Methyl bromide, solvolysis

The real world of Sn reactions is not quite as simple as the discussion has so far suggested. The preceding treatment in terms of two clearly distinct mechanisms, SnI and Sn2, implies that all substitution reactions will follow one or the other of these mechanisms. This is an oversimplification. The strength of the dual mechanism hypothesis and its limitations are revealed by these relative rates of solvolysis of alkyl bromides in 80% ethanol methyl bromide, 2.51 ethyl bromide, 1.00 isopropyl bromide, 1.70 /er/-butyl bromide, 8600. Addition of lyate ions increases the rate for the methyl, ethyl, and isopropyl bromides, whereas the tert-butyl bromide solvolysis rate is unchanged. The reaction with lyate ions is overall second-order for methyl and ethyl, first-order for tert-butyl, and first- or second-order for the isopropyl member, depending upon the concentrations. Similar results are found in other solvents. These data show that the methyl and ethyl bromides solvolyze by the Sn2 mechanism, and tert-butyl bromide by the SnI mech-... 

As a result of the inductive and hyperconjugative effects it is to be expected that tertiary carbonium ions will be more stable than secondary carbonium ions, which in turn will be more stable than primary ions. The stabilization of the corresponding transition states for ionization should be in the same order, since the transition state will somewhat resemble the ion. Thus the first order rate constant for the solvolysis of tert-buty bromide in alkaline 80% aqueous ethanol at 55° is about 4000 times that of isopropyl bromide, while for ethyl and methyl bromides the first order contribution to the hydrolysis rate is imperceptible against the contribution from the bimolecular hydrolysis.217 Formic acid is such a good ionizing solvent that even primary alkyl bromides hydrolyze at a rate nearly independent of water concentration. The relative rates at 100° are tertiary butyl, 108 isopropyl, 44.7 ethyl, 1.71 and methyl, 1.00.218>212 One a-phenyl substituent is about as effective in accelerating the ionization as two a-alkyl groups.212 Thus the reactions of benzyl compounds, like those of secondary alkyl compounds, are of borderline mechanism, while benzhydryl compounds react by the unimolecular ionization mechanism. 

A new Y solvolysis scale has been developed for benzylic species with extensive charge delocalization, based upon the solvolyses of some benzhydryl bromides and /-butyl(2-naphthyl)methyl bromides.39 Chlorides have negative salt effects on the ionization of benzhydryl bromide in 7-butyrolactone.40 The X-ray structure of the dimerization product of l,8-bis(dhnethylammonio)-4-naphthyl(phenyl)methyl carbocation has been determined it appears to be formed via a 4n + 2n-cycloaddition mechanism 41... 

Earlier work (la) on rates of SN2 reactions in a variety of solvents in the literature abounds. In hydroxylic solvents, some classic and valuable data are available on various nucleophiles with methyl bromide, used as a basis for measuring nucleophilic character (lb). Another valuable and relevant study compares dipolar aprotic solvents with hydroxylic solvents (2). Finally, numerous systematic studies (la, 3a-3d) of solvolysis reactions have been made. 

Antimony.— The kinetics of ligand exchange reactions in a number of antimony(m) complexes have been measured. The elimination of methyl bromide from [PhaSbMej+Br" in acetonitrile has been studied, and after allowance for salt effects and for ion-pairing the results fit second-order kinetics. The solvolysis of PhiSbAr (Ar = p-tolyl, /n-chlorophenyl- or /i-anisyl) in octan-2-ol occurs by parallel iSNl(Sb) mechanisms, with the formation of benzene and ArH. The presence of the alkoxide ion of octan-2-ol causes a marked change in the product ratio, and a bimolecular process may then be involved. 

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. 

Cycloaddition of phosphorus(III) halides to 1,3-dienes (such as 1,3-butadiene, 1,3-pentadiene, and 2-methyl-1,3-butadiene) followed by solvolysis is known to produce cyclic unsaturated phosphorus heterocycles, i.e., 2- and 3-phospholene 1-oxide derivatives 94 and 95, respectively, depending on the chloride and bromide of the phosphorus halides (Fig. 6) [34-36]. 

On the other hand, when the nucleophile reacts with the carbenium ion after it has separated from the leaving group, the reaction takes place with complete racemization. This is the case with more stable and consequently longer-hved carbenium ions. For example, the a-methyl benzyl cation, which is produced in the rate-determining step of the solvolysis of R-phenethyl bromide in a water/ethanol mixture, is such a cation (Figure 2.14). As in the solvolysis of Figure 2.13, the nucleophile is a water/ethanol mixture. 

When ferf-butyl bromide is placed in boiling methanol, methyl ferf-butyl ether can be isolated from the reaction mixture. Because this reaction takes place with the solvent acting as the nucleophile, it is called a solvolysis (solvo for solvent, plus lysis, meaning cleavage ). 

Whistler, R L, van Es, T, Solvolysis of methyl 5-thio-D-xylopyranosides and 2,3,4-tri-0-acetyl-5-thio-a-D-xylopyranosyl bromide, J. Org. Chem., 28, 2303-2304, 1963. 

The reactivity of halides is increased by coordination with Lewis acids. For example, silver ion accelerates solvolysis of methyl and ethyl bromide in 80 20 ethanol water by more than 10 In Section 4.4.1, we will see that the powerful Lewis acids SbFj and SbCl5 also assist in the ionization of halides. 

An S l reaction is illustrated by the solvolysis reaction of 2-bromo-2-methylpropane (fert-butyl bromide) in methanol to form 2-methoxy-2-methylpropane terthutyl methyl ether). You may notice that the second step of the mechanism is identical to the second step of the mechanism for the addition of hydrogen hahdes (H—X) to alkenes (Section 5.3A) and the acid-catalyzed hydration of alkenes (Section 5.3B). 

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