Alkyl bromides kinetics

One first selects a chain reaction that can be initiated photochemically and that is terminated by the recombination and disproportionation of interest. An example would be the tin hydride reduction of an alkyl bromide, which proceeds according to Scheme 5. Kinetic analysis (see p. 493) yields a relation between rate... 

The EPR spectra of the NHC boryl radicals that were generated through HAT to the ferf-butoxyl radical clearly show the delocalized 7i-type nature of these intermediates postulated to be essential by calculations [10, 12]. It was also demonstrated that the decay of the EPR signals could be fitted to a second-order decay having 2kt = 9 x 106 M-1 s-1. In agreement with this kinetic analysis, the NHC boryl radicals ultimately dimerize to give bis-NHC diborane derivatives. With the aid of EPR spectroscopy it was also established that the NHC boryl radicals readily abstract bromine atoms from primary, secondary, and tertiary alkyl bromides. However, chlorine atom abstraction is much slower and useful only for benzyl chloride. 

Giese and coworkers determined on the basis of experimental results obtained by Scheffold et al. [268, 299] that addition reactions of alkyl bromides 249 to a,p-unsaturated nitriles or esters 248 catalyzed by cobalamine 247 are free radical reactions (Fig. 61) [300], This conclusion was based on the similar cis/trans-selectivities in addition reactions of the 4-tert-butylcyclohexyl radical to different electron-poor alkenes 248 using 247 as a catalyst on one hand and classical tributyltin hydride conditions on the other. The kinetics of the radical addition was determined. 

It is a known fact that the gas-phase pyrolysis kinetics of alkyl bromides have not been extensively investigated due to the experimental difficulties as well as to the complexity of concurrent unimolecular and radical chain mechanisms. However, when these organic bromides are pyrolyzed under maximum inhibition, the reaction in the presence of a free radical suppressor is a molecular elimination. Sometimes, these organic bromides are pyrolyzed under maximum catalysis with HBr gas, and the process may proceed by an autocatalytic molecular mechanism. 

Kinetic data for the HBr elimination of secondary alkyl bromide, i.e., a-substituted ethyl bromide, in the gas phase are shown in Table 11121. Contrary to primary -substituted ethyl bromides, the rate constants for these secondary halides could not be correlated by the use of the Taft equation. This limitation arised because the corresponding olefin products underwent rapid isomerization with HBr gas acting as a catalyst. The lack of a kinetic control prevented evaluation of the factor by which the Z substituent in ZCH(Br)CH3 affected the direction of elimination. However, as the electron-releasing effect of Z increases (Table 11), a small but significant increase in the overall rate constant was obtained. In view of the catalysis by HBr in the isomerization process of the olefin products, a general mechanism for this process was suggested (equation 40). 

Hughes, E. D. Ingold, C. K. Mackie, J. D. H. Mechanism of substitution at a saturated carbon atom. XLHI. Kinetics of the interaction of chloride ions with simple alkyl bromides in acetone, J. Chem. Soc. 1955, 3173-3177. 

The rate of reaction shows first order dependence on the concentration of iron and ethyl bromide, but is independent of the concentration of ethylmagnesium bromide. The rate, however, varies with the structure of the Grignard reagent, and disproportionation usually results except when the alkyl group is methyl, neopentyl or benzyl which possess no g-hydrogens. The reactivities of the alkyl bromides (t-butyl >i-propyl >n-propyl) as well as the kinetics are the same as the silver-catalyzed coupling described above and suggest a similar mechanism ... 

Alkeuyl-3-methyl-2-eyclohexene-t-ones. The kinetic enolate of 3-methyl-3-cyclohexene-l-one is alkylated only by reactive halides such as allyl bromide. An alternate route to compounds of type 2 starts with the 2-diethy1aminoethyl ether of 2,5-dihydro-m-cresol (1), obtained by Birch reduction. (The corresponding methyl ether is not readily metalated.) n-Butyllithium-HMPT in THF can metalate 1. The resulting carbanion is alkylated by a variety of alkyl bromides to give, after acid hydrolysis, cyclohexenones 2 in yields of 80-90%. ... 

