Solvolysis of benzylic halides

While we can make substitutions on the aromatic ring of the nucleophile or the leaving group for methyl transfers, it is clearly impossible to do the same for the transferring group. The solvolysis of benzyl halides (Bennett and Jones, 1935) gives curved Hammett plots. The interaction of the aromatic ring with the reaction centre makes this a more complicated reaction. Hence we will not consider these reactions in detail. Thus the simplest system we can consider is... 

It is generally agreed that the mechanism of the solvolysis of benzyl halides lies near the region which marks the transition from Sjfl to Sjf2 solvolysis. Considerations of the kinetic chlorine isotope effect have recently led to the conclusion that even 4-nitrobenzyl chloride undergoes Sul solvolysis (Hhl and Pry, 1962) but an examination of the data suggests that this effect does not represent a sensitive test of solvolytic mechanism (Kohnstam, 1967). On the other hand, the values oi AC, AC I AS, and AS show that the solvolysis of the parent compoimd has the characteristic featvues of an 8 2 reaction (Tables 5, 6), and other evidence also supports this conclusion (see Bensley and Kohnstam, 1957). 

The solvolysis of isopropyl halides is generally considered to occur near the mechanistic border-line which marks the beginning of entirely Sul reaction, like the solvolysis of benzyl halides. Results relevant to the present discussion are available only for reaction with water and have been included with those for other Su2 processes in Table 6 since AC is less negative than would have been expected for entirely Sul solvolysis no complicating features which might reverse this conclusion have been envisaged. 

TABLE 9.4 Relative Rates of Solvolysis of Benzylic Halides Substrate MeCI PhCH CI Ph CHCI Ph3CCI... 

Isopropyl chloride and bromide show values of JjS which are intermediate between those found for the corresponding primary and tertiary compounds, unlike benzyl halides whose solvolytic behaviour is close to that of n-alkyl halides (see Tables 5, 6). This suggests that the solvolysis of isopropyl halides occurs by a process which is not uni-molecular but nevertheless shows more of the features of reaction by... 

Reminiscent of the effects encountered in the corresponding allylic systems (Section 14-3), benzylic resonance can affect strongly the reactivity of benzylic halides and sulfonates in nucleophilic displacements. For example, the 4-methylbenzenesulfonate (tosylate) of 4-methoxyphenylmethanol (4-methoxybenzyl alcohol) reacts with solvent ethanol rapidly via an SnI mechanism. This reaction is an example of solvolysis, specifically ethanolysis, which we described in Chapter 7. 

Despite our failure to find any supporting spectral evidence, the suspected presence of a-bromo-o-xylene and the absence of o-methyl-benzyl acetate in the oxidation products from o-xylene suggest a solvolysis rate for this benzylic halide lower than for the isomeric methylbenzyl bromides. 

The unimolecular solvolysis of a-(pentamethyldisilanyl)benzyl halides provides evidence for stabilization of positive charge by hyperconjugation with a... 

Here the transition state, for which the degree of charge separation is estimated to be 0.7 [15], is well on the way to the fully ionised species in which the halide ion is strongly stabilised by hydrogen bonding and the cation is stabilised to some extent by coordination to the lone pair of the solvent. More recent studies by Creary [16] emphasise the dramatic effect of solvent on the rates of ionisation processes. He studied the solvolysis of a benzylic mesylate, known to occur via an ionisation mechanism (equation 12.7). 

Polystyrene-bound allylic or benzylic alcohols react smoothly with hydrogen chloride or hydrogen bromide to yield the corresponding halides. The more stable the intermediate carbocation, the more easily the solvolysis will proceed. Alternatively, thionyl chloride can be used to convert benzyl alcohols into chlorides [7,25,26]. A milder alternative for preparing bromides or iodides, which is also suitable for non-benzylic alcohols, is the treatment of alcohols with phosphines and halogens or the preformed adducts thereof (Table 6.2, Experimental Procedure 6.1 [27-31]). Benzhy-dryl and trityl alcohols bound to cross-linked or non-cross-linked polystyrene are particularly prone to solvolysis, and can be converted into the corresponding chlorides by treatment with acetyl chloride in toluene or similar solvents (Table 6.2 [32-35]). 

Aliphatic alcohols do not undergo solvolysis as readily as benzylic alcohols, and are generally converted into halides under basic reaction conditions via an intermediate sulfonate. Because of the hydrophobicity of polystyrene, however, nucleophilic substitutions with halides on this support do not always proceed as readily as in solution (Table 6.3). Alternatively, phosphorus-based reagents can also be used to convert aliphatic alcohols into halides. 

Nucleophilic substitution reactions that occur under conditions of amine deamination often differ significantly in stereochemistry, compared with that seen in halide or arenesulfonate solvolysis. The results of four key substrates are summarized in Table 5.13. It can be seen (entry 1) that displacement of nitrogen on the 1-butyldiazonium ion is much less stereospecific than the 100% inversion observed on acetolysis of the corresponding brosylate. Similarly, the secondary system (entry 2) affords 2-butyl acetate with only 28% inversion of configuration. Furthermore, a crossover to net retention of configuration is observed as the alkyl group becomes better able to stabilize a carbonium ion. The small net retention (10%) observed in deamination of 1-phenylethylamine increases to 28% retention in the tertiary benzylic system 2-phenyl-2-butylamine. 

MeO , OH , or EtO and methyl fluoride, anisole, and 4-fluoroanisole on the gas-phase 5 2 reactions between dimethylmethylphosphonate and methylformate and HOO versus HO , MeO , or EtO in an attempt to discover the origin of the O -effect on the Sf 2 reaction of carbanions with 4-substituted benzyl chlorides in liquid ammonia " on the solvolysis reaction and the 2 reaction between phenoxide ions, and both neutral and negatively charged amines and 4-substituted benzyl chlorides in liquid ammonia on the ionization rates (the step) of the 5" 1 reactions of many substituted trityl halides and carboxylates in aqueous acetone and in aqueous and pure acetonitrile in the presence of piperidine on the ionization rates ( i) of the 5 reactions of various diarylmethyl chlorides in the presence of piperidine, pyridine, or PPh3 in several dipolar aprotic solvents on the solvolyses of X,Y-substituted benzhydryl acetates in various aqueous MeOH and EtOH solutions and on the dispersions observed in Grunwald-Winstein correlations of 5 solvolyses of substrates with substituents containing adjacent tt-electrons. ... 

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