With its two lone electron pairs and slight excess of negative charge, the carbonyl oxygen has a natural attraction for a proton in acidic solution. A proton is attracted to the O atom (a), and uses one of the oxygen lone pairs to form a covalent O-H bond (b). Since the lone pair thereby is pulled toward the H atom, a small positive charge is created on the oxygen atom. This positive charge exerts an even greater pull on the electrons of the double bond, turning it into a single bond and another O lone pair (c), and transferring the positive charge to the carbon.

The positively charged carbon atom in turn has an attraction for the unpaired electrons of the water molecules surrounding it. One of these can form a bond to the carbon (d), thereby shifting the positive charge to the incoming water oxygen.

Step d represents a short-lived intermediate in the reaction, which cannot be isolated for study at leisure, but which is detectable by rapid spectroscopic and magnetic resonance methods. From d the reaction can go in more than one direction. It could reverse itself through Steps c, b, and a, or one of the protons could dissociate and take the positive charge with it.

One of the possibilities is a rapid proton transfer from the charged oxygen to the bridge oxygen (e), taking the positive charge with it.

Substance e, in turn, could break down in a series of Steps f-h, which are mirror images of Steps c-a, but with HOC₂H₅ playing the role of HOH.

Step g involves a protonated acetic acid molecule instead of protonated ethyl acetate as in b, and the proton falls away again in the last step, h. The proton is used at the beginning and regenerated at the end; thus it facilitates the reaction but is not destroyed by it.

Hydrogen ions help to catalyze this reaction in two ways. The binding of a proton in Step a places a positive charge on the molecule and makes it more susceptible to attack by electron pairs of a water molecule (or any other electron-rich or negatively charged entity).

The other thing that a proton can do better than other ions in aqueous solution is to move about rapidly from one part of a molecule to another. The fast shift between Steps d and e could not be accomplished by a sodium ion or any other substance. The reason why proton transfer in water solution is so easy and so fast is that protons can cascade domino fashion along a row of hydrogen-bonded water molecules, with each proton moving only from one oxygen atom to its neighbor, and with the proton that comes out at the end of the cascade different from the one that went in. This is illustrated on the previous page.