[justswimmingly]

I’m tutoring ochem this summer, and my students had a question from the professor about ranking the acidity of various compounds. Most were easy enough, but they were confused about whether 2-chloropentanoic acid or 3-fluoropentanoic acid was more acidic. Their professor said 3-fluoro was more acidic, and they asked me why, and I didn’t really have a good answer. I told them it’s because fluorine is so electronegative that even 2 positions away, the inductive effect still makes it a stronger acid than chlorine. But the truth is, if I had been answering the question, I probably would have said that 2-chloropentanoic acid is the most acidic, because I was always taught that distance is the most important factor with the inductive effect. Is the professor right? I was trying to find pKa values online to answer for myself but couldn’t find any. And what’s the best way to approach these ranking questions where the acids differ in both EWG and position of EWG?

[Borek]

I agree this is dubious, two competing factors changing at the same time. The only way to say for sure is to compare pKa data (no, I don't have a source at hand).

[Meter]

Hard to say, but my intuition strongly suggests that distance loses to electronegativity. You are looking at the change in electronegative effects over the distance (approximately) of a C-C bond length (~1.5 Å), and electrostatic effects decay quite fast with distance. 

[justswimmingly]

Hard to say, but my intuition strongly suggests that distance loses to electronegativity. You are looking at the change in electronegative effects over the distance (approximately) of a C-C bond length (~1.5 Å), and electrostatic effects decay quite fast with distance.

This is what I was thinking. I decided to look online again to see if the pKas were anywhere and I did find values for 2-chlorobutanoic acid (2.95) and 3-fluorobutanoic acid (3.85). Those were from someone’s random Chegg assignment, so who knows how accurate they are, but it does seem like distance wins.

[rolnor]

I think he is dead-wrong, the distance kills the effect of fluorine. It would be nice if someone could find the pKa for these two acids.

[Babcock_Hall]

The pK_a of 2-chloroacetic acid is 2.86 (thermodynamic), and the pK_a of 2-fluoroacetic acid is 2.66.  This makes me think that the proximity of the halogen is the more important effect.

[rolnor]

The pK_a of 2-chloroacetic acid is 2.86 (thermodynamic), and the pK_a of 2-fluoroacetic acid is 2.66.  This makes me think that the proximity of the halogen is the more important effect.

Great, yes, the 3-fluoro can hardly be stronger acid than the 2-chloro then