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I'm getting conflicting answers when I want to find out the acidity of 3-fluoropropanoic acid and formic acid.

I argue that 3-fluoropropanoic acid is stronger acid than formic acid as it has a -I group (although at a distance) whereas formic acid doesn't. However looking at the pKa table suggests 3-fluoropropanoic acid is weaker that formic acid!

Can anyone tell me what other factors have I missed?

enter image description here

Harikrishnan M
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Akilan SS
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1 Answers1

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Factors influencing the acid strength of carboxylic acids

For starters, it should have been considered that the acid strength of carboxylic acids depends on

  1. electron releasing groups (which weaken the acid) / electron withdrawing groups (which strengthen the acid).

  2. Number of such groups (higher number of ERGs will lead to weaker acids, while higher number of EWGs will lead to stronger acids)

  3. The distance of the group from the carboxyl group (an EWG kept closer will lead to a stronger acid; an ERG kept further away will lead to a stronger acid.)

Inductive effect, due to the $\ce{-\color{red}{F}}$ group, is distance dependent, which means that the effect will become weaker as the group is placed further away. Check out the below example:

enter image description here

Apply a similar logic to the situation.

The reduction in $\mathrm{\color{red}{-I}}$ effect of fluorine is much more and because of that the formic acid turns out to be stronger.

Also, 3-fluoropropanoic acid has one EWG, but some ERGs before it [$\ce{\color{navy}{F}-\color{blue}{C2H4}-\color{magenta}{COOH}}$], and the alkyl group in between has some electron releasing tendency. There is competition between those two effects and $\ce{-\color{red}{F}}$ being further away from the COOH group leaving a $\mathrm{\color{green}{+I}}$ group in between acts as the differentiating factor.

Thus the order turns out in this fashion.

I would suggest you to read these paragraphs from our Chemistry reader: Effect of substituents on the acid strength enter image description here

Image sources:

  1. https://shorturl.at/qzFSY (a Google image)

  2. https://ncert.nic.in/textbook.php?lech2=3-5

This answer is derived from my learnings from NCERT, Class XII, Chemistry Volume II.

Harikrishnan M
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    But in general, there is some EWG in 3-fluoropropanoic acid, however there is no EWG in formic acid. So shouldn't fluoropropanoic acid be slightly more acidic than formic acid? And how do we account for 'wee bit stronger' formally? – Akilan SS Dec 25 '23 at 17:23
  • Question is edited to clarify. Also the quantifier 'wee bit' has been removed for prevention of confusion. – Harikrishnan M Dec 25 '23 at 17:35
  • But we don't tell the same argument when we want to find the acidity of Chloracetic acid or fluoroacetic acid right? We never say the -CH2- group's inductive effect alters the acidity of the acid. – Akilan SS Dec 25 '23 at 17:45
  • It depends on the context what factor we consider. In this case the halogen is attached to the compound as $\ce{X-CH2-COOH}$. Only the -I order matters in this case, as both are the same distance away.

    I will upload a snapshot from my chemistry textbook that gives the complete order of all carboxylic acids. Read whatever factors I have said, understand those factors completely and then have a look at the order.

     – Harikrishnan M Dec 26 '23 at 01:20
  • The edited answer should be able to satisfy all concerns. – Harikrishnan M Dec 26 '23 at 01:31