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Which is more reactive towards electrophilic substitution, meta-cresol or para-cresol?

Both oxygen and methyl are activating groups. I don't know on what basis do I compare their reactivity. There's just a change in the position of one of the group.

My book says meta-cresol is more reactive and the reason given was

If two groups are working for the same effect at the same time, then their effect decreases.

I don't understand the reasoning and hence the answer. Any help would be appreciated.

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  • $\begingroup$ Mono- and double-substitutied benzenes react in pattern of preference during electrophilic substitutions. Assuming ortho-cresol, you still have four different sites where the reaction may happen. So we speak about which position(s) here? $\endgroup$
    – Buttonwood
    Nov 1, 2019 at 16:34
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    $\begingroup$ @Buttonwood Well, nothing was mentioned about the positions in the question. It just asked which is more reactive towards electrophilic substitution. Maybe it considers all the positions to be equivalent(At least for answering this question). By the way, we are talking about meta-cresol and not ortho-cresol. $\endgroup$
    – Tony
    Nov 2, 2019 at 10:09
  • $\begingroup$ @Tony: Agreed about my err on o, m, p-cresol. In terms of positions and their relative ease to react, I had in mind a scheme like «rates on nitration» on www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/benzrx1.htm. Unable to localize a source now, I recall there are similar ones for already double-substituted benzene (like the cresols) undergoing a third electrophilic substitution since sometimes the influence of the two may be synergistic. $\endgroup$
    – Buttonwood
    Nov 2, 2019 at 13:21
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    $\begingroup$ Can anybody explain why is my post on "hold"? $\endgroup$
    – Tony
    Nov 3, 2019 at 6:42
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    $\begingroup$ @Tony A reference/source is always required when something is quoted, otherwise this may be considered a plagiarism. It doesn't matter whether the textbook is available online or not. Every textbook should at least have an author, title, publisher, year and page numbers. $\endgroup$
    – andselisk ♦
    Nov 3, 2019 at 14:32

1 Answer 1

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To start, it is important that you understand that the methyl and hydroxy groups are activating only in the ortho and para positions. For example, if I were to substitute plain phenol, which contains an activating hydroxy group, which position would my electrophile (in this a bromine electrophile) add to? You can see that the intermediary sigma complex involves a positive charge. When the bromine attacks at the ortho or para (I did not include a diagram for the para attack, but it is similar to the ortho attack) positions, it is stabilized by an additional resonance structure provided by the oxygen lone pair. In the case of the meta attack, as you can see in the diagram below, this does not happen, and the intermediate is left with one less resonance structure.

enter image description here

enter image description here

Most activating groups will have the same directing effects as shown with the activating hydroxyl group above (para-ortho directors), so a methyl group, which contains no lone pair (and thus no additional resonance structures) will still stabilize the intermediate by donating electron density in much the same way as if an additional resonance structure was present.

Now getting back the para and meta cresol, the meta cresol contains two activating groups

enter image description here

Both of these activating groups activate the same positions, so this combination of activating effect will make the meta cresol more reactive than para-cresol. In para-cresol, the two groups do not work to activate the same positions, so, no matter which carbon is substituted, it will not be activated by both groups simultaneously like in meta-cresol.

enter image description here

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