-amine and especially pyridine (cf. 9). The same effect occurs in the reaction of iV -methylaniline with 2-iV -methylanilino-4,6-dichloro-s-triazine. In benzene solution, the amine hydrochloride is so insoluble that the reaction could be followed by recovery. of the salt. However, this precluded study mider Bitter and Zollinger s conditions of catalysis by strong mineral acids in the sense of Banks (acid-base pre-equilibrium in solution). Instead, a new catalytic effect was revealed when the influence of organic acids was tested. This was assumed to depend on the bifunctional character of these catalysts, which act as both a proton donor and an acceptor in the transition state. In striking agreement with this conclusion, a-pyridone is very reactive and o-nitrophenol is not. Furthermore, since neither y-pyridone nor -nitrophenol are active, the structure of the catalyst must meet the conformational requirements for a cyclic transition state. Probably a concerted process involving structure 10 in the rate-determining step... 

Square brackets in kinetic equations signify the effective concentrations of the bracketed species, these being the equilibrium forms actually taking part in the rate-determining step. Parentheses are used for stoichiometric concentrations. Thus (ArNH2) is the total amount of an amine present in the system, even if it... 

The rate-determining step in the diazotization of aniline in aqueous perchloric acid below concentrations of 0.05 m (pH >0.7) is the formation of N203. The following A-nitrosation step is faster (rate equation of Scheme 3-12). However, with aromatic amines that are weaker nucleophiles than aniline, e.g. 4-nitroaniline, nitrosation is slower than the formation of N203, and the rate is second-order with respect to nitrous acid and first-order in amine (Scheme 3-13, Larkworthy, 1959). 

As discussed in the three preceding sections, the key intermediate in diazotizations is the A-nitroso derivative of the primary amine, the formation of which is usually the rate-determining step of diazotization. The subsequent steps are faster and therefore not easily accessible to study. The sequence of protonation, deprotonation, protonation, and dehydration in Scheme 3-36 seems to be the most reasonable mechanism. 

This possibility warrants more detailed consideration. For the reaction of 2,4-dinitrochlorobenzene with an amine it is highly probable that the rate-determining step is intermediate formation. One possible mode of base catalysis in this system would involve transfer of the proton in the step in which the intermediate is formed. 

The different reactivity mentioned above also proves the validity of inequality ki, k3> >k4 used in the simplification of our model. On the contrary, in the presence of CHA less than one equivalent the signals of both the la and Ih appear, a large extent of deuteration at C-3 is observed both in the cis and tram isomers and in the product flavone (2). Using an excess of amine both isomer gave 2 deuterated at C-3 to an extent ca. 80-85 %. Considering the kinetic profile of the interconversion we conclude that it takes place via an enolate where the rate determining step is the deprotonation at C-3. 

If the formation and breakdown steps of a mechanism involving a tetrahedral intermediate respond differently to changes in pH or catalyst concentration, then one can find evidence from plots of rate versus pH or rate versus catalyst concentration for a change in rate determining step and thus for a multistep mechanism. An example would be the maximum seen in the pH rate profile for the formation of an imine from a weakly basic amine (such as hydroxylamine). On the alkaline side of the maximum, the rate determining step is the acid-catalyzed dehydration of the preformed carbinolamine on the acid side of the maximum, the rate determining step is the uncatalyzed addition of the amine to form the carbinolamine. The rate decreases on the acid side of the maximum because more and more of the amine is protonated and unable to react. 

In most of the cases discussed so far, the rate-determining step of the reaction is nitrosation of the free amine. When diazotisation is carried out in concentrated acids, the nitrosonium cation, NO +, is the nitrosating species. In this case, an exchange mechanism has been proposed, as... 

Examination of a series of imines of differing electronic properties showed that a change in the rate-determining step of this stoichiometric C=N hydrogenation occurs as the imine becomes more electron-rich. Hydrogenation of N-iso-propyl-(4-methyl)benzilidene amine led to an amine complex of ruthenium. In addition, the C=N hydrogenation was accompanied by isomerization of the imine to a ketimine (Eq. (47)). 

Furthermore, in many cases, changes in the mechanism have also been observed and they will be discussed in a later section. Nevertheless, by selecting a system that exhibited the same rate-determining step in a variety of solvents it would be possible to assess how the rate of a given process may be affected by a solvent transfer. Such is the case of the reaction of l-chloro-2,4-dinitrobenzene with piperidine, where the rate dependence with amine concentration has been studied in 12 aprotic solvents483 as well as in 10 protic solvents4815. It was found that the reaction does not exhibit base catalysis in any of the solvents studied that is, addition of piperidine is rate-limiting in all the... 

When the decomposition of the zwitterionic intermediate is rate-determining, the effect of the solvent is crucial since it may produce changes in the mechanisms and in the rate-determining step. A recent study of the kinetics of the reactions of 1-chloro-, 1-fluoro- and l-phenoxy-2,4-dinitrobenzene with piperidine, n-butylamine and benzylamine in ethyl acetate and THF indicated that these reactions resemble those in dipolar aprotic solvents when primary amines are the nucleophiles (i.e. that shown in equation 1, with... 

Most of the novel mechanisms hitherto presented were based on the observation of overall fourth-order kinetics (third-order in amine). Nevertheless, this result gives an account only of how many molecules are involved in the rate-determining step. It cannot distinguish, e.g., between three mechanisms that could be depicted as equations 38-40. 

The amine-catalyzed mercaptan-epoxide reaction (Equation 4) proceeds exothermally at room temperature (27. 28). The order of average relative nucleophile-displacement rates (Table II) further suggests that mercaptans react significantly faster than amines and that the addition of the mercaptlde (RS ) ion to the epoxide group is the rate determining step (30). 

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