Solvolysis reaction

The earliest report of the solvolysis of a halocyclopropane to an allyl alcohol was presented by Roberts and Chambers in 1951.10 These early examples did not utilize silver to assist in halide departure. Instead, the halocyclopropane was simply dissolved in solvent and allowed to react at elevated temperatures. For example, in 

In an effort to overcome this issue, the authors decided to approach the synthesis of 15 in a different manner, inducing solvolysis of the gcw-dibromocyclopropane with silver acetate in the presence of acetic acid to furnish an allylic acetate. The acetate 

From the results of a kinetic study of a series of X-phenyl trifluoroacetates (11 X = 4-Me, H, 4-F, 4-Cl, 3-Cl), it was concluded that, as is the case for aryl acetates and formates, aryl trifluoroacetates are hydrolysed by a concerted mechanism. Another 

Products of attack by OH radicals rather than hydrolysis by supercrihcal water occur when 4-nitrophenyl acetate is sonolysed in argon-saturated water. Hydrolyses of substituted benzoic acid esters in near-critical water (250-300 °C) show autocatalytic kinetic behaviour and surprisingly give the same rate constant regardless of substituent, suggesting that an acid-catalysed mechanism predominates.  

Rate coefficients have been measured for the alkaline hydrolysis of methyl 2-trichloro- and 2-tribromo-acetylbenzoate (18 X = Cl, Br), and also the alkaline 

In studies in aqueous dioxane, kinetic evidence has been obtained for an oxydian-ionic tetrahedral intermediate in the alkaline hydrolysis of methyl salicylate (20).  

A study of imidazolysis of 4-nitrophenyl acetate in water, acetonitrile, propylene carbonate, and dioxane at different temperatures concluded that two competitive pathways operate, the bimolecular nucleophilic substitution of the phenol moiety by imidazole and general-base catalysis by a second imidazole molecule.  

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The Lewis base that acts as the nucleophile often is but need not always be an anion Neutral Lewis bases can also serve as nucleophiles Common examples of substitutions involving neutral nucleophiles include solvolysis reactions Solvolysis reactions are substitutions m which the nucleophile is the solvent m which the reaction is carried out 8olvolysis m water (hydrolysis) converts an alkyl halide to an alcohol... 

When applied to the synthesis of ethers the reaction is effective only with primary alcohols Elimination to form alkenes predominates with secondary and tertiary alcohols Diethyl ether is prepared on an industrial scale by heating ethanol with sulfuric acid at 140°C At higher temperatures elimination predominates and ethylene is the major product A mechanism for the formation of diethyl ether is outlined m Figure 15 3 The individual steps of this mechanism are analogous to those seen earlier Nucleophilic attack on a protonated alcohol was encountered m the reaction of primary alcohols with hydrogen halides (Section 4 12) and the nucleophilic properties of alcohols were dis cussed m the context of solvolysis reactions (Section 8 7) Both the first and the last steps are proton transfer reactions between oxygens... 

Solvolysis reaction (Section 8 7) Nucleophilic substitution m a medium m which the only nucleophiles present are the solvent and its conjugate base... 

A scale for solvent ionizing power, K+, applicable in solvolysis reactions of cationic substrates, has been developed. For example,... 

Winstein suggested that two intermediates preceding the dissociated caibocation were required to reconcile data on kinetics, salt effects, and stereochemistry of solvolysis reactions. The process of ionization initially generates a caibocation and counterion in proximity to each other. This species is called an intimate ion pair (or contact ion pair). This species can proceed to a solvent-separated ion pair, in which one or more solvent molecules have inserted between the caibocation and the leaving group but in which the ions have not diffused apart. The free caibocation is formed by diffusion away from the anion, which is called dissociation. 

Stabilization of a carbocation intermediate by benzylic conjugation, as in the 1-phenylethyl system shown in entry 8, leads to substitution with diminished stereosped-ficity. A thorough analysis of stereochemical, kinetic, and isotope effect data on solvolysis reactions of 1-phenylethyl chloride has been carried out. The system has been analyzed in terms of the fate of the intimate ion-pair and solvent-separated ion-pair intermediates. From this analysis, it has been estimated that for every 100 molecules of 1-phenylethyl chloride that undergo ionization to an intimate ion pair (in trifluoroethanol), 80 return to starting material of retained configuration, 7 return to inverted starting material, and 13 go on to the solvent-separated ion pair. 

