[jezaza]

I'm on the amino acids section of the course and it's describing using a benzyl ester as a protecting group for the carboxylic acid. As a means to remove it, they say to use HBr in acetic acid.

I just can't seem to wrap my head around how that would work. I've tried protonating the double bonded O and pushing charges around to no avail.

[crawlingmcedge]

What happens when you protonate the other oxygen? Not the double bonded one. After you protonate, what is the other species left from HBr

Think ether cleavage.

[Babcock_Hall]

Deprotections of esters of carboxylic, phosphoric, and phosphonic acids can take at least two paths.  One can envision attack at the carbonyl or phosphoryl atom, or one can envision an attack elsewhere.