Carbonium ions solvolysis reactions

Winstein Robinson (1958) used this concept to account for the kinetics of the salt effects on solvolysis reactions. They considered that carbonium ions (cations) and carbanions could exist as contact ion-pairs, solvated ion-pairs and as free ions and that all these forms participated in the reactions and were in equilibrium with each other. These equilibria can be represented, thus ... 

This synthetic approach involves rearrangement of the incipient carbonium ion derived from the readily available six-membered ring compounds. Acid catalyzed 15,16) and solvolysis 17) reactions of 17a and 17b, respectively, afforded dibenzo[6/]-thiepin (9) which was also obtained by reaction of thioxanthylium ion (18) with diazomethane 18,19). 

For (XX), L py, it is likely that the major reaction path involves initial skeletal isomerization to give (XXI) followed by rapid solvolysis of this isomer. The solvolysis of this isomer is strongly metal-assisted since the intermediate carbonium ion is stabilised by the metal-alkene resonance form as shown in the Scheme. The product is the 1-D2 isomer. Now, the skeletal isomerization of (XX) is expected to be retarded by free pyridine and cannot occur when L2 = 2,2 -bipyridyl C7). Hence under these conditions the reaction must occur by solvolysis of (XX) giving largely the 3-D2 isomer. However, the product formed under these conditions is still about 30% of the 1-D2 isomer (Table I). 

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. 

This reaction proceeds via the transition state illustrated in Fig. 10.2. An Sn2 reaction (second order nucleophilic substitution) in the rate limiting step involves the attack of the nucleophilic reagent on the rear of the (usually carbon) atom to which the leaving group is attached. The rate is thus proportional to both the concentration of nucleophile and substrate and is therefore second order. On the other hand, in an SnI reaction the rate limiting step ordinarily involves the first order formation of an active intermediate (a carbonium ion or partial carbonium ion, for example,) followed by a much more rapid conversion to product. A sampling of a and 3 2° deuterium isotope effects on some SnI and Sn2 solvolysis reactions (i.e. a reaction between the substrate and the solvent medium) is shown in Table 10.2. The... 

K, J. Morgan, and F. J. Chloupek Structural effects in solvolytic reactions. I. The role of equilibrating cations in carbonium ion chemistry. Nature of the intermediates involved in the solvolysis of symmetrically substituted 3-phcnylethyl derivatives. J. Amer. chem. Soc. 87, 2137 (1965). 

The relative rate data closely parallel the results obtained in the solvolysis studies. Such a result might be expected from reactions proceeding through similar transition states. The observed order of relative rates may result from better overlap as the size of the central metal atom and the polarizability of its electron shell increase. This would result in increased stabilization and therefore ease of formation of the carbonium ions, proceeding from lighter to heavier metal complexes. 

The detailed mechanism, or mechanisms, of the solvolysis of acid chlorides is still a matter of dispute. There are at least four possible mechanisms, (a)-(d) below, all of which have been proposed either to act separately or in various combinations, and there is a unified mechanism, that of Minato93 which will be discussed later. The bimolecular mechanisms (a) and (b) differ in that (a) includes a tetrahedral intermediate whereas (b) does not. The former is commonly accepted as the most likely for the bimolecular mechanism and the arguments against (b) have been stated in the introduction. There is, however, good evidence for (A), at least in the case of the hydrolysis of chloracetyl chloride94. The acylium ion mechanism (c) and the hydrated carbonium ion mechanism (d) are both unimolecular mechanisms. Whereas the acylium ion XXVII has never been directly observed in hydrolysis or alcoholysis reactions, it is favoured as an intermediate by many workers, although it is kinetically indistinguishable from XXVIII. 

Hydride shifts can take place directly, without the intervention of a carbonium ion intermediate, if the geometry of the system is favorable. For example, in the solvolysis of cyclohexyl-2,6-d2 tosylate in 97 percent acetic acid, 1,3-hydride shifts have been reported to account for 33 percent of the product.124 If this is so, it must be because the reaction is made facile by the proximity of the 3-axial hydrogen to the empty p orbital. 

The work of Swain, de la Mare, and Roberts indicated that caution was necessary because of the differences in the intermediates. However, the resonance-stabilized carbonium ion (12) formed in the rate-determining step of the solvolysis offered an attractive model for the benzenium ions (13) formed in the slow step of the substitution reaction. The preliminary correlation of rate data for solvolysis of substituted phenyl-dimethylcarbinyl chlorides and aromatic substitution reactions revealed... 

