Alternative Proton Donors

H3O + has been the overwhelming proton donor of choice in PTR-MS. The reasons for this were given earlier and include the proton affinity of H2O, which lies conveniently between those of the common inorganic constituents of air and those of most VOCs, making the latter but not the former group susceptible to proton transfer from H3O+. 

Since proton transfer from NH4 + is out of the question for most organic compounds on energetic grounds, the chemistry is dominated by association reactions. The association reactions lead to adducts of the type M.NH4 +. The factors affecting the reaction outcome have been explored in detail by Keough and DeStefano [47]. Studies of the reactions of NH4 + with a variety of organic reagents in both SiFT-MS and PTR-MS have also been reported [48,49]. 

While the scope for applications of NH4 + in SIFT-MS and PTR-MS is limited, other proton donors derived from conjugate bases with proton affinities between those of H2O and NH3 might have more value. In the case of PTR-MS, the production of such intermediate proton donors, which might be one of any number of protonated organic molecules. 

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The presence of alternative proton donors (HA) or acceptors (B ) increases the rate of the reaction. 

Propiolic esters combine readily with pyridines to give first a zwitter-ion of type 95, but the only recorded case of subsequent addition of a second mole of the ester to give a quinolizine is with 6-methylphenan-thridine (see Section V,M). Zwitterion 95 usually abstracts a proton from either another molecule of alkyl propiolate or an alternative proton donor, and further reactions ensue. Some of the reactions are very difficult to reproduce and are very dependent on trace impurities and yield complex mixtures. 

With very electrophilic olefins, an alternative hydrogen fluoride addition process is often preferred This process, involving reaction of the olefin with fluoride ion in the presence of a proton donor, is applicable to certain perhalogen ated alkenes [/] and substrates with other electron attracting groups attached to the double bond [i5, 36] (equations 4 and 5)... 

In the case of a slow protonation rate (with inefficient proton donors and/or low concentrations of the proton source), the alternative could be an EECC mechanism through a disproportionation process, still at the potential of the first step. 

The inertness of phenols and phenoxy phenols toward Na/liq. NH3 can be attributed to the fact that phenols are powerful proton-donors in this system, and resistance of the resultant anions toward reduction is believed to result from stabilization by resonance (10). While alkylation of low-rank coals before treatment with Na/liq. NH3 therefore offers means for establishing the presence of phenoxy phenol ethers in them, an alternative is afforded by the observation that some phenols can be reduced by concentrated solutions of lithium (11). If this latter reaction also reduces phenoxy phenols in coal, a second treatment should then cause ether-cleavage. 

Since 1-octanol has certain limitations (see Section 1.3) many alternative lipophilicity scales have been proposed (see Figure 1.8). A critical quartet of four solvent systems of octanol (amphiprotic), alkane (inert), chloroform (proton donor) and propy-... 

This analogy is plausible on energetic grounds, since the decreased base strength of the proton acceptor should be approximately compensated by the increased acid strength of the proton donor. In view of the different species involved, however, it is reasonable to expect appreciable differences in the configurations of the transition states and hence in the activation barriers for the two paths. Therefore, the failure to observe an acid-catalyzed exchange reaction cannot be taken as conclusive evidence in favor of the alternative (hydride ion) mechanism. 

Tanaka and Mika 42) suggest that the higher basicity of amine relative to epoxide makes the formation of an amine-proton donor adduct more likely, and they proposed the following equations as an alternative to Eqs. (3-12) and (3-13). 

Alternative b is the most probable for cytochrome-c-peroxidase complex. Here, the N2 atom is the basic site of the catalytic act (which promotes specificity of the cytochrome-c-peroxidase complex) and the proton donor, simultaneously. It should be noted that in other heme-containing enzymes N2 atom of His 552 (fragment a) may be of the amphoteric type. 

Thus, the formation of an alcohol from a cathodic reduction of a ketone is favored in media where a single two-electron wave is observed, i.e., in the medium pH range in SSE s of high water contents, and in the presence of small amounts of proton donors in non-aqueous SSE s. Alternatively, in cases where two one-electron waves are observed, the reaction should be performed at a potential corresponding to the plateau of the second wave if the cathodic limit of the SSE allows this to be reached in preparative runs (which may be difficult in aqueous SSE s). 

The alternative reverse addition procedure can give incomplete reduction of the alkyne (33). An increase in the ratio of liquid ammonia to alkyne (34), the addition of co-solvents (23), the use of lithium rather than sodium, or the use of a higher temperature in an autoclave are advisable for the reduction of high molecular weight alkynes to overcome solubility problems which can also result in incomplete reduction. The resulting olefin is usually very pure ji isomer containing no detectable Z isomer. Use of an alcohol as a co-solvent and proton donor can accelerate the reduction, but the resulting olefin then contains a minor amount of the Z isomer. Polymer-bound alkynes can not be successfully reduced with sodium in liquid ammonia (35). 

Among possible alternative isomers, the preferential formation of diene 11, with the indicated location of the double bonds, is determined by the structure of the initially formed, most stable intermediate, radical-anion 14. Thus, the reduction of a single bond of toluene, as is represented in equation 5, requires the presence of an electron source (sodium), a solvent capable of electron solvation (liquid ammonia), and a proton donor (alcohol). 

Perhaps another litmus test of the ability of the alkynic C—H group to donate a proton in a H-bond arises when a molecule of this type is paired with a hydrogen halide, HX. One then has two distinct possibilities. The X atom, although a weak proton acceptor by nature, can form a complex of the C—H—XH type. An alternative would have the XH acting as the proton donor, with the electron-rich alkyne triple bond acting as the acceptor. Experimental measurements indicate the latter is the more stable of the two alternatives. Indeed, a similar sort of geometry is adopted when HE approaches the n system of ethylene , even though the electron source in this double bond is less rich than in the triple bond of an alkyne. 

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