Weak Base/Acid does not have Strong Conjugate Per Se

[capostat]

I was tripped by a TBR Orgo problem in which the answer for choosing the strong base, was CH3CH2Li over NaOH.

It said that that since CH3CH3 is a weak acid, then that means CH3CH2- is a strong base, and compared it to NaOH where the conjugate of that is H20 water.

I'm assuming the point was that water not being that much of a weak conjugate, is what makes NaOH not that much of a strong acid. *shrug*

1)Is that fair?

I always looked at it that NaOH was a strong base no matter what.


2) how do we know when to say a weak acid or base DEFINITELY has a strong conjugate base or acid? You need to know the Ka (pka) for this, no?

I try to think that the MCAT will steer clear of these philosophies since a book like EK will get you to just know the shortlist of strongs of acids and bases, and not have you deal with the is it weak/weak or weak/strong for the uncommon acids/bases.

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[Geekchick921]

I was tripped by a TBR Orgo problem in which the answer for choosing the strong base, was CH3CH2Li over NaOH.

It said that that since CH3CH3 is a weak acid, then that means CH3CH2- is a strong base, and compared it to NaOH where the conjugate of that is H20 water.

I'm assuming the point was that water not being that much of a weak conjugate, is what makes NaOH not that much of a strong acid. *shrug*

1)Is that fair?

I always looked at it that NaOH was a strong base no matter what.


2) how do we know when to say a weak acid or base DEFINITELY has a strong conjugate base or acid? You need to know the Ka (pka) for this, no?

I try to think that the MCAT will steer clear of these philosophies since a book like EK will get you to just know the shortlist of strongs of acids and bases, and not have you deal with the is it weak/weak or weak/strong for the uncommon acids/bases.

I have, too.

I googled organolithium compounds, and the Wiki page (TOTALLY trustworthy, right?) has a section on the use of organolithium compounds. http://en.wikipedia.org/wiki/Organolithium_reagent

Apparently, organometallic compounds have a pKa of a whopping 50 or so. NaOH is about 16. I supposed this is just something we should know. I am not through my O-chem review but I'm not sure how thoroughly that was covered when I took the class.

 

[loveoforganic]

organometals can generally be seen as carbanions with a metal counterion. At any rate, I still consider NaOH strong.

 

[capostat]

Thanks guys,

I'll keep it simple with the list of strongs, and when presented with a new or uncommon acid or base, consider conjugates/pKas, etc...

 

[BerkReviewTeach]

I was tripped by a TBR Orgo problem in which the answer for choosing the strong base, was CH3CH2Li over NaOH.

It said that that since CH3CH3 is a weak acid, then that means CH3CH2- is a strong base, and compared it to NaOH where the conjugate of that is H20 water.

I'm assuming the point was that water not being that much of a weak conjugate, is what makes NaOH not that much of a strong acid. *shrug*

1)Is that fair?

I always looked at it that NaOH was a strong base no matter what.


2) how do we know when to say a weak acid or base DEFINITELY has a strong conjugate base or acid? You need to know the Ka (pka) for this, no?

I try to think that the MCAT will steer clear of these philosophies since a book like EK will get you to just know the shortlist of strongs of acids and bases, and not have you deal with the is it weak/weak or weak/strong for the uncommon acids/bases.

Forgive me for a different tack here, but I think you might be missing the big picture. You don't have to memorize strong and weak lists. You have to be able to make choices based on periodic trends.

There are two basic ideas here:

• the stronger the acid, the weaker its conjugate base

• Oxygen is more electronegative than carbon, so an O-H bond is more acidic than a C-H bond and an O- is more basic than a C- (given that they have the same hybridization)

As for your first question, NaOH is a strong base and water (its conjugate) is a weak acid. However, because R-H is an even weaker acid than water, R- must be an even stronger base than OH-.

As for your second question, an acid is said to be strong if it fully dissociates, which would lead to a very large Ka (Ka > 1). This would result in a negative pKa (pKa < 0). So strong acids have negative pKas and strong bases have negative pKbs. But for questions like this, you don't need to know pKa values; you need to know periodic trends to predict relative acidity.

I have, too.

I googled organolithium compounds, and the Wiki page (TOTALLY trustworthy, right?) has a section on the use of organolithium compounds. http://en.wikipedia.org/wiki/Organolithium_reagent

Apparently, organometallic compounds have a pKa of a whopping 50 or so. NaOH is about 16. I supposed this is just something we should know. I am not through my O-chem review but I'm not sure how thoroughly that was covered when I took the class.

