Source of Protons

Note Chemi-ionization means ionization of internally excited molecules upon interaction with other neutrals by electron detachment. This corresponds to M + X MX + e . Chemi-ionization is different from Cl in that there is no ion-molecule reaction involved (cf. Penning ionization. Chap. 2.1.3) [8,16]. 

The occurrence of [Mh-H] ions due to bimolecular processes between ions and their neutral molecular counterparts is called autoprotonation or self-CI. Usually, autoprotonation is an unwanted phenomenon in EI-MS. [M-i-1] ions from autoprotonation become more probable with increasing pressure and with decreasing temperature in the ion source. Furthermore, the formation of [M-i-1] ions is promoted if the analyte is of high volatility or contains acidic hydrogens. Thus, self-CI can mislead mass spectral interpretation either by leading to an overestimation of the number of carbon atoms from the C isotopic peak (Chap. 3.2.1) or by 

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In all cases, water and carbonic acid, the latter of which is the source of protons, are the main reactants. The net result of the reaction is the release of cations (Ca " ), Mg ", K", Na" ) and the production of alkalinity via HCO. When ferrous iron is present in the lattice, as in biotite, oxygen consumption may become an important factor affecting the weathering mechanism and the rate of dissolution. 

Several other polypyridyl metal complexes have been proposed as electrocatalysts for C02 reduction.100-108 For some of them HCOO- appears as the dominant product of reduction. It has been shown for instance that the complexes [Rhin(bpy)2Cl2]+ or [Rh n(bpy)2(CF3S03)2]+ catalyze the formation of HCOO- in MeCN (at —1.55 V vs. SCE) with a current efficiency of up to 80%.100,103 The electrocatalytic process occurs via the initially electrogenerated species [RhI(bpy)2]+, formed by two-electron reduction of the metal center, which is then reduced twice more (Rlr/Rn Rh°/Rh q. The source of protons is apparently the supporting electrolyte cation, Bu4N+ via the Hoffmann degradation (Equation (34)). 

When trifluoroacetic acid is used as the source of protons, it is known that rapid formation of trifluoroacetate esters precedes reduction to hydrocarbons.134,204 206 Use of acetic acid in place of trifluoroacetic acid, for example, would be expected to fail to produce good conversion to reduced product because of the combination of decreased acidity and increased nucleophilicity of acetic acid relative to... 

Protons in soil are associated with soil water molecules to form hydronium ions, which can also be on an exchange site in soil (see Figure 8.5). Additionally, protons are bonded with different strengths to both organic and inorganic components in soil. All of these are potential sources of protons for chemical reactions. They also produce buffering of soil pH such that the pH of soil is hard to change. 

These three sources of protons are illustrated in Figure 10.4, in which H30+s designates hydronium ions in solution, H30+e designates hydronium ions on... 

Bronated acid chem A chemical species which can act as a source of protons. Also known as proton acid protonic acid. bron-steth or bren.sted as-ad )... 

When the addition is carried out in acetonitrile-dj, mono and di-deuterated [l-i F]l,l,l,2-tetrafluoroethanes are obtained, suggesting that the solvent acts as a source of protons. This reaction was the first demonstration of an efficient addition of [ F]fluoride to a fluoro-olefin it achieves the equivalent of the addition of hydrogen [ F] fluoride. 

These are standard reactions, and it does not matter which you do first. Resist the urge to do both at the same time it s unlikely to occur, and will certainly complicate your structures and mechanisms. Note that we have to include a number of protonations to make the mechanisms work. This is quite legitimate, and although the question has not specified any conditions, you can assume there will be a source of protons. In any case, you will observe that protons are first released to solvent, then transferred back to an alternative site in the molecule. 

FIGURE 11.2 A buffer solution contains a sink for protons supplied by a strong acid and a source of protons to supply to a strong base being added. The joint action of the source and sink keeps the pH constant when strong acid or base is added. 

In an analogous way, electron transfer to an acceptor-substituted aromatic like 25 produces a radical anion ot type 26. This is proto-naled in the ipso position to give intermediate 27. A second electron transfer and prolonation leads similarly to product 28. In most cases BuOH proves to be a good source of protons. 

Brpnsted acid a chemical species that can act as a source of protons. 

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