When textbooks (or your teacher/TA/tutor) start writing down reaction mechanisms, sometimes you’ll see hell of a lot of curved arrows. The curved arrow notation is useful – it depicts how pairs of electrons flow from areas of negative charge to positive charge, and you can even use a single-barbed arrow to depict radical reactions. Most of the time you’ll see your professor / textbook / TA write out the full arrow-pushing mechanism. But sometimes you’ll see something like this:
What does “proton transfer” mean? Well, it’s shorthand for this:
It’s an intramolecular acid-base reaction.
Why don’t we draw out the whole thing? Well, we could. But we’re lazy. And it’s a boring reaction. You’re taking a proton off and moving it to a different part of the molecule. The thing is, it isn’t really important HOW the transfer happens, it’s just important that it happens. For example, the proton could also be moved onto the hydroxyl group by the following mechanism:
Which proton transfer mechanism is right? Well, they both operate to some extent. It doesn’t matter that much because the focus is on teaching (in this instance) how acetals form.
So in summary, just saying these two magic words accomplishes two important things.
1) it sacrifices a little bit of rigor in order to reduce what can potentially be a major (and ultimately inconsequential) source of confusion – the mechanism of the proton transfer.
2) it’s less work
I predict that the Magic Wand of Proton Transfer will be with us for decades to come.