Participation of carbon-carbon double bonds in solvolysis reactions is revealed in some cases by isolation of products with new carbon-carbon [Pg.313]

Many other cations besides the norbomyl cation have nonclassical structures. Scheme 5.5 shows some examples which have been characterized by structural studies or by evidence derived from solvolysis reactions. To assist in interpretation of the nonclassical stmctures, the bond representing the bridging electron pair is darkened in a corresponding classical stmcture. Not surprisingly, the borderline between classical stmctures and nonclassical stmctures is blurred. There are two fundamental factors... 

The Yukawa-Tsuno r values have been measured for the solvolysis reactions fonning benzyl cations and several a-substituted derivatives, 6-3IG charges and bond orders have been calculated for the presumed cationic intermediates. Analyze the data for relationships between r and the structural parameters. (Hint. Plot r versus the bond orders and the charges at C-1, C-2, C-3, and C-4.)... 

AC is interpreted as the difference in heat capacities between the transition state and the reactants, and it may be a valuable mechanistic tool. Most reported ACp values are for reactions of neutral reactants to products, as in solvolysis reactions of neutral esters or aliphatic halides. " Because of the slight curvature seen in the Arrhenius plots, as exemplified by Fig. 6-2, the interpretation, and even the existence, of AC is a matter of debate. The subject is rather specialized, so we will not explore it deeply, but will outline methods for the estimation of ACp. 

If (A i[X ]/A 2[Y ]) is not much smaller than unity, then as the substitution reaction proceeds, the increase in [X ] will increase the denominator of Eq. (8-65), slowing the reaction and causing deviation from simple first-order kinetics. This mass-law or common-ion effect is characteristic of an S l process, although, as already seen, it is not a necessary condition. The common-ion effect (also called external return) occurs only with the common ion and must be distinguished from a general kinetic salt effect, which will operate with any ion. An example is provided by the hydrolysis of triphenylmethyl chloride (trityl chloride) the addition of 0.01 M NaCl decreased the rate by fourfold. The solvolysis rate of diphenylmethyl chloride in 80% aqueous acetone was decreased by LiCl but increased by LiBr. ° The 5 2 mechanism will also yield first-order kinetics in a solvolysis reaction, but it should not be susceptible to a common-ion rate inhibition. 

Table 8-14. Regression Parameters for Tosylate Solvolysis Reactions"...

It is important to note that the one-step conversion of 27 to 28 (Scheme 4) not only facilitates purification, but also allows differentiation of the two carbonyl groups. After hydrogenolysis of the iV-benzyl group (see 28—>29), solvolysis of the -lactone-ring in 29 with benzyl alcohol and a catalytic amount of acetic acid at 70 °C provides a 3 1 equilibrium mixture of acyclic ester 30 and starting lactone 29. Compound 30 can be obtained in pure form simply by washing the solid mixture with isopropanol the material in the filtrate can be resubjected to the solvolysis reaction. 

The principal solvolysis reactions for PET are methanolysis with dimethyl terephthalate and ethylene glycol as products, glycolysis with a mixture of polyols and BHET as products, and hydrolysis to form terephthalic acid and ethylene glycol. The preferred route is methanolysis because the DMT is easily purified by distillation for subsequent repolymerization. However, because PET bottles are copolyesters, the products of the methanolysis of postconsumer PET are often a mixture of glycols, alcohols, and phthalate derivatives. The separation and purification of the various products make methanolysis a cosdy process. In addition to the major product DMT, methanol, ethylene glycol, diethylene glycol, and 1,4-cyclohexane dimethanol have to be recovered to make the process economical.1... 

Important solvolysis reactions for nylons are hydrolysis, methanolysis, glycolysis, aminolysis, ammonolysis, transamidation, and acidolysis.17 Hydrolysis of nylon-6 with steam in the presence of an acid catalyst to form caprolactam is tlie preferred depolymerization approach. However, when recycling carpet face fibers, file fillers in the polymer may react with file acid catalyst and lower the efficiency of the catalyst. 

Nylon-6,6 monomers, recycling, 544 Nylon-6,6 products, depolymerized, 533 Nylons, 19, 135. See also Nylon PA entries Polyamides (PAs) depolymerization of, 566-571 solvolysis reactions for, 535... 

