this is just a general question and im not exactly sure it matters what the compounds are other than knowing if they are strong or weak bases. I don't have a specific example. I am just wondering if there is something inherently wrong with using a 4 step process as I have not seen it in my prep books
sigh, well, the problem is that you didn't give us any information about any of the compounds. anyway, since I used to teach for TPR, I vaguely remember that the hyperlearning book has a flowchart showing extraction with 4 compounds. they were something like this:
naphthalene (two fused benzene rings), some sort of benzoic acid, some sort of aniline derivative, and some sort of phenol derivative.
so the naphthalene has no significant acid/base properties. the benzoic acid derviative is a strong acid (relatively). the aniline derivative is a weak base. and the phenol derivative is a weak acid.
So they do something like, add a strong acid to pull the aniline into aqueous solution, then add a weak base to pull the benzoic acid into the water, then add a strong base to pull the phenol out. You're left with naphthalene in the organic layer.
so let's see what you're suggesting. you add a weak acid, but in this example, nothing comes out (since a weak acid wouldn't be good enough to protonate aniline), so you just wasted some (admittedly cheap) reagent and maybe 10 minutes of your time. Then you add a strong acid and you pull the aniline out. Then you add a weak base to pull the benzoic acid out, then a strong base to pull the phenol out. Is that what you're asking? Are you just saying that you add a weak acid as the first step, and then do the rest of the protocol like the review books are saying?
If that is what you're saying, there is no point to adding that extra step. It does absolutely nothing in this example. Now, if you had a strong base like ethylamine or something in solution, then yeah, you're going to need to add a weak acid sometime before you add a strong acid. The reason these books don't suggest a four step protocol is because there are only three steps necessary.
But I would strongly caution you against thinking about it like that. It sounds like you're attempting to memorize a protocol for doing solvent extraction (sorry if this is an incorrect observation, it just sounds like it), when you should just understand it. It's pretty simple. You're trying to separate compounds based on their acid/base properties. If the reagent you're adding will change the protonation state of a compound you're trying to extract so that you have a salt instead of a neutral compound, then the compound will be more soluble in the aqueous layer and will be found mainly in that layer. So if you have multiple ionizable compounds in your organic layer, you want to add your extraction reagents in an order that will allow only one compound to come into the aqueous layer at a time. If multiple compounds come into the aqueous layer at the same time, you may have separated them from the rest of the compounds, but you'll still need to separate those compounds from each other.
