Substituent effects benzyl bromide

The rates of bromine atom abstraction by tris(trimethylsilyl)silyl radicals from a range of /Jara-substitutcd benzyl bromides has indicated that the silyl radical is nucleophilic. In addition both the polar and spin-delocalization effects of the substituents play a role in the abstraction reaction with the latter effect greater than for H-atom abstractions.166 The perfluoroalkylation of aromatics and alkenes has been investigated using C4F9I as the source of C,. Measurement of rate constants indicated that perfluoroalkyl radicals were 2-3 orders of magnitude more reactive than the corresponding alkyl radicals. This was attributed primarily to the reaction enthalpy and far less to the electrophilic nature of the radicals.167... 

The intriguing combination of an a-silyl and a /1-silyl substituent in the disilanyl group (—SiR2SiR3) prompted Shimizu, Tsuno and coworkers to study the effect of a /3-Si—Si bond on the stability of carbocations86. They studied the solvolysis of the benzylic bromides 213-215 and found that the /i-SiMe3 substituent in 213 increases the solvolysis rate by a factor of 1.07 x 105 compared with the f-butyl-substituted reference compound 214 (Table 7). 

Substituent effects have also been reported28 for the iodinolysis of substituted a-carbethoxybenzylmercuric bromides by iodine in the presence of cadmium iodide, using solvent toluene plus 1.5 vol. % methanol. Details are given in Table 6. It may be seen from Tables 5 and 6 that substituent effects in the a-carbethoxy-benzyl compounds are rather different from those in the simple benzylmercuric chlorides. The mechanism of iodinolysis of the a-carbethoxybenzylmercuric bromides was suggested28 to be in the boundary region between SE1 and SE2. 

Substituent effects have been reported previously and work on this topic continues. The half wave potentials of a series of substituted benzyl chlorides and bromides gave excellent correlations with Hammett o- substituent constants48. The positive p values from these Hammett LFERs (p = 5.0 and 2.8, respectively for chlorides and bromides) suggest that the potential-determining electrochemical process involves the formation of radical anion intermediates. 

The bimolecular process of the Menschutkin reaction of benzyl [21] bromides with dimethylanilines in methanol also shows a similar concave correlation of the substituent effect to that described above. 

For styrene-based random copolymers, functional groups can be introduced into the polymer chains via copolymerization with functional styrene derivatives, because the electronic effects of the substituents are small in the metal-catalyzed polymerizations in comparison to the ionic counterparts. Random copolymer R-6 is of this category, synthesized from styrene and />acetoxystyrene.372 It can be transformed into styrene// -vinylphenol copolymers by hydrolysis.380 The benzyl acetate and the benzyl ether groups randomly distributed in R-7 and R-8 were transformed into benzyl bromide, which can initiate the controlled radical polymerizations of styrene in the presence of copper catalysts to give graft copolymers.209 Epoxy groups can be introduced, as in R-9, by the copper-catalyzed copolymerizations without loss of epoxy functions, while the nitroxide-mediated systems suffer from side reactions due to the high-temperature reaction.317... 

Neopentyl (2,2-dimethylpropyl) systems are resistant to nucleophilic substitution reactions. They are primary and do not form carbocation intermediates, but the /-butyl substituent effectively hinders back-side attack. The rate of reaction of neopentyl bromide with iodide ion is 470 times slower than that of n-butyl bromide. Usually, the neopentyl system reacts with rearrangement to the /-pentyl system, although use of good nucleophiles in polar aprotic solvents permits direct displacement to occur. Entry 2 shows that such a reaction with azide ion as the nucleophile proceeds with complete inversion of configuration. The primary benzyl system in entry 3 exhibits high, but not complete, inversion. This is attributed to racemization of the reactant by ionization and internal return. 

Analysis of the structure-reactivity cross-correlations shows the existence of isoparametric relationships in the reactions of Y-substituted benzyl bromides with X-substituted anilines in dioxane and in its mixtures with DMSO at 40 The kinetics of hydrolysis of 1-arylethyl ethers of salicyclic acid, catalysed intramolecularly by 0-CO2H, have been studied. Analysis of substituent effects in both arylethyl and leaving groups provides the most detailed available mechanistic insight into a reaction involving efficient intramolecular proton-transfer catalysis. The mechanism is very different from classical general acid-base catalysis. ... 

The quaternization of pyridine in non-polar solvents has been studied as a means of clarifying the much debated nature of the displacement reaction under such circumstances . Swain and Eddy i deduced evidence for their theory of specific solvation from the reaction of pyridine and methyl bromide in benzene containing various hydroxylic solutes. Swain and Langs-dorf45ii> found the Hammett plot for the reaction of substituted benzyl bromides with pyridine in acetone to be markedly concave, and indeed to fall into two separate lines for meta- and / zm-substituents. The curvature and the division illustrate the effects of substituents upon reactions of intermediate character. Ingold and his co-workers from reactions in sulphur dioxide,... 

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