The Velocity of Acid-Base Reactions

In the preceding sections we have frequently postulated reactions which take place at a measurable speed, which are formally acid-base reactions involving the transfer of a proton from one molecule to another. Since such reactions are commonly thought of as being very fast, we must examine how far the idea of acid-base reactions of measurable speed is consistent with information outside the field of acid-base catalysis. 

It is common experience that many acid-base reactions (e.g., dissociation, hydrolysis, neutralization) take place too rapidly to be followed by ordinary methods, even when very weak acids and bases are involved. However, a few instances are known in which a slow reaction can be observed, the best known of which is the reaction of nitroparaffins with hydroxyl ions. For example, nitromethane, CH3N02, is neutralized at a measurable speed by hydroxyl ions to given an anion whose structure 

Hantzsch (123), who introduced the term pseudo-acid to describe the nitroparaffins and similar substances. Hantzsch supposed that the slow change was the transformation of the nitroparaffin into the act form, e.g., OH 

The rate-determining steps thus involve simultaneously a proton transfer and a change in the number of molecules, thus showing analogy with the usual picture of a pseudo-acid. 

The term pseudo-base has been used less widely and less consistently than pseudo-acid. Logically it should be applied to a species which undergoes a change of structure when it adds on a proton. This would include the anions of the pseudo-acids discussed above, and a few uncharged species such as coloring matters (e.g., anthocyanins and flavones) derived from 7-pyrone, where the addition of a proton involves the reaction 

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