Since the solvolysis reactions of bridgehead substrates are mechanistically uncomplicated (i.e. competing elimination is highly unlikely187 ns-H9) an(j backside solvent participation is impossible), they provide ideal models for limiting carbonium ion behavior. Adamantyl derivatives have become the substrates of choice for this purpose due to their availability and convenient reactivity. 

Side-chain carbonium ion reactions Solvolysis 6. Of 1 -arylethyl-p-nitrobenzoates -6.0 1.0 x 10s 2.9 x 102 b... 

A direct comparison between 1-methylindole and 1-methylpyrrole is possible only for an a-carbonium ion reaction, the solvolysis of... 

The principal exception to this statement involves tertiary carbon, allylic carbon, and benzyl carbon atoms atl ached to X. In such cases it is often found that the second-order reaction with solvate ion such as RO (in ROH solvent) is much slower than apparent first-order reaction with solvent. These systems may be recognized as those giving quite stable carbonium ions, and the solvolysis has b( en ascribed to an Sifl mechanism. 

A comparison of rates of acetolysis in a series of cholestanyl tosylates [ii] revealed differences (Table 20) which were interpreted in terms of relief of steric strain as the geometry at the reaction site changes from tetrahedral towards the planar configuration of the carbonium ion. These results extend a concept developed earlier from studies of the solvolysis of 4-tert-butylcyclohexyl and other monocyclic tosylates with more or less rigid conformations [12]. In monocyclic systems the observed relative rates for axial (i) and equatorial tosylates (2) were explained in a serai-quantitative manner in terms of strain in the ground state of an axial tosylate, due to Van der Waals interactions between the axial oxygen atom... 

As already stated (p. 240), sr-bond participation does not occur in the solvolysis of e f-cholesteryl tosylate (i). Whatever the reaction conditions the major product is cholesta-3,5 diene, probably resulting from a normal raws-diaxial elimination with the 4/6-proton. Powerfully nucleophilic reagents like pyridine may also give a little of the g/S-substituted product by an Sn2 process [34], A curious side-reaction observed during buffered methanolysis [y ] or hydrolysis [y6] of efi cholesteryl tosylate has been interpreted as a rearrangement of the classical C(S) carbonium ion (2) by hydride migration from C(4), to give the resonance-stabilised ylic cation (3). The structure of this cation was revealed when the methanoly-... 

Some new examples of cyclosteroid formation by homo-allylic participation have appeared recently. Hydrolysis of the A -19-mesylate (4) proceeded with z -electron participation to give the 5j(5,i9-cyclo-6 -ol (5) [6sa] or the related A -olefin (6) [64] according to the reaction conditions. Further studies on these compounds [63,65,66,6 ]] revealed other complicated transformations which must proceed through carbonium ion intermediates. Recent work by Tadanier [66,6]] indicates that two distinct non-classical carbonium ions are involved. Buffered solvolysis of the A -ig-mesylate (4) gives the ion represented by resonance between the canonical structures (A), or by the non-classical structure (B). Stereoelectronic factors of the type discussed for i-steroids ensure 6jS-attachment of a nucleophile in forming the 5jS, 19-cyclosteroid (5). This appears to be the initial product of a kinetically-controlled process, for a further rearrangement occurs in weakly acidic... 

Solvolysis of (S)-2,6-dimethyloct-5-yl toluene-p-sulphonate gives a tetrahydro-linalool, (i )-2,6-dimethyloctan-6-ol, with about 60% retention of asymmetry. Kirmse and Arold have described several other similar reactions and suggest that a hydrophobic anchimeric interference of alkyl residues persists during the rearrangement, giving chirality to the carbonium ion. ... 

The same products, but in different ratios, occur in the gas phase eliminations of HCl from bornyl and isobornyl chlorides and from the pyrolysis reaction of bornyl and isobornyl benzoates and methyl xanthates . The isobornyl reaction is appreciably faster than that of the bornyl ester ((iso-B/B) = 6.8 at 345 °C), but proceeds at a slightly slower rate than that of cyclohexyl acetate, (CH/iso-B) = 2.1 at 600 °K. By analogy with the nonclassical carbonium ion interpretation of the solvolysis rates of bornyl and isobornyl chlorides, the participation of nonclassical carbonium ion intimate ion-pairs, e.g. 

There is no added strong nucleophile and so, for many compounds, solvolysis falls into the category we have called S l that is, reaction proceeds by two— or more—steps, with the intermediate formation of a carbonium ion. It is this intermediate that lies at the center of the problem its nature, how it is formed, and how it reacts. In studying solvolysis one is studying all SnI reactions and, in many ways, all reactions involving intermediate carbonium ions. 

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