I know it's tempting to research facts when in doubt, but hopefully you see the trend between the compounds, because that's how you'll have to attack questions like this on the MCAT. And just to be accurate, it's the conjugate acid of NaOH (H2O) that has a pKa of about 16, not NaOH itself. Water has a pKa of 15.7.

 

[Geekchick921]

I know it's tempting to research facts when in doubt, but hopefully you see the trend between the compounds, because that's how you'll have to attack questions like this on the MCAT. And just to be accurate, it's the conjugate acid of NaOH (H2O) that has a pKa of about 16, not NaOH itself. Water has a pKa of 15.7.

You're totally right. My mistake.

 

[capostat]

Forgive me for a different tack here, but I think you might be missing the big picture. You don't have to memorize strong and weak lists. You have to be able to make choices based on periodic trends.

There are two basic ideas here:

• the stronger the acid, the weaker its conjugate base

• Oxygen is more electronegative than carbon, so an O-H bond is more acidic than a C-H bond and an O- is more basic than a C- (given that they have the same hybridization)

As for your first question, NaOH is a strong base and water (its conjugate) is a weak acid. However, because R-H is an even weaker acid than water, R- must be an even stronger base than OH-.

As for your second question, an acid is said to be strong if it fully dissociates, which would lead to a very large Ka (Ka > 1). This would result in a negative pKa (pKa < 0). So strong acids have negative pKas and strong bases have negative pKbs. But for questions like this, you don't need to know pKa values; you need to know periodic trends to predict relative acidity.

My problem is how can you say that R-H is even weaker and so R- must be an even stronger base? Because a weak acid doesnt necessarily translate to a strong base, unless like you said--it's weaker than water.

Is it possible then that anything that is a weaker acid than water will be a stronger base that water's conjugate base, OH-?

Why is there a caution against viewing weak acids and bases as absolutely having strong conjugates? Does this have to do with instances where you are NOT looking to factor in H-O-H or OH- partners as a comparison of strength.

 

[loveoforganic]

Acetic acid. Weak acid. Acetate. Weak base.

The weaker the acid, the stronger the conjugate base. All it's saying is an acid being weak, in and of itself, does not mean that the conjugate base must be strong.

 

[UCB05]

My problem is how can you say that R-H is even weaker and so R- must be an even stronger base? Because a weak acid doesnt necessarily translate to a strong base, unless like you said--it's weaker than water.

Is it possible then that anything that is a weaker acid than water will be a stronger base that water's conjugate base, OH-?

Why is there a caution against viewing weak acids and bases as absolutely having strong conjugates? Does this have to do with instances where you are NOT looking to factor in H-O-H or OH- partners as a comparison of strength.

I'm sorry, am I just missing something? I was always taught that the weaker the acid, the stronger the conjugate base. In what cases does this not necessarily correlate? or is it a matter of threshold pk values that you would consider weak and strong as such?

 

[capostat]

Acetic acid. Weak acid. Acetate. Weak base.

The weaker the acid, the stronger the conjugate base. All it's saying is an acid being weak, in and of itself, does not mean that the conjugate base must be strong.

Ok so it's in relative terms and not absolute. So something can be a weak acid, and it's conjugate can be a stronger base, but that stronger base is still weak compared to say, OH-.

i think

 

[capostat]

I'm sorry, am I just missing something? I was always taught that the weaker the acid, the stronger the conjugate base. In what cases does this not necessarily correlate? or is it a matter of threshold pk values that you would consider weak and strong as such?

i ran into this in EK Chem book, and audio Osmosis.

loveoforganic put an example there.

 

[UCB05]

i ran into this in EK Chem book, and audio Osmosis.

loveoforganic put an example there.

I think I get what you're trying to say now. While it is true that the weakER the acid, the strongER its conjugate base, but your contention is that a weak acid does not mean strong conjugate base, which is true especially of the middle-of-the-road acids which are not quite considered strong, but not terribly weak and therefore their conjugate bases are not considered strong bases. Am I getting that part right?

 

[loveoforganic]

I didn't put an example of that. The example I posted follows the normal trend. I'm not aware of any deviations from that trend.

@UCB - spot on.

 

[capostat]

I didn't put an example of that. The example I posted follows the normal trend. I'm not aware of any deviations from that trend.

@UCB - spot on.

ok sorry, so you were confirming that weak acids can have weak conjugate bases

 

[loveoforganic]

Yes 🙂 That's all that TBR is trying to emphasize there.

 

[capostat]

i'm gonna be very careful when i see NaOH as an answer choice, or any metal hydroxide

 

[loveoforganic]

Metal hydroxides, as long as they're soluble, are generally considered strong bases.