And solvolysis

In addition to simple dissolution, ionic dissociation and solvolysis, two further classes of reaction are of pre-eminent importance in aqueous solution chemistry, namely acid-base reactions (p. 48) and oxidation-reduction reactions. In water, the oxygen atom is in its lowest oxidation state (—2). Standard reduction potentials (p. 435) of oxygen in acid and alkaline solution are listed in Table 14.10- and shown diagramatically in the scheme opposite. It is important to remember that if or OH appear in the electrode half-reaction, then the electrode potential will change markedly with the pH. Thus for the first reaction in Table 14.10 O2 -I-4H+ -I- 4e 2H2O, although E° = 1.229 V,... 

Experimentally, there are two approaches to the elucidation of the structure of vinyl cations first, preparation and solvolysis of systems where because of geometric restrictions the intermediate vinyl cation by necessity is bent and second, by a careful examination of the stereochemistry of solvolysis of appropriate acyclic substrates. 

The technical advantages of the chemical recycling of PETP bottles are discussed, and some developments in depolymerisation processes are examined. Particular attention is paid to glycolysis, hydrolysis and solvolysis processes respectively developed by TBl, Tredi and Eastman Chemical. 

Polymers with hetero-atoms in the chain are suitable for chemical recycling of waste materials. In addition to depolymerisation (nylon 6) and solvolysis (nylon 6,6, PETP, PU) the degradation of aliphatic polyamides with dicarboxylic acids, diamines and cyclic anhydrides, especially trimellitic anhydride, becomes more and more important. The utilisation of the obtained fragments is described. 

Scheme 12.15 gives some examples of both acid-catalyzed and nucleophilic ring openings of epoxides. Entries 1 and 2 are cases in which epoxidation and solvolysis are carried out without isolation of the epoxide. Both cases also illustrate the preference for anti stereochemistry. The regioselectivity in Entry 3 is indicative of dominant bond cleavage in the TS. The reaction in Entry 4 was studied in a number of solvents. The product results from net syn addition as a result of phenonium ion participation. The ds-epoxide also gives mainly the syn product, presumably via isomerization to the... 

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). 

Optically active 19 was prepared, and solvolysis products obtained in various solvents were examined. The rearranged product 23 was always the main product and the chirality of the substrate 19 was completely transferred to the product 23 in all solvents employed, ranging from ethanol to HFIP (l,l,l,3,3,3-hexafluoropropan-2-ol).12Typical results are shown in eq 8. 

Organoboranes react with ethyl 4-nitrobenzenesulphonyloxycarbamate under basic two-phase conditions in the presence of benzyltriethylammonium chloride or Aliquat to yield ethyl (V-alkylcarbamates [38]. The reaction probably proceeds via the initial formation of the nitrene, which reacts with the borane to form a B -N+ zwitterion. Subsequent rearrangement and solvolysis leads to the product. Aliquat is the better catalyst for the higher-molecular-weight boranes. 

Cram et al.ss> have studied the stereochemical course and solvolysis reactions of optically active l-tosyloxy[2.2]paracyclophane 91 (see scheme 91—95). Reactions 92-+91, 92- -93, 92- -94, and 92- -95 were all shown to proceed as expected, with complete retention of configuration. 

We have examined the competing isomerization and solvolysis reactions of 1-4-(methylphenyl)ethyl pentafluorobenzoate with two goals in mind (1) We wanted to use the increased sensitivity of modern analytical methods to extend oxygen-18 scrambling studies to mostly aqueous solutions, where we have obtained extensive data for nucleophilic substitution reactions of 1-phenylethyl derivatives. (2) We were interested in comparing the first-order rate constant for internal return of a carbocation-carboxylate anion pair with the corresponding second-order rate constant for the bimolecular combination of the same carbocation with a carboxylate anion, in order to examine the effect of aqueous solvation of free carboxylate anions on their reactivity toward addition to carbocations. 

Z)-awh-4-Hydroxy-l-aIkenyl carbamates 363, when subjected to substrate-directed, vanadyl-catalysed epoxidation , lead to diastereomerically pure epoxides of type 364 (equation 99)247,252,269 qqjggg epoxides are highly reactive in the presence of Lewis or Brpnsted acids to form -hydroxylactol ethers 366 in some cases the intermediate lactol carbamates 365 could be isolated . However, most epoxides 364 survive purification by silica gel chromatography . The asymmetric homoaldol reaction, coupled with directed epoxidation, and solvolysis rapidly leads to high stereochemical complexity. Some examples are collected in equation 99. The furanosides 368 and 370, readily available from (/f)-0-benzyl lactaldehyde via the corresponding enol carbamates 367 and 369, respectively, have been employed in a short synthesis of the key intermediates of the Kinoshita rifamycin S synthesis . 1,5-Dienyl carbamates such as 371, obtained from 2-substituted enals, provide a facile access to branched carbohydrate analogues . 

With the methylated PAHs, another bioactivation pathway leading to benzylic carbocations becomes available through side chain oxidation to form a benzylic alcohol, followed by esterification and solvolysis. Thus, benzylic sulfate ester formation (via initial formation of benzyl alcohol) constitutes an additional route that could contribute to metabolic activation (Fig. 2). ... 

Roller, P.P., Shimp, D.R. and Keefer, L.K., "Synthesis and Solvolysis of Methyl (acetoxymethy 1) nitrosamine. Solution Chemistry of the Presumed Carcinogenic Metabolite of Dimethylnitros-amine". Tetrahedron Letters, 1975, 2065-2068. 

Other evidence advanced in favor of alkyl nitrenium ions came from Ag+ promoted isomerization and solvolysis of various cyclic and bicyclic A-chloroamines (e.g., A representative example is shown in Figure 13.12. It was argued... 

Figure C. Nucleophilic discrimination parameters, s, and solvolysis constants, log ks, for several Pt(II) complexes (2)...

An important field of application of MS is the detection of impurities and products of unwanfed side reactions, for example, producfs of oxidafive processes and solvolysis, in ILs. 

---