The combination of glacial acetic acid and sulfuric acid creates one of the strongest known superacids. The reaction between the two substances results in a protonation process that changes the structure of acetic acid. The stoichiometric ratio for the best behavior of the superacid is 2:1.
Acetic acid and sulfuric acid are, respectively, an organic carboxylic acid and an inorganic or “mineral” acid. The combination of glacial acetic acid and sulfuric acid, one of the strongest simple acids, resulted in one of the first two known “superacids.” This term, superacid, was introduced by chemists Hall and Conant in 1927 and refers to two groups by definition. Any acid stronger than 100% sulfuric acid is a Brönsted-variety superacid, while any acid stronger than anhydrous aluminum trichloride is a Lewis-variety superacid. The combination of acids from these two groups produced the strongest known superacids, including the famous “magic acid” composed of fluorosulfonic acid and antimony pentafluoride.
Since it is a moderately weak acid, it might appear that the combination of acetic acid and sulfuric acid should result in a liquid of intermediate acidity. This is not the case, because that action does not simply result in the formation of a solution; rather, a reaction takes place between the two substances. To understand as fully as possible the reaction between acetic acid and sulfuric acid, it is necessary to focus closely on the actual structure of a carboxyl group.
The oxygen atoms within the carboxyl group (-COOH), although drawn adjacently, are not bonded to each other. Actually, the leftmost oxygen is bonded only to the carbon atom, forming a carbonyl group (-C=O), while the rightmost oxygen is bonded only to the carbon and hydrogen atoms, -C- OH. This allows us to visualize the reaction CH3COOH + H2SO4 → CH3C(OH)2+. For clarification, this structure can also be written: CH3C(OH)(OH). In theory, one acetic acid molecule protonated by sulfuric acid would result in the production of HSO4-, whereas if a second acetic acid molecule is protonated by that remaining bisulfate anion, the mechanism is CH3COOH + HSO4- → CH3CO(OH)2 + SO4-2.
Once protonated, this structure is thought to change in a variety of ways, including the restoration of one carbonyl group, with the second branch becoming -C-OH2+. Acetic acid structures modified in one form or another would unite into polymer-like segments with one of three substances: sulfuric acid molecules, HSO4 ions, or SO4-2 ions. What has certainly been determined is that the stoichiometric ratio between acetic acid and sulfuric acid for the best behavior of the superacid is, in the protonation phase, 2:1. At the level of formation of the final product or segment, the ratio changes to 2:3.
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