[Peanuts]

I am not entirely certain as to why acetic acid eliminates streaks in TLC, but my best guess is that it when acetic acid is added to the developing solvent it deprotonates. So, any acidic solution on the TLC plate that deprotonates can be protonated by the free-floating H+ ions. Any help will be appreciated, Thanks!

[wildfyr]

Usually I've seen triethylamine added to eliminate streaking by amines because silica gel is acidic and the amines get protonated, however I guess you could use acetic acid to force a very acidic group to stay protonated, and thus keep its band narrow.

[Peanuts]

Thx for the reply. I understand how bases react with the slightly acidic silica gel and that triethylamine reacts with the silica gel to prevent this from happening, however, I am not entirely certain how acetic acid or formic acid can prevent streak. Because my instructor told me to use acetic acid in the developing solvent for an acidic or very polar compound. I understand that the acetic acid makes a developing solution containing ethyl acetate and hexane more polar which pushes the polar compound, thus reducing streak. But for an acidic compound, I can't think of anything besides acetic acid protonating the acidic compound in case it becomes deprotonated.

[Arkcon]

But for an acidic compound, I can't think of anything besides acetic acid protonating the acidic compound in case it becomes deprotonated.

Very good.  Now.  Consider this: what do you expect from a compound with no acidic or basic groups, and what do you expect if you you have a slightly deprotonated compound mixed with fully protonated in the same lane on a plate?