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This was a practice question from an MCAT book whose answer I didn't really understand:

The normal reactivity of methyl benzoate is affected by the presence of certain substituents. Which of the following substituents will decrease methyl benzoate reactivity making it safer for transport?

a) $\ce{NO2}$

b) Hydrogen

c) $\ce{Br}$

d) $\ce{CH3}$

They give the answer as (a) because "in order to decrease the reactivity of benzene molecule, an electron withdrawing deactivating substituent must be used." However, I thought the reactive part of the molecule would be the carboxylic acid, and so I would've expected (d) to decrease the reactivity of the molecule because it's electron donating.

I suppose if the question meant the reactivity of the ring itself, then the answer is (a) since electron withdrawing substituents on a benzene ring lower its reactivity.

So my question is: How would one reasonably infer that the reactive part of this molecule is the benzene ring itself instead of the carboxylic acid?

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  • $\begingroup$ Methyl Benzoate is an ester, not a carboxylic acid. As far as reasonably infering the reactive part of the molecule, you can assume they are talking of ring reactivity when they ask which "ring substituent" will decrease reactivity, although they didn't specify ring in the question. I wouldn't think much of it as I've noticed a lot of ambiguity in my PCAT study book as well (it's a Kaplan). But technically speaking, electron withdrawing groups will decrease the reactivity of the ring portion of the molecule, thereby decreasing the reactivity as a whole. $\endgroup$ – KeatonB Aug 18 '16 at 21:14
  • $\begingroup$ You can also make the inference by looking at the answer choices, NO2 is the only strongly deactivating group as Br is capable of both donating and withdrawing electron density, CH3 is activating, and H is neither (it's what EWG and EDG are measured against) $\endgroup$ – KeatonB Aug 18 '16 at 21:18
  • $\begingroup$ @Ki11akd0g Whoops, even wrote it down as an ester but changed it in my head. I'll assume the question will be much less ambiguous then on the actual exam. Appreciate your insight! $\endgroup$ – Brenton Aug 18 '16 at 22:34
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    $\begingroup$ Methyl benzoate isn’t even something I would consider dangerous to transport, so the entire question is somewhat moot. $\endgroup$ – Jan Aug 18 '16 at 23:01
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    $\begingroup$ Toluene is dangerous to transport... let's add a nitro group to it to reduce the reactivity of the benzene ring! But why stop at one nitro group? Surely three nitro groups should be even better... $\endgroup$ – orthocresol♦ Aug 19 '16 at 8:46
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I’ve already addressed this in a comment, but now I went the lengths to check out the material safety data sheets of these compounds from Sigma Aldrich. My suspicions have proven correct: methyl benzoate is not a dangerous compound to transport so any recommendations to reduce its reactivity by adding a substituent must be moot.

In short, section 14 of the MSDSs as I access them contains transport information; and this entry for the compounds methyl benzoate (CAS 93-58-3), methyl 3-nitrobenzoate (CAS 618-95-1), methyl 2-methylbenzoate (CAS 89-71-4) and methyl 2-bromobenzoate (CAS 610-94-6) contains nothing but ‘not dangerous goods’ and the required iterations and variations of that theme to the different categories there.

Therefore, the question must be rejected as totally pointless: the starting compound is not considered a dangerous good for transportation and neither is any of the three possible substituted ones (one generic placeholder for each; I’m not going to check an additional six MSDS sheets).

Naturally, the only answer you can remove by deduction is b, because a hydrogen ‘substituent’ does not change the properties of the benzoate residue since hydrogen is already present there five times.

Considering we have found out that the compound is not dangerous to transport, how can we approach this question? Well, it’s hard. Concerning the reactivity of electrophilic aromatic substitutions — which is low anyway because of the deactivating ester substituent — of course an electron-withdrawing substituent would help; the nitro group suggests itself. On the other hand, concerning the reactivity of the ester group’s carbonyl function, only electron-donating substituents would help; the strongest being the inductive effect of the methyl group. So we are left with two possibilities and it is unclear which one we should follow.

Of course, neither the reaction with electrophiles nor the reaction with nucleophiles is inherently more dangerous than the other. We have again reached a vicious circle in which we cannot proceed at the risk of taking the wrong path.

Therefore, the best advice I can give is to ignore the question in the book. If it is given in an exam, challenge the exam for there is no reasonable answer to it.

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