Acetic acid protonation

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. 

Notice in the list of Lewis bases just given that some compounds, such as carboxylic acids, esters, and amides, have more than one atom with a lone pair of electrons and can therefore react at more than one site. Acetic acid, for example, can be protonated either on the doubly bonded oxygen atom 01 on the singly bonded oxygen atom. Reaction normally occurs only once in such instances, and the more stable of the tw o possible protonation products is formed. For acetic acid, protonation by reaction with sulfuric acid occurs on... 

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. 

Reaction normally occurs only once in such instances, and the more stable of the two possible protonation products is formed. For acetic acid, protonation occurs on the doubly bonded oxygen. 

Acetic acid protonated by fluorosulphonic acid in the crystalline state at 90K produces a symmetrical species with symmetric and anti-symtetric C-0 stretches at 1615 and 1560 cm-1 [11]. 

The authors speculate that the stereochemical divergence may be related to the ability of the electrophile to coordinate with the lithium, coupled with the presence or absence of a low-lying LUMO. Curiously, protonation by methanol proceeds with retention whereas protonation with either acetic acid or triphenyl methane proceeds with inversion. The authors speculate that, in acetic acid, protonation of the TMEDA nitrogen and internal return (c/. Schemes 3.2 and 3.24) may occur instead of direct protonation [184]. Presumably, direct protonation is the only mechanistic course with weak acids such as methanol and triphenylmethane and steric effects dictate inversion for the latter. Hoppe also noted that the enantiomeric purity of the products also depended on the solvent. In THF, the products were nearly racemic, and the enantiomeric purity of several of the other alkylation products was variable in solvents such as ether and pentane. This variability is due, at least in part, to the degree of covalency of the C-Li bond. In donor solvents such as THF, racemization is more facile. 

Similarly, acetic acid protonates water, as shown earlier in this section, but is protonated by stronger acids such as HBr ... 

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