A problem we see students constantly running into is that they do not readily recognize strong acids in organic chemistry class. This is a terrible mistake and should never happen; you will need to quickly recognize strong acids and understand which atom they will be protonating. As far as strong acids go, you should immediately identify the Magnificent Seven:
STRONG ACIDS: HI, HCl, HBr, HNO3, H2SO4, HClO3, H3PO4 (and H3ClO4)
Just think that that it’s the three hydrogen halides and the four acids that have N, S, Cl, and P as central atoms. Many students remember the other four acids with the mnemonic, something you would never say to a girlfriend: “Never Say ‘Please Clean’ ”.
Once you have recognized that you have a strong acid present in your reaction, it is necessary to determine what it is protonating. Remember that H+ is electron-deficient (Lewis acidic) and will look for an electron-rich (Lewis basic) atom to protonate. This could be a nitrogen atom, such as an amine, or an oxygen atom in a carbonyl or an alcohol. Most often, the atom being protonated will have a lone pair somewhere on it.
Take Home Message: Know the strong acids, the mnemonic “Never Say Please Clean” might help