Additional evidence for the SnI mechanism, in particular, for the intermediacy of carbocations, is that solvolysis rates of alkyl chlorides in ethanol parallel carbocation stabilities as determined by heats of ionization mea.sured in superacid solutions (p. 219). It is important to note that some solvolysis reactions proceed by an Sn2 mechanism." ... 

A complete picture of the possibilities for solvolysis reactions in a solvent SH (ignoring the possibilities of elimination or rearrangement, see Chapters 17 and 18) is the following, though in any particular case it is unlikely that all these reactions... 

We have previously discussed the possibilities of racemization or inversion of the product RS of a solvolysis reaction. However, the formation of an ion pair followed by internal return can also affect the stereochemistry of the substrate molecule RX. Cases have been found where internal return racemizes an original optically active RX, an example being solvolysis in aqueous acetone of a-p-anisylethyl p-nitrobenzoate, while in other cases partial or complete retention is found, for example, solvolysis in aqueous acetone of p-chloro benzhydryl p-nitrobenzoate. the pathway RX R+X some cases where internal return involves racemization, it has been shown that such racemization is faster than solvolysis. For example, optically active p-chlorobenzhydryl chloride racemizes 30 times faster than it solvolyzes in acetic acid. ... 

For ion-molecule pairs in other solvolysis reactions, see Thibblin, A. J. Chem. Soc., Perkin Trans. 2, 1987, 1629. 

Essentially similar results and conclusions were obtained by Peterson and Indelicato (158) in the solvolysis of the corresponding tosylates and brosylates, 171 b (R = p-CHj Cfi H4 or p-BrCg H4 ) and 171 c (R = p-CHj 5 H4, p-BrCg H4 ), in 50% aqueous methanol at 130°. In this case, the trans isomer was found to react at a rate 10 times that of the cis isomer. Furthermore, the trans isomer gave 95% 2-butyne and 5% 2-butanone, whereas the cis isomer gave 72% 2-butyne and 28% 2-butanone as products. Also, as expected (vide supra) for a unimolecular solvolysis reaction, the cis brosylate reacts at a rate four times that of the corresponding tosylate. 

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 ... 

Acyclic phosphoranes, ArnP(OR)5 n with n = 0 - 3 have been shown to hydrolyse by an SN1(P) mechanism for n. = 1, 2 and 3 but by an S 2(P) or addition-elimination mechanism for n. = O23. This duality of mechanism is analogous to the classical S l vs S 2 mechanisms observed for solvolysis reactions at tetrahedral carbon. 

In this type of reaction the active drug undergoes decomposition following reaction with the solvent present. Usually the solvent is water, but sometimes the reaction may involve pharmaceutical cosolvents such as ethyl alcohol or polyethylene glycol. These solvents can act as nucleophiles, attacking the electropositive centers in drug molecules. The most common solvolysis reactions encountered in pharmaceuticals are those involving labile carbonyl compounds such as esters, lactones, and lactams (Table 1). 

The studies of Havinga and co-workers have resulted in the discovery of many solvolysis reactions which appear to occur via polar or heterolytic pathways 1361 ... 

Photochemical solvolysis reactions are directly analogous to ordinary solvolysis reactions except that the absorption of light energy is necessary for reaction to occur ... 

The chemical nature of the main-chain linkages of step-growth polymers makes this class of polymers particularly reactive to a wide variety of chemical species. Solvolysis reactions break the C-X bond at the polymer linkage bonds. These types of reactions are often pH-dependent, so the stability of the polymer is highly dependent on the acidity or basicity of the prodegradant. 

In a solvolysis reaction, attack on R by a solvent molecule, e.g. H20 , in (26) is likely to lead to inversion, as attack can take place (by the solvent envelope) on the back side of R , but not on the front side where there are no solvent molecules, and which is shielded by the Bre gegen ion. Attack in (27) is more likely to lead to attack from either side, leading to racemisation, while attack on (28) can clearly happen with equal facility from either side. Thus the longer the life of R , i.e. the longer it escapes nucleophilic attack, the greater the proportion of racemisation that we should expect to occur. The life of R is likely to be longer the more stable it is—(a) above—but the shorter the more powerfully nucleophilic the solvent—(b) above. 

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