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In Electrophilic Aromatic Substitution ($\text{S}_{\text{E}}\text{Ar}$) reactions groups that can donate an electron pair into the aromatic ring, e.g. $\ce{-OH}$ or $\ce{-NH2}$, are ortho-/para-directing.

In the case of $\ce{-NH2}$-groups one has to be cautious because if the $\text{pH}$ value of the solution is too low the $\ce{-NH2}$-group will be present mostly in its protonated form, $\ce{-NH3^+}$, which lacks a free electron pair and has a meta-directing effect because of its strong $-I$ effect. This brings up the question: What is the situation like with protonated $\ce{-OH}$ groups, $\ce{-OH2^{+}}$? In contrast to $\ce{-NH3^+}$ this group still has a free electron pair which could, in principle, interact with the aromatic ring ($\Rightarrow$ ortho-/para-directing) but it should also have a very powerful $-I$ effect ($\Rightarrow$ meta-directing).

I'd say, if the electron pair can be effeciently delocalised into the aromatic ring then its ortho-/para-directing power should outweigh the meta-directing power of the inductive effect. But I wonder if there even is any appreciable degree of delocalisation present because the mesomeric structures don't look very favourable:

Thinking about it from a frontier orbital perspective I would say: The orbital of the $\ce{-OH2^{+}}$ group's free electron pair will be much lower in energy than that of the $\ce{-OH}$ group's free electron pair. Thus, the energy difference between free electron pair's orbital and the aromatic ring's $\pi$-system might be too large to facilitate a good interaction. Therefore, I'd expect a protonated $\ce{-OH}$ group to be meta-directing.

But what is the actual situation when phenols or phenol ethers undergo ($\text{S}_{\text{E}}\text{Ar}$) reactions in strongly acidic solutions? Does one get the ortho/para or the meta product?

Edit:

I looked up the $\mathrm{pK}_{\mathrm{a}}$ value for protonated phenol: It is approximately $-7$. This makes me wonder if there are any reasonable (real world) situations where the solution is acidic enough to protonate phenol (or phenol ethers) completely (even $\ce{HI}$ with its $\mathrm{pK}_{\mathrm{a}}$ of $-10$ shouldn't be enough for that, right). I'm not sure if even sulfonation or nitration reach such low $\text{pH}$ values. But @Uncle Al's answer made me aware that I forgot about Lewis acids and I'd say that Lewis acids like $\ce{AlCl3}$ that coordinate strongly to oxygen might lead to a state that will be very similar to a $\ce{-OH2^{+}}$ group (please, correct me if I'm wrong and the coordination of Lewis acids is not strong enough to be compared to the $\ce{O-H}$-bond). If this is the case, then what about Friedel-Crafts reactions which make heavy use of Lewis acids: Would those Lewis acids interfere with the ortho-/para-directing properties of phenols? Does anyone have experience with the directional properties of a $\ce{-OH}$ group in the presence of a strong Lewis acid, maybe even in the context of Friedel-Crafts reactions?

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    $\begingroup$ Interesting question. Regarding the possibility of quantitatively protonating phenol, it's certainly doable with, say, fluorosulphuric acid or carborane superacids. Whether anyone does that with regularity, I don't know. I don't think that comparing values of $pK_a$ works too well because they are rather poorly defined for extremely strong or extremely weak acids/bases. It's probably best to compare them with some extension such as the Hammett acidity function. $\endgroup$ – Nicolau Saker Neto Feb 15 '14 at 16:51
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    $\begingroup$ This article from the master of superacidic media George Olah will probably have some insight into phenol protonation and reactivity. And here's what seems to be an even more precise reference. $\endgroup$ – Nicolau Saker Neto Feb 15 '14 at 16:52
  • $\begingroup$ @NicolauSakerNeto Thanks for the hints. I agree that $\mathrm{pK}_{\mathrm{a}}$ values in this regime shouldn't be taken too seriously. But I think, they are at least evidence that under usual reaction conditions, phenols won't be protonated completely and in those situations I don't have to worry about a possible meta-directing property of a phenol. $\endgroup$ – Philipp Feb 15 '14 at 17:01
  • $\begingroup$ @NicolauSakerNeto Thanks again. The second reference you provided seems to be exactly what I was looking for. It supports my view that protonated $\ce{-OH}$ groups are meta- rather ortho- / para-directing. But still information about the influence of Lewis acids on the directional properties of the $\ce{-OH}$ group would be interesting. Especially, since it would throw some light on how strong the coordination between oxygen and the Lewis acid is. $\endgroup$ – Philipp Feb 15 '14 at 17:13
  • $\begingroup$ Excellent, I'm happy to help. Perhaps you could consider making your own answer with what you've learned! $\endgroup$ – Nicolau Saker Neto Feb 15 '14 at 17:16
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O-protonated phenols direct in meta, according to an article on that topic. As it occurs, phenols can be, depending on acid, protonated on oxygen, but also on carbon, and even diprotonated (but not twice on the same atom), when most powerful mixtures of HF and SbF5 are used. O-protonated forms can be brominated, hydroxylated and alkilated. C-protonated forms don't posses such reactivity and are essentially Meisenheimer's complexes.

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A four-coordinate $\ce{sp^2}$ oxygen with two positive charges is a very high energy state. Aniline is a much weaker base than an aliphatic primary amine for leaking electron density into the ring. Phenol in turn will be a much weaker base than a primary alcohol. Deactivating the phenol will require a very Brønsted acidic medium indeed.

Lewis acids would be more effective. Anhydrous $\ce{AlCl3}$ is a powerful oxophile. Adding a mole would turn off all the phenols. One might have quite a time getting the phenol free of the aluminum during workup. Note the large number of LAH workup recipes to avoid intractable aluminum oxohydroxide gels during workup.

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  • $\begingroup$ Thanks for your answer. It made me aware of the very low pH necessary to protonate phenols completely. But unfortunately, you don't say anything concerning the ortho- / para- or meta-directing properties of protonated/Lewis-acid-coordinated Phenols. Could you add something concerning this topic to your answer. Please, have a look at the Edit in my question. $\endgroup$ – Philipp Feb 15 '14 at 16:38

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