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Of these two molecules, which one would be expected to have the most deshielded signal?

First off, I think the wording of this question is wack because neither of these molecules has only one NMR signal.

In any case it's clear that the question is asking about which molecule has the highest degree of electron withdrawal ...

I chose E, because I was looking at the benzene's signals. This one has an alkyl group, which is a slight inductive donor, thereby shielding the benzene's protons (slightly). This one also has a benzylic carbonyl group, which, according to my teacher, is only a resonance withdrawer and not in any form a resonance donator. Based on this, I chose E as the more de-shielded.

The other group similarly has a C=O resonance withdrawing group, but a resonance donating -OH group. Given that the resonance effect usually trumps the inductive effect, I said that E is still the more deshielded of the two.

I realize I might have now erred in limiting my analysis of shielding to only the hydrogens on the benzene. In E, the methyl's hydrogens are fairly well-shielded still, being connected to a carbon (albeit an sp2 hybridized carbon). On the other hand molecule F has a long linear chain; the hydrogens toward the end of that linear chain will have well-shielded signals! F's linear chain is also longer than E's linear chain ...

Frankly I think this is a horrible question but in any case the answer is apparently "F" ... do you agree?

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    $\begingroup$ I'm fairly sure that the proton giving the signal with the highest chemical shift (i.e., the most deshielded proton) is in fact the phenolic proton of molecule F. The presence of an EWG in the para- position likely contributes to it having a particularly downfield signal. $\endgroup$
    – Greg E.
    Commented Sep 29, 2014 at 2:59
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    $\begingroup$ Perhaps the question means, in each molecule, which proton is most deshielded? Draw resonance structures involving the carbonyl and the aromatic ring (E and F) and the hydroxyl group and the aromatic ring (F). Which aromatic carbon atoms wind up with a positive charge? Which protons are attached? These protons are probably the most deshielded. $\endgroup$
    – ron
    Commented Sep 29, 2014 at 3:00
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    $\begingroup$ @GregE. 's comment is correct. But if that is the answer they were looking for then, IMO, it is a bad question. OH signals are all over the place, only with great care and attention to detail can you see them way (10-12 ppm) downfield. $\endgroup$
    – ron
    Commented Sep 29, 2014 at 3:13
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    $\begingroup$ @Dissenter, aldehydic protons can sometimes be seen in that range (10-12 ppm), as can phenolic protons when the ring is conjugated to strong EWGs, along with those attached to various other less common functional groups (some imines, enols, etc.). $\endgroup$
    – Greg E.
    Commented Sep 29, 2014 at 3:27
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    $\begingroup$ This is the typical result for phenol where traces of water haven't been removed, see chemicalbook.com/SpectrumEN_108-95-2_1HNMR.htm $\endgroup$
    – ron
    Commented Sep 29, 2014 at 3:30

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I don't believe there is anything really wrong with this question. Yes, both molecules have multiple signals, but one of them contains a signal which corresponds to it being the most deshielded. In fact, one of these, molecule F, contains the most deshielded signal AND the most shielded signal.

"In any case it's clear that the question is asking about which molecule has the highest degree of electron withdrawal ..." not necessarily so. The correlation between electron withdrawing groups and shielding/deshielding is a strong rule of thumb, but can become complicated with aromatic systems and competing electronegativities of substituents. In any case, you need to be looking at parts of the molecule; shielding is quite a localised effect, and as I said above, molecule F actually has the most shielded AND deshielded protons.

In any case, if you consider the electronegativity of oxygen; a proton attached to this will be strongly deshielded. Stick them on an aromatic ring para to a carbonyl group, and you will generate a resonance system like below:

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In this case, the phenolic proton is by far the most deshielded of all protons in the molecules shown; I'd expect a chemical shift of over 10ppm. In response to some of the comments about -OH signals being over the place; it is important not to confuse raw chemical shift and averaged chemical shift resulting from chemical exchange with other labile protons, such as the (often observed) exchange of phenol protons with water. In these cases, the resulting chemical shift depends on the relative concentrations of the exchanging species, the exchange rate, and the original chemical shifts of the non-exchanging species.

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