A problem we see students constantly running into is that they do not readily recognize strong acids. This is a terrible mistake and should never happen. You will need to quickly recognize strong acids and which atom they will be protonating. As far as strong acids go, you should immediately recognize the Magnificent Seven:
STRONG ACIDS: HI, HCl, HBr, HNO3, H2SO4, HClO3, H3PO4
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.
Take Home Message: Know the strong acids, the mnemonic Never Say ‘Please Clean’ might help