most acidic

[premed8]

---

Members do not see this ad.

Hi I have a question about acidity of these.. thanks for you kind explanation.
s in advance.

Which of the following compounds is expected to be
most acidic?
A. CCl3COOH
B. CH3OH2+
C. NH4+

 

[flyingforever]

Hi I have a question about acidity of these.. thanks for you kind explanation.
s in advance.

Which of the following compounds is expected to be
most acidic?
A. CCl3COOH
B. CH3OH2+
C. NH4+

I am not answering pre-med (jk). A is the correct answer. You can look at the acidity problems in four ways:
1. Atom
2. Resonance
3. Induction
4. Orbital

When you pull off the proton from carboxylic acid of the CCl3COOH, you get a stable negative charge. Then you have induction effect of having 3 Chlorine atoms around the central carbon.

 

[Charmm]

Wrong

The answer is B.
B is a protonated alcohol with negative pka, very acidic. A is a carboxylic acid ( pka ~ 4.5) with three electronegative chlorines attached. Remember, positively charged oxygen is very unhappy and wants to give up the H to become neutral;
NH4+, has a pka ~ 11, the least acidic in the series

 

[Charmm]

bump

 

[thewobblytooth]

if it was just a regular alcohol, it would have been A though right?

 

[rnbaxter]

Wrong

The answer is B.
B is a protonated alcohol with negative pka, very acidic. A is a carboxylic acid ( pka ~ 4.5) with three electronegative chlorines attached. Remember, positively charged oxygen is very unhappy and wants to give up the H to become neutral;
NH4+, has a pka ~ 11, the least acidic in the series

Correct
1) PKA between 1-4 estimating because of the chlorine withdrawers
2) PKA negative very acidic H
3) PKA 9-10

 

[rnbaxter]

if it was just a regular alcohol, it would have been A though right?

si senior

 

[Ibraiz]

Wrong

The answer is B.
B is a protonated alcohol with negative pka, very acidic. A is a carboxylic acid ( pka ~ 4.5) with three electronegative chlorines attached. Remember, positively charged oxygen is very unhappy and wants to give up the H to become neutral;
NH4+, has a pka ~ 11, the least acidic in the series

But looking at the inductive effect and the conjugate to determine the acidity wouldn't A be more acidic because Cl (in this case 3 Cl) are using their electron withdrawing characteristic to withdraw the electron from the carbanion that is formed after donating the 'H' thus making it more stable (weak) conjugate base and a stronger acid?

Only reason I would lean towards answer choice B is because it has a positive charge and it has 2 H's to give away but not sure if that can compete with the explanation provided for answer choice A.

 

[orgohacks]

But looking at the inductive effect and the conjugate to determine the acidity wouldn't A be more acidic because Cl (in this case 3 Cl) are using their electron withdrawing characteristic to withdraw the electron from the carbanion that is formed after donating the 'H' thus making it more stable (weak) conjugate base and a stronger acid?

Only reason I would lean towards answer choice B is because it has a positive charge and it has 2 H's to give away but not sure if that can compete with the explanation provided for answer choice A.

Answer B is correct, it's a protonated alcohol. However it's a really poor question IMO since the difference in acidity between A (pka ~1) and B (pKa ~ -1.7) is very small. Acetic acid would have been a better choice over Cl3COOH.

 

[Ibraiz]

CH3OH2+ is not an acetic acid though. CCl3COOH is an acetic acid. CCl3COOH is protonated too. Is CH3OH2+ more acidic because it has 2 'H' in it. With that being said H2SO4 would be a stronger acid than HCl04 because H2S04 has 2 H in it to deprotonate?

 

[Charmm]

CH3OH2+ is not an acetic acid though. CCl3COOH is an acetic acid. CCl3COOH is protonated too. Is CH3OH2+ more acidic because it has 2 'H' in it. With that being said H2SO4 would be a stronger acid than HCl04 because H2S04 has 2 H in it to deprotonate?

No, it is not the number of hydrogens we care about. An alcohol is less acidic than a carboxylic acid because the conjugate of acid is stabilized by resonance and inductive effect of an extra oxygen in carboxylic acid. however, in CH3OH2+ the oxygen has a formal charge of +1. A positive charge on oxygen is the key here. so here is how I teach the acidity or basicity

When you are comparing acidic hydrogens attached to the SAME ELEMENT
1) charge is the most important 2) resonance in conjugate is the second 3) inductive effect
So NH4+ is a stronger acid relative to NH3 and CH3O- is a stronger base than CH3OH ( it is also true for bases)
when comparing acidic hydrogens attached to different elements in a column of periodic table , size is the most important hence HI is stronger than HCl

When comparing acidic hydrogens attached to different elements in a row of periodic table , electronegativity is the most important so H2O is a stronger acid that NH3

 

[Charmm]

CH3OH2+ is not an acetic acid though. CCl3COOH is an acetic acid. CCl3COOH is protonated too. Is CH3OH2+ more acidic because it has 2 'H' in it. With that being said H2SO4 would be a stronger acid than HCl04 because H2S04 has 2 H in it to deprotonate?

No, both of these acids conjugates are stabilized by resonance
Compare S and Cl and tell me which acid is stronger😉

 

[Ibraiz]

No, both of these acids conjugates are stabilized by resonance
Compare S and Cl and tell me which acid is stronger😉

S is stronger acid as it can deprotonate easier than Cl as it is less electronegative, correct?

Also, for the original question on this post, are you suggesting that CH3OH2+ is more acidic because of its positive charge as charge is the first priority we look at. Even though CCl3COOH has more resonance (between C=0 and C-0) and inductive effect (because of 3 Cl attached) it still not stronger than CH3OH2+ because it lacks a positive charge?

Thanks

 

[Charmm]

S is stronger acid as it can deprotonate easier than Cl as it is less electronegative, correct?

Also, for the original question on this post, are you suggesting that CH3OH2+ is more acidic because of its positive charge as charge is the first priority we look at. Even though CCl3COOH has more resonance (between C=0 and C-0) and inductive effect (because of 3 Cl attached) it still not stronger than CH3OH2+ because it lacks a positive charge?

Thanks

Nope, since cl is more electronegative than s , HClO4 is a stronger acid

Regarding the original question, when comparing a charged and non charged , charge is even more important than resonance and inductive effect but there are exceptions always in chemistry, just try to understand the process don't memorize ,
Best of luck