Primary Oxonium Ions

Fig. 24. Acid-catalysed enolisation of acetone O, carboxylic acids, pyridinium ions and oxonium ion , primary and secondary amines [3].

This is an Sn2 process, since inversion is found at R. Another good leaving group is NTS2 ditosylamines react quite well with acetate ion in dipolar aprotic solvents RNTs2 4- OAc — ROAc. Ordinary primary amines have been converted to acetates and benzoates by the Katritzky pyrylium-pyridinium method (p. 447). Quaternary ammonium salts can be cleaved by heating with AcO in an aprotic solvent. Oxonium ions can also be used as substrates RsO -f R COO —> R COOR R2O. 

The use of iodotrimethylsilane for this purpose provides an effective alternative to known methods. Thus the reaction of primary and secondary methyl ethers with iodotrimethylsilane in chloroform or acetonitrile at 25—60° for 2—64 hours affords the corresponding trimethylsilyl ethers in high yield. The alcohols may be liberated from the trimethylsilyl ethers by methanolysis. The mechanism of the ether cleavage is presumed to involve initial formation of a trimethylsilyl oxonium ion which is converted to the silyl ether by nucleophilic attack of iodide at the methyl group. tert-Butyl, trityl, and benzyl ethers of primary and secondary alcohols are rapidly converted to trimethylsilyl ethers by the action of iodotrimethylsilane, probably via heterolysis of silyl oxonium ion intermediates. The cleavage of aryl methyl ethers to aryl trimethylsilyl ethers may also be effected more slowly by reaction with iodotrimethylsilane at 25—50° in chloroform or sulfolane for 12-125 hours, with iodotrimethylsilane at 100—110° in the absence of solvent, " and with iodotrimethylsilane generated in situ from iodine and trimcthylphenylsilane at 100°. ... 

The inactivation of an ion (a) by its transformation into a less reactive ion by reaction with a neutral base, e.g., water, which thus gives a primary oxonium ion or (b) by reaction with an anion to give one or more neutral molecules. 

In case of alkanols, the methylene oxonium ion, CH2=OH, m/z 31, deserves special attention. Resulting from a-cleavage, it undoubtedly marks spectra of primary alkanols, where it either represents the base peak or at least is the by far most abundant of the oxonium ion series (Fig. 6.8). [32] The second important fragmentation route of aliphatic alcohols, loss of H2O, is discussed in Chap. 6.10. 

Rupture of the C-C bond adjacent to oxygen in ether ROR leads to the formation of oxonium ions, which are stabilised by resonance (Fig. 16.26). A similar situation is encountered in amines (formation of the iminium ion at mass 30 for primary amines CH2 = NH2). Cleavage of the C O bond in ethers is also observed and leads to the cations R + and R +. 

Carbocation-oxonium ion equilibria are obvious complicating factors in studies of the kinetics of initiation of polymerisation and useful thermodynamic data for such equilibria involving Ph3C+ and a variety of linear and cyclic ethers have been reported by Slomkowski and Penczek (132). A dramatic increase in rates of initiation of polymerisation of THF induced by Ph3C+ salts is observed on addition of small amounts of epoxides such as propylene oxide (113a,b), which compete favourably with THF in the primary carbocation-oxonium ion equilibria and simplify the initiation reaction ... 

Primary Oxonium Ions [ROH2+]. It has been shown49 that methyl and ethyl alcohol in sulfuric acid give stable solutions of the corresponding alkyl hydrogen sulfates 2 [Eq. (4.1)]. Many other alcohols show similar initial behavior, but the solutions are not stable at room temperature. 

The primary addition products A now undergo SN1 reactions (Figure 9.12). A reversible protonation of the OH group of A leads to oxonium ions D. Water is ejected from D without the nucleophile getting involved. Thereby the intermediate oxocarbenium ions C (Nu = OR3), thiocarbenium ions C (Nu = SR3), or iminium ions C (Nu = NR3R4) are formed. These combine with the second equivalent of the nucleophile to form an 0,0-acetal B (Nu = OR3), an S,S-acetal B (Nu = SR3), or an N,N-acetal (Nu = NR3R4). 

Kinetics of the reactions of Me3Si+ with water (57) and alcohol (31,32) were investigated by ICR mass spectrometry. Proposed mechanisms involve formation of an oxonium ion as the primary product, which in the case of the methanol adduct is eventually decomposed to methoxy-silylenium ion according to Eq. (9). Reactions of halide transfer to silylenium ion [Eqs. (10) and (11)] have been studied by FT mass spectrometry (59) and by tandem mass spectrometry (44). Hydride transfer... 

This cation can be drawn either as an oxonium ion or as a primary carbenium ion. The oxonium ion structure is the more realistic. Primary carbenium ions are not known in solution, let alone as isolable intermediates, and the proton NMR spectrum of the cation compared with that of the isopropyl cation (this is the best comparison we can make) shows that the protons on the CH2 group resonate at 9.9 p.p.m. instead of at the 13.0 p.p.m. of the true carbenium ion. 

Chemical shifts of protons in secondary oxonium ions differ substantially ftom chemical shifts of protons in the primary hydroxy groups. One can expect a fast proton exchange between these two spwies. However, if the individual chemical shifts are known, then the observed chemical drift (due to exchange) permits the determination of the actual proportions of the condary oxonium ion IV.l and tertiary oxonium ion IV.4. 

Under typical polymerization conditions, the total concentration of growing species is e.g. 10 mole F and the concentration of polymer is equal to e.g. 2.5 mole 1. Under these conditions, taking into account the value of Kg = 3 10 mole", we would have 10" mole 1" of oxonium ions (assuming that dimethoxymethane is a suitable model fmonomer molecules converted into polymer, only 10 wouU be added through carbenium ions and the rest throu oxonium ions. The actual data for 1,3-dioxolane are not known at pr nt and may differ from values found for dimethoxymethane. Nevertheless, of primary importance is the finding that the reactivities of alkoxycarbenium ions and tertiary oxonium ions toward linear acetals, expressed through the corresponding rate constants, differ for the discussed above conditions only by 10 times. 

Ionization after an initial reaction that converts one functional group into a leaving group, as in protonation of an alcohol to give an oxonium ion or conversion of a primary amine to a diazonium salt, both of which ionize to the corresponding carbocation ... 

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