Pyridinium ions—continued

Criterion b demands detectability of the chemisorbed species of the poison. This point is particularly important in the case of proton acids, since the lifetime of protonated species may be very low due to the high mobility of surface protons. Thus, the pyridinium ion cannot be detected on silica surfaces, although some protonated species must have been formed (399), as can be shown from a continuous absorption in the infrared spectra. Protons that can hardly be detected directly by protonated probe molecules may well initiate catalytic reactions due to their polarizing action during their fluctuations (349,350). 

Figure 12.25 shows how acetals can be brominated electrophihcally because of the (weakly) acidic reaction conditions. Proper acidity and electrophihcity is ensured by the use of pyri-dinium tribromide (B). This reagent is produced from pyridinium hydrobromide and one equivalent of bromine. Pyridinium tribromide is acidic enough to cleave the acetal A into the enol ether G. This cleavage succeeds by way of an El elimination like the one encountered in Figure 9.32 as an enol ether synthesis. The enol ether G reacts with the tribromide ion via the bromine-containing oxocarbenium ion H and the protonated acetal D to form the finally isolated neutral bromoacetal C. (The reaction can be conducted despite the unfavorable equilibrium between the acetal A and the enol ether G, since G continuously reacts and is thus eliminated from the equilibrium.)... 

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