Between 3-chlorophenol and 3-fluorophenol, which one is more acidic?

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**


1. 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](https://doi.org/10.1021/jo00182a020).
2. 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](https://doi.org/10.1021/j100553a009).


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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.


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