Electra
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Order and Speed of deprotonation of substances of differing acidities?
If there are two acidic compounds with pKa's of 15 and 25 respectively, in a solution with a base that does not fully deprotonate either of them, will
there be a higher concentration of the anion of the more acidic compound? In other words, does the more acidic compound get deprotonated at a faster
rate than the less acidic compound? If so, is there a way to calculate how much faster? For example, would a 25 pka compound react xx
times slower with a base than a the pka 15 compound?
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DraconicAcid
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Acidity is determined by thermodynamic factors; kinetic factors are irrelevant. There will be a higher concentration of the anion of the more acidic
compound *at equilibrium*. Ka or pKa values say nothing about how fast that equilibrium is reached.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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Electra
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If kinetic factors are irrelevant, then is steric factors and physical hindrance irrelevant as well?
At any given point, is it more likely that the base will deprotonate the more acidic compound over the less acidic one, if there will always a higher
concentration of the more acidic one at equilibrium? I would assume this is how equilibrium is reached?
We will be doing an experiment in a week involving multiple ketones with different pKa values that can both react with the substrate, so I hope to
anticipate what the major product/reaction will be. Either substrate + highly acidic ketone via base, or substrate + lesser acidic ketone via base.
If pKa/acidity plays a role in which reaction is more likely to occur, then is this likely hood in anyway proportional to the differences in the pKa's
of the competing molecules?
I recall glossing over one reaction where a molecule with a pKa 10 points less than its competitor reacted 2x slower. Although, the more acidic
compound was much more bulky, indicating that kinetic/steric factors do participate.
[Edited on 7-5-2014 by Electra]
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DraconicAcid
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Okay, so you have two ketones, which will react with a base, and then the deprotonated ketones will react with a substrate, and you're going to
determine which reacts faster by the amount of each product formed.
The acid-base reactions themselves tend to be fairly rapid (depending on the solvent, at least- they are very fast in aqueous solution)- the pKa
values will determine the concentrations of the anions formed. These concentrations will affect the rate of the reactions of the anions with the
substrate- but steric factors and hindrance will also play a role in those rates. If you have significantly more of anion A than B, but B reacts a
lot faster, you may yet get more of the B-derived product.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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Electra
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Well based on the limited information I know this reaction won't be too complicated. The desired ketone that we want to react is much less stericly
hindred, and is about 10 pKa values lower. From what I recall glimpsing the lab sheet, the undesired ketone has 9-12 more carbons than the desired
one, which may or may not have an aromatic ring or two. It was a very funky looking molecule, very bulky, from what I hardly remember.
Acid-Base reactions are indeed very fast, but are they effected by steric hindrance to a great extent?
From what I am gathering and I hope my assumption is correct, is that the bulky molecule will barely react with the substrate seeing as the competing
ketone is much more acidic and much less sterically hindered. Hopefully I can find out what the specific rates are.
edit:
By the way, can you suggest any specific keywords/readings to learn more about this specific topic. Haven't encountered much information specifically
about the equilibrium of enolates of compounds of different acidities. Am I right to assume the equilibrium behaves in the way that it does because
the base is able to react slightly more quickly with the more acidic compound, and/or is more likely to?
[Edited on 7-5-2014 by Electra]
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DraconicAcid
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I want to say no, but I'm used to aqueous solutions and small bases. It's possible that in organic solvents (where you have to worry about solvent
cages and such things) with bulky bases, there may be some problems with steric hindrance. What base and solvent are you using?
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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Electra
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Base is supposed to be standard sodium hydroxide. As far as solvent, it will be dissolved in water.
One interesting fact I just found out is that the bulkier ketone is not soluble in water, while the [other] ketone is, yet, since the [other] ketone
is also an organic solvent itself, the bulky ketone may be soluble in it, and thus make it soluble in the water, though I'm not sure if
cross-solubility works like that.
That's sounds confusing. I wish I could get the names for these. Since I just found out about the solubilities, I am going to assume that since the
target ketone is 20-100x more soluble in water than the bulky one, then it will be the bulk of the deprotonated/reacting compound. All signs seem to
point to the desired reaction but I am not sure to what extent.
My professor won't give me anymore information for right now  .
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