Second-order rate constant alkyl bromides

Ratio of second order rate constant k for indicated alkyl bromide to k for isopropyl bromide at 25 C... 

Second-order rate constant for hydrolysis of ethyl iodide by H2O ( ) used as a surrogate for kH2o,sN for DMIH. Rationale for this approach comes from an analogy with the hydrolysis of n-alkyl bromides, since kn20 N for MeBr, EtBr, n-PrBr and n-HexBr all agree within a factor of two at 25°C (see Table V). 

The second-order rate constants for reactions of Co(I)(BDHC) with alkyl halides were determined spectrophotometrically at 400 nm (17). These rate constants are listed in Table VII along with those for Co(I)(corrinoid)(vitamin Bi2s) in methanol at 25°C (35). These data indicate that the SN2 mechanism is operative in the reaction of Co(I)(BDHC) the iodides are more reactive with the cobalt complex than the bromides, and the rate decreases with increasing bulkiness of the alkyl donor. The steric effect is more pronounced for Co(I)(BDHC) than for vitamin B12s, which is confirmed by the rate ratios for... 

Interestingly, no correlation could be observed from their monomer ion-pair acidities (pAT0 in THF) and the second-order rate constant for the monomer in their reaction with m-chlorobenzyl bromide (Table 2, right), a linear relationship occurs when the corresponding cesium salts are alkylated with methyl tosylate. On the other hand according to the authors, this accounts for the fact that the lithium cation is as important as the basicity of the enolate. 

Aliphatic and aromatic nucleophilic substitution reactions are also subject to micellar effects, with results consistent with those in other reactions. In the reaction of alkyl halides with CN and S Oj in aqueous media, sodium dodecyl sulfate micelles decreased the second-order rate constants and dodecyltrimethylammonium bromide increased them (Winters, 1965 Bunton, 1968). The reactivity of methyl bromide in the cationic micellar phase was 30 to 50 times that in the bulk phase and was negligible in the anionic micellar phase a nonionic surfactant did not significantly affect the rate constant for n-pentyl bromide with S2O3-. Micellar effects on nucleophilic aromatic substitution reactions follow similar patterns. The reaction of 2, 4-dinitrochlorobenzene or 2, 4-dinitrofluorobenzene with hydroxide ion in aqueous media is catalyzed by cationic surfactants and retarded by sodium dodecyl sulfate (Bunton, 1968, 1969). Cetyltrimethylammonium bromide micelles increased the reactivity of dinitrofluorobenzene 59 times, whereas sodium dodecyl sulfate decreased it by a factor of 2.5 for dinitrochlorobenzene, the figures are 82 and 13 times, respectively. A POE nonionic surfactant had no effect. 

The second-order rate constants for the substitution reactions of alkyl halides generally decrease as the aUcyl halide varies from methyl to 1° to 2° to 3°. This trend is illustrated in Table 8.9, which summarizes kinetic data for alkyl bromides (R-Br) reacting with bromide ion. ° This trend is usually ascribed to increasing steric hindrance to back-side attack of the nucleophile as hydrogens bonded to the C—Br carbon atom are replaced with larger alkyl groups. Theoretical calculations and gas phase studies have provided support for this explanation. It is notable that neopentyl bromide is less reactive than... 

The kinetics of the Menshutkin reaction of pyridine with benzyl bromide in methanol, acetone, and acetonitrile have been studied. " Second-order rate constants for the reactions of ethyl a-haloacetate and triethylamine have been determined in aprotic dipolar ketonic and halogenated solvents, nitrogen-containing solvents, and alcoholic solvents at 30 °C. Simple and multiple regression on various solvent parameters was applied to the results. The same group made a similar study of the reactions of ethyl a-haloacetate with triethylammonium carboxylates in a wide range of protic and aprotic solvents. The kinetics of alkylation of sodium acetate by butyl bromide have been investigated in dipolar non-hydroxylic solvents. Hexamethylphosphoramide is the best solvent for this reaction. 

Table III. 10-Lithio- Second-Order Rate Constants for Reaction of 9-Alkyl- 9.10-Dihvdroanthracene with Alkvl Bromides ...

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