Proton acetate

G Addition of a second equivalent of alcohol gives a protonated acetal. 

Capon et al. considered three possible mechanisms [equation (47)] for hydrolysis of o-methoxymethoxybenzoic acid. Mechanisms (a) and (b) involve nucleophilic attack by the carboxylate anion on the protonated acetal, and mechanism (c) proceeds with intramolecular gener2il acid catalysis. Capon was able to rule out mechanisms (a) and (b) since the synthetically prepared intermediate in (a) is stable under the reaction conditions. That in (b) leads to products at... 

Acetic acid is a very weak proton acceptor and thus does not compete effectively with weak bases for protons. Only very strong acids will protonate acetic acid appreciably according to the equation shown below ... 

Tiedemann and Riveros (1974) first discussed the gas-phase reactions which are equivalent to an esterification reaction. An icr study of alcohols and acetic acid revealed that the formation of protonated acetic acid by ion-molecule reactions of fragment ions with acetic acid is followed by the rapid reaction (72). 

For the esterification reaction, there is of course no way of distinguishing between the two proposed mechanisms. The fact that protonated acetic acid reacts with H2lsO to produce CH3C(OH)18OH+ in a thermoneutral exchange with displacement of H20 can be accommodated by either of the mechanisms. Failure to observe the alcoholysis process (76) cannot be rationalized in terms of one mechanism or the other. 

In effect this equation defines an acidity function, Hs = mH0, for the ionization of protonated acetate esters. It is an experimental observation that Hs is the same linear function of H0 for several of the esters concerned, so that the approximations involved are greatly reduced from those implicit in the use of // itself, while the treatment remains more general than the experimentally unattainable ideal of measuring the acidity function directly for every substrate. In practice the excellent results obtained with aliphatic esters are in contrast to the slightly less clear-cut picture for aryl esters (see below) and suggest that H describes the protonation behaviour of substituted phenyl acetates less than perfectly. However, there is little doubt that Hs will be a linear function of H0 for any given ester, since this appears to be the case for the acidity functions defined for a wide variety of neutral substrates. Yates and McClelland37 show that all available acidity functions measured in sulphuric... 

The X-ray crystal structure of protonated formic acid, acetic acid, and methyl formate has recently been determined by Minkwitz and co-workers. In agreement with NMR data discussed above, the nearly equal C—O bond lengths of protonated formic acid (1.239 and 1.255 A),573 protonated acetic acid 275 (1.251-1.291 A for various salts),574 and protonated methyl formate 276 (1.260 and 1.264 A)575 show efficient delocalization of the positive charge. Minkwitz et al.576 have also studied protonation of oxalic acid in HF-SbF5 and isolated the hexafluoroantimonate of the mono- and diprotonated acid at 75°C and 40°C, respectively. Structural characteristics are very similar to those of the other cations discussed. 

The formation or the hydrolysis of an acetal function proceeds by the mechanism described in Fig. 16 in which oxonium ions and hemiacetals occur as intermediates. It has also been established (76) that the rate determining step in acetal hydrolysis is generally the cleavage of the C—bond of the protonated acetal 100 to form the oxonium ion 111, This ion is then rapidly hydrated to yield the protonated hemiacetal 112 which can give the aldehyde product after appropriate proton transfers. It is pertinent therefore to find out if stereoelectronic effects influence the rate determining step (110 111) of this hydrolysis reaction. 

A second molecule of alcohol attacks the carbonyl carbon that is forming the protonated acetal. 

Acetic acid, protonated acetic, diprotonated acetic acid (89), acetyl cation, and the protioacteyl cation (46) were calculated at the MP2/6-31G //GIAO-MP2/tzp/dz level of theory nitric acid, nitronium cation, and the protionitronium cation (3) were calculated at the HF/6-31G // II//6-31G level of theory hydronium ion and the tetrahydridooxonium ion (90) were calculated at the MP2/6—31 G //GIAO-MP2/tzp/dz level of theory. 

Regioselective debenzylation can be achieved by treatment with Lewis acids such as ferric chloride and S11CI4 or under acetolysis conditions with acetic anhydride and sulfuric acid, and several examples are depicted in Scheme 2.3.9 Acetolysis results in cleavage of the most acid-sensitive benzyl group. In general, primary benzyl ethers can be selectively acetolysed in the presence of secondary benzyl ethers. The regioselectivity of the reaction can be explained as follows sulfuric acid protonates acetic anhydride followed by the formation of an acetyl ion and acetic acid. The acetyl ion reacts with the sterically most accessible oxygen which is at... 

A germanium complex with ethylenediaminetetraacetic acid (Hedta) 532 has a pen-tadentate edta ligand with one protonated acetate group which does not coordinate, forming a six-coordinate complex1150. 

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.)... 

This means that, to a certain degree, all compounds are amphoteric. For example, hydrochloric acid will protonate acetic acid. 

We considered the mechanism for this reaction in Chapter 14 but did not then concern ourselves with a label for the first step. It has, in fact, an S l style of rate-determining step the decomposition of the protonated acetal to give an oxonium ion. If you compare this step with the decomposition of the chloroether we have just described you will see that they are very similar, hydrolysis of acetals - SN1 mechanism for the first step... 

The presence of two heteroatoms complicates the mechanism. In fact, propyl acetate yields protonated acetic acid as a base peak, whereas methylbutyrate yields an (RCOOH + H)+ peak that is hardly detectable the proton is derived from the alcohol. Experiments on propyl acetate labelled with deuterium at various positions on the propyl chain yield the following percentages concerning the origin of the proton during the rearrangement. This distribution seems fairly statistical ... 

The equilibrium depicted in Figure 1.16 can be shifted by adding a strong acid, such as HCl, to the beaker as shown in Figure 1.17. This increase in fhe concentration of (HjO" ) ions shifts the equilibrium to increase the concentration of acetic acid and decrease the concentration of acetate ion. The increase in the concentration of molecules that leave the aqueous phase increases the rate of volatilization from the beaker to the air. Addition of strong acid does not increase the tendency of any particular molecule of acetic acid to enter the atmosphere. The strong acid merely increases the overall concentration of protonated acetic acid (at the expense of the ionized acetic acid). Protonated acetic acid hasp much greater tendency than ionized acetic acid to leave the water phase. 

The reaction involves the formation (step I) of the ion I, which then combines (step 2) with a molecule of alcohol to yield the protonated acetal. As we can see,... 

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