[Mustafa28]

Hi guys, I was hoping you could help me out.

I missed 3 classes due to being unwell, and so I'm kind of lost as to the new assignment we have.

We have assignments every week online, so its not a particularly big part of the grade so have no fears that your helping me cheat  or something

Anyway, I have attached an image of the question to this post. Please check it out.

For question 1 for example. "Draw the structure of the predominant form of CH3COOH (pKa = 4 .8 ) at pH = 3"

I am kind of confused as to what they are asking here. We looked at the whole "ranking the most acidic proton" topic early in the course. It involved considerations like electron donating groups, conjugate base stability, etc. But that was because we were learning to predict which side of an acid-base reaction the equillibirum would favour.

However, until 3 classes ago, we were getting into aromatic reactions and spectroscopy. So i'm kind of confused as to why this acid-base stuff has popped up again, and why we have a full 15 question assignment on this topic.

So my questions are

1) Can you think of any possible link between such questions and aromatic's or spectroscopy?

2) How would I go about approaching this question? CH3COOH at pka 4.8 is a pretty strong acid. Its most acidic proton is on the carboxyl group, and not the Methyl group hydrogen (obviously). But what exactly do they mean by "predominant form", and why are they telling us the pH of it? How does the pH factor into the question?

Any pointers in the right direction would be very much appreciated. Perhaps even some recommended reading, or what chapter in an organic chemistry textbook this kind of material would be covered.

Thanks!

[Dan]

Hint: If you consider acetic acid, at what pH is it 50% dissociated? So at what pH does [AcOH] = [H⁺]?

[Borek]

If you have mixture of different forms, spectrum of each one will show together with others. Knowing which is a dominating form can help record spectrum of this particular form, unobscured by others.

Think about it - this is how two forms of methyl orange look like in solutions of different pH:



If you are interested in the red form, what pH will you select to record the spectrogram? High one, or low one?

[Mustafa28]

Thanks for the responses guys. It made me realize some general chemistry was going to be needed for these kinds of questions, and helped me pinpoint what specific areas in general chem I needed to review.  

I've written up a brief break-down of my thinking below. Please let me know any potential pitfalls I might face later or if this line of thinking covers my bases.
----------------------------------------------------------------------------------------------------------------
(1) The fact that they specify the pH hints that the pH is buffered.

(2) In a buffered solution, the pH will remain steady regardless of how much acid or base you throw into the mix.

(3) The Henderson-Hasselbalch equation helps us deal with buffered solutions, by setting up the relationship below.

pH = pKa + log  ( [conjugate acid]  /  [acid]  )

Where the pH refers to the pH of the buffered solution. pKa is a constant for the acid in question, and must be looked up because it depends on factors like temperature and pressure.

(4) In order to answer the types of questions I asked in the first post, it would make sense to rearrange the the Henderson-Hasselbalch equation.

[conjugate acid]  /  [acid]   = 10^{pH - pKa}

(5) So for example, in question 1 "Draw the structure of the predominant form of CH3COOH (pKa = 4 .8 ) at pH = 3", we wold first need to determine what the acid and the conjugate base is.

The acid is CH3COOH
The conjugate base is CH3COO-

In other words, the ability to determine the most acidic proton is a pre-requisite for these types of questions.


(6) Since we have been told the pKa of CH3COOH is 4.8, and the the pH of the solution is 3, we enter those into the rearranged Henderson-Hasselbalch equation

[RCOO-]  /  [RCOOH]   = 10^{3 - 4.8} = 10^-1.8 = 0.016

This means the denominator (rcooh) is larger than the numerator.

Therefore, in the buffered solution, there are more moles of RCOOH than there are RCOO-

And therefore, the equillibrium favours the re-formation of RCOOH rather than RCOO-

Therefore the Predominant form is RCOOH.

---------------------------------


Have I solved it?
on the right track?
or heading in the wrong direction?

Thanks

[Dan]

Yeah, that looks good to me - except you are calling AcO^- the conjugate acid - AcO^- is the conjugate base of AcOH.

Qualitatively, you can show from the HH equation that 50% dissociation occurs when pH = pK_a. Therefore if the pH<pK_a, then the protonated form (acid form) predominates, and if pH>pK_a then the conjugate base predominates.