Acids Bases

[predental89]

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I had a quick question please help explain!!!

Consider acetic acid, pKa= 4.34. when pH = 2 which is present?

a) Acetate Ion
b) Protonated acid
c) Neutral Ion
d) Zwitterion
e) None of these









Answer is C, but I don't understand how this works!! 😕

 

[needzmoar]

Consider acetic acid, pKa= 4.34. when pH = 2 which is present?

a) Acetate Ion
b) Protonated acid
c) Neutral Ion
d) Zwitterion
e) None of these


Firstly, I think they meant Neutral Acid for answer choice C.
Secondly, I'm going to assume that there are only molecules of acetate and acetic acid present in the solution.

Use the Henderson–Hasselbalch equation and you'll see that in order for the solution's pH to be 2, there must be more molecules of acid than base.

pH = pKa + log (acetate ion / acetic acid)

 

[predental89]

ok thank you also one more question...can you explain a little about how aromaticity and acidity and basicity are connected? I don't understand how something would be more acidic just because when you deprotonate it results in an aromatic ion.

 

[Demps]

ok thank you also one more question...can you explain a little about how aromaticity and acidity and basicity are connected? I don't understand how something would be more acidic just because when you deprotonate it results in an aromatic ion.

It depends on the substituents on the aromatic ring.

Electron donating group (EDG) makes it more basic
Electron withdrawing group (EWG) makes it more acidic

watch Chad's, it will be very clear.

 

[JustwantDDS]

so where does the 'neutral' in answer choice c come from?

Consider acetic acid, pKa= 4.34. when pH = 2 which is present?

a) Acetate Ion
b) Protonated acid
c) Neutral Ion
d) Zwitterion
e) None of these


Firstly, I think they meant Neutral Acid for answer choice C.
Secondly, I'm going to assume that there are only molecules of acetate and acetic acid present in the solution.

Use the Henderson–Hasselbalch equation and you'll see that in order for the solution's pH to be 2, there must be more molecules of acid than base.

pH = pKa + log (acetate ion / acetic acid)

 

[Demps]

I kinda just memorized this but

for Carboxylic Acid or alcohol group. They are neutral when pKa > pH, but ionized or deprotonated when pKa < pH

For Amino group. They are protonated when when pKa > pH, but neutral when pKa < pH

 

[needzmoar]

ok thank you also one more question...can you explain a little about how aromaticity and acidity and basicity are connected? I don't understand how something would be more acidic just because when you deprotonate it results in an aromatic ion.

Are you asking how substituents affect acidity/basicity or how aromaticity affects acidity/basicity

 

[predental89]

Are you asking how substituents affect acidity/basicity or how aromaticity affects acidity/basicity

how aromaticity affects acidity/ basicity

 

[needzmoar]

how aromaticity affects acidity/ basicity

Ok, lets look at the classic example of pyrollidine and its aromatic cousin, pyrrole.

View attachment predent.gif​

The basicity of amines are determined by how available the lone pairs are to serve as a lewis base. If the electrons are 'tied' to some sort of system, they're not as available. Pyrole is not very basic at all, its pKa is around 17-20. It's not basic because the lone pair of nitrogen is tied to the 6 pi aromatic system. Pyrollidine, on the other hand, has a freely available pair of electrons. It is very basic.

edit: avaible = available ...I'm too lazy to fix it lol

 

[predental89]

Ok, lets look at the classic example of pyrollidine and its aromatic cousin, pyrrole.

View attachment 17539​

The basicity of amines are determined by how available the lone pairs are to serve as a lewis base. If the electrons are 'tied' to some sort of system, they're not as available. Pyrole is not very basic at all, its pKa is around 17-20. It's not basic because the lone pair of nitrogen is tied to the 6 pi aromatic system. Pyrollidine, on the other hand, has a freely available pair of electrons. It is very basic.

edit: avaible = available ...I'm too lazy to fix it lol

wow thank you so much, the pictures deff helped with my understanding I really appreciate it!! So how do you know if the lone pair is tied to a system? if there are other double bonds within the ring?

 

[yappy]

Okay, I dont get how the convo went from acetic acid to aromatic compounds but when acetic acid is protinated it is not an ion like hydronium it is a neutral compound. Hence C.

 

[needzmoar]

wow thank you so much, the pictures deff helped with my understanding I really appreciate it!! So how do you know if the lone pair is tied to a system? if there are other double bonds within the ring?

Well, if you have a heteroatom within in a ring, is has a choice. If the pair of electrons are 'tied' to the system, does it lead to greater stability (I.E. will it be aromatic)? If it destabilizes the system, the lone pairs would much rather sit in an sp3 hybridized orbital (not tied to pi-cloud). Since I can't sleep here are some examples along with an acid example. I'm sure there are typos so if you don't understand anything just ask. good night sdn

View attachment predent-aromatic tutorial.pdf

 

[predental89]

Sweet thank you so much! I really appreciate it, I will deff let you know if anything else comes up!