pKa values and predicting organic reaction

[kevo246]

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just a quick reminder needed in reference to predicting reactions. So a few pKa values: strong acids=0 or below; -COOH~5; phenols~10; H2O, OH~15. The question I have is what reacts to form what? A low pKa value reactant will take the proton from a higher pKa substrate...correct?

 

[ilovemcat]

just a quick reminder needed in reference to predicting reactions. So a few pKa values: strong acids=0 or below; -COOH~5; phenols~10; H2O, OH~15. The question I have is what reacts to form what? A low pKa value reactant will take the proton from a higher pKa substrate...correct?

Stronger Acid + Stronger Base ---> Weaker Acid + Weaker Base

Stronger Acid = Low pKa
Stronger Base = Low pKb

Weaker Acid = High pKa
Weaker Base = Low pKb

 

[GreenRabbit]

A low pKa value reactant will take the proton from a higher pKa substrate...correct?

No, and it's also not so simple. The lower the pKa the more it wants to donate the proton. But you need to compare the pKa of the donor to the pKa of the product of H+ and the recipient (e.g. if you want to know if H2O will pull a H+ off COOH, compare the pKas of COOH and H3O+).

 

[kevo246]

No, and it's also not so simple. The lower the pKa the more it wants to donate the proton. But you need to compare the pKa of the donor to the pKa of the product of H+ and the recipient (e.g. if you want to know if H2O will pull a H+ off COOH, compare the pKas of COOH and H3O+).

So, the pKa of -COOH~5 and H3O+~-2.....what does this tell us about predicting the reaction? It will not proceed because of the low pKa of H30+, right. The lower pKa will NOT form

 

[GreenRabbit]

So, the pKa of -COOH~5 and H3O+~-2.....what does this tell us about predicting the reaction? It will not proceed because of the low pKa of H30+, right. The lower pKa will NOT form

Well it's not a choice between whether it proceeds or doesn't proceed: its going to hit an equilibrium. Some H3O+ will form, but there will be more much more COOH than H3O+ (meaning also more COOH than COO-). Which makes sense because COOH is an organic acid, not a strong acid like HCl. You expect only a small fraction to dissociate due to the higher pka of COOH. On the other hand, HCl has a pkA of -7, so almost all of the HCl will donate a proton to water to form H3O+ (pkA of "only" -2).

 

[kevo246]

Well it's not a choice between whether it proceeds or doesn't proceed: its going to hit an equilibrium. Some H3O+ will form, but there will be more much more COOH than H3O+ (meaning also more COOH than COO-). Which makes sense because COOH is an organic acid, not a strong acid like HCl. You expect only a small fraction to dissociate due to the higher pka of COOH. On the other hand, HCl has a pkA of -7, so almost all of the HCl will donate a proton to water to form H3O+ (pkA of "only" -2).

Alright....I definitely understand. I was considering mentioning the equilibrium distinction, but didn't want to type the explanation out. I really appreciate your help Rabbit!! All Clear Now!!

 

[kevo246]

any guesses on what values of pKa we should be familiar with for this MCAT!!?? I've got the basics down as stated in prior posts within this thread....but not esters and amines. any other thoughts?

 

[GreenRabbit]

What you're familiar with now is good. They'll give you the pka value in the question or the passage if you need it.

 

[ridethecliche]

If you're looking at it from the reaction point of view, then the pka of the products matter as well.

Are you trying to conceptualize this as zwitterions?
Then it's the pH of the solution and the pka of the amino acids that matter.