Alkylation of phthalimide anion can be carried out under solid-liquid phase-transfer conditions, using phosphonium salts or ammonium salts. In the reaction systems using hexadecyltributylphosphonium bromide, alkyl bromides and alkyl methanesulfonate are more reactive than alkyl chlorides. Octyl iodide is less reactive than the corresponding bromide and chloride. ( )-2-Octyl methanesulfonate was converted into (S)-2-octylamine with 92.5% inversion. Kinetic resolution of racemic ethyl 2-bromopro-pionate by the use of a chiral quaternary ammonium salt catalyst has been reported. Under liquid-liquid phase-transfer conditions, A -alkylation of phthalimide has been reported to give poor results. ... 

Many E2 reactions fit into this category. A simple base-catalysed elimination of an alkyl bromide will normally give the E-alkene because there are two diastereotopic hydrogen atoms which could be lost and the one which gives the E-alkene is lost more quickly because that transition state has the lower energy. This is a kinetic effect on a reaction in which the transition state resembles the product. 

R = bornyl) and aldehydes R CHO has been reported " the reaction of the lithium enolate of the optically active oxazolidinone (386) with alkyl bromides or aldehydes likewise proceeds with high kinetic diastereo-selectivity. ... 

Mark and Rechnitz [3] systematized a vast amount of experimental material that can be used directly in KGCM. Some data are presented here that show the wide differences in organic compounds with regard to their kinetic characteristics. Table 2.1 [14] gives the relative rates of reaction of olefins with perbenzoic acid and Table 2.2 summarizes the rates of the etherification reaction of carboxylic acids with diphenyldiazomethane [15]. The tabulated data are indicative of large differences in organic compounds as far as their reactivity is concerned. The rates of reaction of some isomers differ so widely that one can, for example, analyse secondary and tertiary alkyl bromides in the presence of primary alkyl bromides in a reaction with silver nitrate [16]. It is possible to differentiate between CIS and trans isomers of 1,3-dienes by their reaction with dienophils (e.g., chloromethylene anhydride) because the cis isomer reacts much more slowly than the trans isomer [17]. 

The normal course of a kinetic investigation involves the posmlation of likely mechanisms and comparison of the observed rate expression with those expected for the various mechanisms. Those mechanisms that are incompatible with the observed kinetics can be eliminated as possibilities. One common kind of reaction involves proton transfer occurring as a rapid equilibrium preceding the rate-determining step, for example, in the reaction of an alcohol with hydrobromic acid to give an alkyl bromide ... 

The three different second-order processes thus exhibit widely different kinetic behaviour towards the varying base concentration at constant buffer ratio. In theory this dependence should provide a means of assigning the mechanism. An advantage over the isotopic exchange approach is that it should be possible to detect carbanion intermediates that eliminate more rapidly than they protonate. Unfortunately, the kinetics are not always clear-cut. The E2 mechanism can, under certain conditions, follow specific base catalysis, especially if one base is of much greater catalytic efficiency than the other bases present (e.g. the E2 reaction of l,l,l-trichloro-2,2-di-p-chlorophenyl-ethane with sodium thiophenoxide in methanol) . Alternatively, the base may be sufficiently powerful to produce a kinetically significant concentration of lyate ions (e.g. the E2 reaction of alkyl bromides with phenoxides in ethanol) " . 

Kinetics studies of the oxidative addition of a variety of unactivated primary alkyl electrophiles to several PdL2 complexes (L=trialkylphosphine) have been described [46]. For the reaction depicted in Eq. 17, the activation parameters are AU =20.8 kcal mol" (20 °C) AH =2A kcal mol" AS =-63 eu. The large negative AS" value that is observed is consistent with an associative pathway for oxidative addition. The rate of addition increases as the solvent polarity increases, as would be expected for the postulated 8 2 mechanism, and is not affected by added P(f-Bu)2Me, indicating that the alkyl bromide oxidatively adds to PdL2 (not PdLi or PdL3) [47]. 

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... 

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