[tappinfool66]

Hi everyone,

I have a quick question about something I'm just not sure about with one of my lab experiments. We did an experiment with Thin Layer Chromatography and some of the samples we were eluting were *supposedly* weakly acidic acid-base indicators (methyl red, methyl orange, methyl violet, bromothymol blue and bromophenol blue). We used two solvents: Ammonium hydroxide in 1-propanol and Glacial acetic acid in 1-propanol. Of course, the experiment went poorly. My professor then asked this question: do acidic compounds travel further in acidic solvents or in basic solvents?

This confused me a lot because I know that the Rf of a compound depends on its polarity (and how polar/nonpolar the plate/solvent are and how strongly the compound adsorbs to whichever phase and all of that good stuff). I didn't know if an acid travels further in a solvent simply because the solvent is acidic or basic though. Does anyone know anything about this? I haven't been able to find anything :/

[dunno260]

Organic chemists have a lot of experience with this because acid/bases on silica gel in particular can be somewhat of a nightmare.  Its more of an issue with basic compounds than it is acidic compounds.

Think of say benzoic acid.  If you react it with a base you get some benzoate salt (so lets say you used sodium hydroxide to get sodium benzoate, a salt).  Which compound is more polar?  Benzoic Acid or sodium benzoate?

You can do the same thing with a base.  Lets choose an organic base (say aniline or 1-amino-benzene).  When you react it with hydrochloric acid you get aniline hydrochloride (a salt).  Which one of those is more polar?

[tappinfool66]

Organic chemists have a lot of experience with this because acid/bases on silica gel in particular can be somewhat of a nightmare.  Its more of an issue with basic compounds than it is acidic compounds.

Think of say benzoic acid.  If you react it with a base you get some benzoate salt (so lets say you used sodium hydroxide to get sodium benzoate, a salt).  Which compound is more polar?  Benzoic Acid or sodium benzoate?

You can do the same thing with a base.  Lets choose an organic base (say aniline or 1-amino-benzene).  When you react it with hydrochloric acid you get aniline hydrochloride (a salt).  Which one of those is more polar?

I'd say the salts because they're ionic. So basically, when an acid reacts with a basic solvent (or a base with an acidic solvent) the resulting 'compound' usually become ionized? Making it more polar than the original acid/base and letting it travel further up the plate? So acids generally travel further in basic solvents and bases generally travel further in acidic solvents? Sorry if I'm not getting it but does that make sense?