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I was reading about $\mathrm{p}K_\mathrm{a}$ values of 2-fluorobenzoic acid and 4-fluorobenzoic acid and found that the latter had a higher value which was surprising to me. I think that intramolecular hydrogen bonding should dominate all the other effects in 2-fluorobenzoic acid as mentioned in the following figure and thus it should have a higher value:

2-fluorobenzoic acid

What is wrong with my logic?

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    $\begingroup$ See ortho effect in benzoic acid. $\endgroup$
    – V.G
    Commented Apr 1, 2021 at 9:29
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    $\begingroup$ @LightYagami I don't think ortho effect has a role to play here. Fluorine has too small an A Value of 0.15 compared to the groups which prominently show ortho effect, and the crystal structure here shows the cooh group almost in the plane of the ring. $\endgroup$
    – Box Box Box Box
    Commented Apr 1, 2021 at 11:15
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    $\begingroup$ @Rishi Also note that 2-Fluorophenol is more acidic than phenol even after having intramolecular H-bonding because of the fact that F has a strong -I effect. So I think even here the "distance-dependent" inductive effect has taken over the H-bonding. $\endgroup$
    – Neha
    Commented May 29, 2021 at 9:30
  • $\begingroup$ @LightYagami ortho effect won't come into play here because of the small steric size of fluorine. $\endgroup$
    – Neha
    Commented May 29, 2021 at 9:32

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You seem to be asking the question "Why does this intramolecular hydrogen bonding not cause a considerable effect to the $\mathrm pK_\mathrm a$ values of these compounds?"

TL;DR That is because this type of bonding is only present in a specific conformer only and this type of conformer of the compound 2-fluorobenzoic acid exists in low population levels, 2%$^{[1]}$ to not affect the ${\mathrm pK_\mathrm a}$ of the compound considerably.

Regarding the conformers of the 2-FluoroBenzoic Acid


A throughout conformational search on the B3LYP/6-311++G(d,p) potential energy surfaces of the studied molecules (the mono- and di- ortho fluoro- or/and chloro- substituted benzoic acids, plus the parent benzoic acid, for completeness) was undertaken. The identified conformers are presented$^{[2]}$enter image description here


Here both the cis conformers are planar and low energy while only one trans conformer is planar while both of them are relatively high energy of which Trans II is of relatively lower energy.

The two cis conformers have rather similar energies, with cis-II (where the fluoro-substituent stays the same side of the molecule as the acid oxygen atom and a C-H . . . O= stabilizing interaction exists), being the most stable form. The cis-I conformer has a relative energy of 2.82 kJ·mol−1 . In this conformer, the C-H . . . O= interaction present in cis-II is replaced by a weaker stabilizing C-H . . . OH interaction. The C-H . . . O= interaction is more efficient in stabilizing cis-II than the C-H . . . OH interaction in stabilizing cis-I due to the more favorable localization of the interacting carbonyl oxygen lone electron pair, which stays in the molecular plane, whereas in cis-I both lone electron pairs of the acid oxygen atom are out of the molecular plane. On the other hand, the interactions between the fluoro-substituent and the oxygen atoms, which are of repulsive nature, also favor a lower energy for cis-II compared to cis-I. This can be rationalized in similar terms as for the interactions involving the ortho hydrogen atom discussed above. In cis-I, the F . . . O= repulsion is stronger than the F . . . OH repulsion in cis-II because the interacting lone electron pair of the carbonyl oxygen atom is in the same plane as the fluorine atom in the former conformer, while the lone electron pairs of the acid oxygen atom in cis-II are both out of the plane in relation to the fluoro-substituent.$^{[2]}$


It is important to note that though the population level of Trans II are low, it is still a stable compound and nevertheless has been detected.$^{[3]}$

So.. Why is 2-fluorobenzoic acid more acidic than 4-fluorobenzoic acid?


I could not find any research for this but I could make an educated guess that the acidity of 2-fluorobenzoic acid comes from the similar stabilization as that in salicylic acid.$^{[4]}$ in the conformer cis II as shown below:-

enter image description here

References:

  1. Kuş, N.; Fausto, R. Effects of the matrix and intramolecular interactions on the stability of the higher-energy conformers of 2-fluorobenzoic acid. J. Chem. Phys. 2017, 146 (12), 124305. [DOI: 10.1063/1.4978794] (https://doi.org/10.1063/1.4978794).
  2. Ogruc Ildiz, G.; Fausto, R. Structural Aspects of the Ortho Chloro- and Fluoro- Substituted Benzoic Acids: Implications on Chemical Properties. Molecules 2020, 25 (21), 4908. DOI: 10.3390/molecules25214908.
  3. Daly, A. M.; Carey, S. J.; Pejlovas, A. M.; Li, K.; Kang, L.; Kukolich, S. G. Gas phase measurements of mono-fluoro-benzoic acids and the dimer of 3-fluoro-benzoic acid. J. Chem. Phys. 2015, 142 (14), 144303. DOI: 10.1063/1.4917031.
  4. Dunn, G. E.; Penner, T. L. Effect of intramolecular hydrogen bonding on the relative acidities of substituted salicylic acids in benzene solution. Can. J. Chem. 1967, 45 (14), 1699–1706. DOI: 10.1139/v67-274.
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