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Between 3-chlorophenol and 3-fluorophenol, which one is more acidic?
In this case, I know that both F and Cl are electron-donating groups. Furthermore, I know that F is more electron-donating than Cl. But I can't figure out whether that makes 3-chlorophenol more acidic, or 3-fluorophenol.
Experimentally, the $\mathrm pK_\mathrm a$ values for 3-chlorophenol and 3-fluorophenol appear to be (perhaps unexpectedly) equivalent.
The table below1 shows the measured $\mathrm p K_\mathrm a$ values of substituted phenols in dimethyl sulfoxide (DMSO):
$$\begin{array}{ccc} \hline \text{substituent} & \sigma & \mathrm p K_\mathrm a \\ \hline \ce{H} & 0 & 18.03 \pm 0.04, \color{silver}{16.4} \\ \ce{p-CH3} & -0.170 & 18.86 \pm 0.05 \\ \ce{m-CH3} & -0.069 & 18.23 \pm 0.01 \\ \ce{p-Cl} & 0.227 & 16.74 \pm 0.03 \\ \color{red}{\ce{m-F}} & \color{red}{0.337} & \color{red}{15.88 \pm 0.07} \\ \color{red}{\ce{m-Cl}} & \color{red}{0.373} & \color{red}{15.83 \pm 0.05} \\ \ce{m-CN} & 0.56 & 14.76 \pm 0.05 \\ \ce{m-NO2} & 0.710 & 14.39 \pm 0.12, \color{silver}{13.75} \\ \hline \end{array} \\ \small\text{(Greyed out values are taken from ref 2.)}$$
In general, halides have slightly unusual effects on aromatic rings. Despite being highly electronegative (and deactivating), they actually donate into the pi system via their lone pair.
This donation means that the phenoxide generated upon deprotonation (i.e. the dissociation being measured by the $\mathrm pK_\mathrm a$ value) isn't as stabilised as it would be with a purely withdrawing group (such as the nitrile or the nitro groups).
References
Bordwell, F. G.; McCallum, R. J.; Olmstead, W. N. Acidities and hydrogen bonding of phenols in dimethyl sulfoxide. J. Org. Chem. 1984, 49 (8), 1424–1427. DOI: 10.1021/jo00182a020.
Chantooni, M. K., Jr.; Kolthoff, I. M. Comparison of substituent effects on dissociation and conjugation of phenols with those of carboxylic acids in acetonitrile, N,N-dimethylformamide, and dimethyl sulfoxide. J. Phys. Chem. 1976, 80 (12), 1306–1310. DOI: 10.1021/j100553a009.
Fluorine can donate its lone pairs to the conjugated ring. Chlorine cannot do that because of ineffective 3p-2p orbital overlap.
For more acidic compound, the electron density on the -OH group must be taken away from it. $-I$ effect (inductive effect) makes a compound more acidic.
Since fluorine and chlorine are both $-I$ groups, the acidic strength of 3-fluorophenol should be higher but due to its contribution to resonance ($+R$ effect) in the aromatic ring, 3-fluorophenol becomes less acidic.
