Halides, alkyl hydrolysis, rates

Organonickel(II) species are believed to be formed during the reaction between [Ni(TMC)] and primary alkyl halides, and subsequently undergo hydrolysis with cleavage of the Ni—C bond. Kinetic data measured in the presence of excess alkyl halide indicate a rate law -dlNi1 (TMC)+]/cft = MNi (TMCr][RX]. The rate constants increase for R and X in the order methyl < primary < secondary < allyl < benzyl halides and Cl < Br < I (133, 140). This suggests that the rate-determining step is electron transfer from the Ni(I) complex to R—X via an inner-sphere atom-transfer mechanism (143). 

The susceptibility of alkyl halides to hydrolysis is a function of the halide (generally, I > Br > Cl > F). In contrast, the rates of aryl halide hydrolysis are generally F > Cl > Br > I. The reversal of the order of rates is because aryl halides hydrolyze via an addition-elimination reaction (March, (129)). The nucleophilic addition step, which is typically the ratedetermining step, is facilitated by the strongly electronegative fluorine. 

Aqueous hydrolysis rate constants for esters, carbamates, epoxides, halomethanes, and selected alkyl halides. Acid- and base-catalyzed rate constants are estimated, but not neutral hydrolysis rate constants. 

Hence the larger the n value, the stronger the nucleophile, and the smaller the [nucl]50o/o. As already pointed out earlier, the [nucl]50o/o values given in Table 2 show that in uncontaminated freshwaters, hydrolysis is by far the most important nucleophilic substitution reaction. Furthermore, since the hydrolysis of a carbon-halogen bond is generally not catalyzed by acids, one can assume that the hydrolysis rate of aliphatic halides will be independent of pH at typical ambient conditions (i.e., pH < 10). In this context it is also important to note that no catalysis of the hydrolysis of alkyl halides by solid surfaces has been observed (El-Amamy and Mill, 1984 Haag and Mill, 1988). In salty or contaminated waters, reactions of organic chemicals with nucleophiles other than water or j hydroxide ion may be important. Zafiriou (1975), for example, has demonstrated j that in seawater ([Cl ] 0.5 M), a major sink for naturally produced methyl j iodide is transformation to methyl chloride j... 

It is apparent that only those aliphatic amides which contain substituents that withdraw electron density from the carbonyl group (e.g., halides), making it more susceptible to nucleophilic attack, will have appreciable hydrolysis rates at pH 7 at 25°C. Alkyl substituents on nitrogen will increase hydrolysis half-lives due to steric effects (compare ti/2 for acetamide to A-methyl- and A-ethylacetamide). 

Additions of acetals and orthoesters to enol ethers probably represent the most intensively studied class of Lewis acid promoted reactions in the chemistry of aliphatic compounds. Since usually catalytic amounts of BFg OEta have been employed, concentration control (rule A) should predominate. Unlike the solvolyses of alkyl halides, the acid catalyzed hydrolyses of acetals and orthoesters do not follow a rate equilibrium relationship so that the corresponding hydrolysis rates cannot be used for the analysis of electrophilic addition reactions. We have, therefore, carried out competition experiments to determine relative reactivities of acetals and orthoesters towards methyl vinyl ether in presence of catalytic amounts of BF3 0Et2 (Figure 11). As the reactivity order towards other ir nucleophiles can be expected to be similar, the krei values of Figure 11 can be used to rationalize or predict the results of acetal and orthoester additions 1 1 Adducts can only be generated selectively if the k ei values of the designed products are smaller than the k Qi values of the reactants. 

In this book the discussion has been restricted to the structure of the normal states of molecules, with little reference to the great part of chemistry dealing with the mechanisms and rates of chemical reactions. It seems probable that the concept of resonance can be applied very effectively in this field. The activated complexes which represent intermediate stages in chemical reactions are, almost without exception, unstable molecules which resonate among several valence-bond structures. Thus, according to the theory of Lewis, Olson, and Polanyi, Walden inversion occurs in the hydrolysis of an alkyl halide by the following mechanism ... 

Strongly influences rates of hydrolysis. Hydrolysis of aliphatic and alkylic halides optimum at neutral to basic conditions.43 Other hydrolysis reactions tend to be faster at either high or low pH.186... 

Then again, if the hydrolysis in aqueous solution of the alkyl halide, RHal, is found to follow the rate equation,... 

Now, to return to the hydrolysis of the secondary alkyl halides, we will call the reactions (1) and (2), where the 1 relates to the SnI reaction and the 2 relates to the Sn2 reactions. (And we write the numbers with brackets to avoid any confusion, i.e. to prevent us from thinking that the T and 2 indicate first- and second-order reactions respectively.) We next say that the rate constants of the two concurrent reactions are k(p and k(2) respectively. As the two reactions proceed with the same 1 1... 

By varying R at C, (which is separated from the reaction site at Q by an oxygen) it is possible to influence the electron density at C in a defined way to change the heterolysis rate constant. Corrected for the incomplete transmission of electronic effects through the C methylene group and assuming that the transmission of the oxygen is 100%, the Taft p value for the effect of substitution on the heterolysis rate constant is equal to — 3.95, comparable to that ip = — 3.29) for the SnI hydrolysis of tertiary alkyl halides [21],... 

Substitution of fluorine for halogen in aromatic and alkyl sulfonyl halides can be carried out with or without using a solvent. An aqueous system such as 70 % aqueous potassium fluoride is also used because the rate of hydrolysis is much slower than the rate of fluorination as reported for the conversion of methanesulfonyl chloride (1) to methanesulfonyl fluoride (2).23... 

When comparing the hydrolysis half-lives of the alkyl halides in Table 13.6, we notice that the reaction rates increase dramatically when going from primary to secondary to tertiary carbon-halogen bonds. In this series, increasing the stabilization... 

Hudson et a/.156 have shown that N,N-dialkylcarbamates decompose in strongly acidic media to carbon dioxide, olefin, alkyl halide and alcohol, the rate of reaction of the secondary esters closely following h0. This fact, together with the variation in the rate of hydrolysis of carbamates of cyclic alcohols with the ring size154, shows that these reactions involve the intermediate formation of carbonium ions. 

Some indication of the importance of the solvent interactions is afforded by the observation that of the thousands of reactions which have been studied in solution, less than 20 have been capable of comparative study in the gas phase. The study of ionic reactions has been almost completely restricted to solutions for reasons which are quite understandable ionic processes are virtually nil in the gas phase at temperatures below 1000 K. However, this accounts for most of the solution reactions studied, since, as we shall see, most reactions between polar molecules involve ionic species as intermediates. Thus such common reactions as the hydrolysis of alkyl halides or of esters do not proceed at measurable rates in the gas phase (at least not at temperatures at which other, competing reactions are not dominant). The only large class of reactions which proceed conveniently in both gas and liquid states is the free radical class, and undoubtedly as... 

Although superoxide ion is a powerM nucleophile in aprotic solvents, it does not exhibit such reactivity in water, presumably because of its strong solvation by that medium (A//hydration, lOOkcalmol" ) and its rapid hydrolysis and disproportionation. The reactivity of 02 - with aUcyl halides via nucleophilic substitution was first reported in 1970. These and subsequent kinetic studies - confirm that the reaction is first order in substrate, that the rates follow the order primary > secondary > tertiary for alkyl halides and tosylates, and that the attack by 02 - results in inversion of configuration (Sn2). 

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