In Steps d and e of the catalytic mechanism, the same proton does not have to move bodily from one oxygen atom to the other. It can transfer to a nearby water molecule and cause that molecule to donate one of its protons to the bridge oxygen. Proton transfer in aqueous solution is faster and easier than any other kind of atom shift.

Any compound that has lone electron pairs on an electronegative atom can attract a proton in acid solution. This includes almost every type of organic compound. A proton can also be attracted to the electrons of a double bond and catalyze the addition of HBr or some other substance across the bond:

There are few reactions of carbon compounds that acid will not catalyze. This is why we have hydrochloric acid in our stomach, to help enzymes digest and hydrolyze proteins. This is also why acid catalysis is so important in the chemical industry, from synthesis of rubbers and organic dyestuffs to pharmaceutical compounds.