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Which one of the following substituents at para-position is least effective in stabilizing the phenoxide ion $\ce{PhO-}$

(a)$\ce{-CH3}$

(b)$\ce{-OCH3}$

(c)$\ce{-COCH3}$

(d)$\ce{-CH2OH}$

According to me it should be $\ce{-OCH3}$ because of its mesomeric effect which will counter the mesomeric effect of $\ce{O-}$.

$\ce{-CH3}$ has a positive inductive effect but, the mesomeric effect of $\ce{-OCH3}$ should be stronger and destabilize the phenoxide ion to a greater extent

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Binary Geek
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1 Answers1

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Which one of the following substituents at para-position is least effective in stabilizing the phenoxide ion $\ce{PhO^{−}}$

Yes, the p-methoxy group is the least stabilizing of the choices. I'm not quite sure what you mean by

its mesomeric effect which will counter the mesomeric effect of $\ce{O^{−}}$ .

so let me add a few words of explanation for the choice.

In the figure below, I've drawn a few of the resonance structures possible for p-methoxyphenoxide (A-C). The methoxy group donates electrons through resonance and resonance structure B illustrates this effect. However, note that in resonance structure B we have placed two negative charges close to each other - this is very destabilizing. Therefore, as the methoxy group donates electrons through resonance it destabilizes the system. In resonance structure C, the electron withdrawing inductive effect of the methoxy group is stabilizing. However the resonance effect outweighs the inductive effect and overall the p-methoxy group is strongly destabilizing in the case of phenoxide.

Resonance structure D illustrates the p-methyl case. Methyl is very mildly electron releasing both through resonance and inductively, so it also destabilizes the phenoxide anion, but since its effect is weak, it is not as destabilizing as the p-methoxy substituent.

Finally, notice in the p-acetyl case we can draw resonance structure E which actually stabilizes the phenoxide anion via resonance. The p-acetyl group is also inductively stabilizing in the phenoxide example.

enter image description here

ron
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  • I meant that $\ce{O-}$ will try to donate electrons to the ring and to the methoxy group (and will be very successful in doing so).The methoxy group will try counter the effects of donation of electrons by $\ce{O-}$ by its mesomeric effect. – Binary Geek Mar 28 '15 at 16:12
  • In this answer, you explain why methoxy at para position will stabilize phenoxide more than methyl at para position. Doesn't this contradict the above answer? – Binary Geek Mar 28 '15 at 16:14
  • I would agree Ron, however the pKa's then seem counter intuitive, since pKa of p-cresol = 10.26 and pKa of p-Methoxyphenol = 10.16. (http://pubchem.ncbi.nlm.nih.gov/compound/p-cresol#section=Surface-Tension and http://research.chem.psu.edu/brpgroup/pKa_compilation.pdf). Or am I not seeing something? There differences are very small anyhow. – Jori Mar 28 '15 at 16:15
  • Thanks for clarifying. That is correct, the phenoxide oxygen does donate to the ring, but it can't donate to the methoxy oxygen. Try to draw a resonance structure to show this and you wind up with 10 electrons around the methoxy oxygen. – ron Mar 28 '15 at 16:16
  • No, I explain why methoxy destabilizes. Resonance structures like B, where the methoxy donates electron density into the ring, place 2 negative charges close to one another - destabilizing. – ron Mar 28 '15 at 16:19
  • @Jori This question is specific to phenoxide anion. Pka's involve comparison of substituent effects on the anion and the neutral phenol (e.g. you must compare both reactant and product, here we're just looking at the "product"). – ron Mar 28 '15 at 16:22
  • I agree that methoxy destabilizes, but the question was to compare the amount of destabilization. In this answer, you explain the reason why p-methoxyphenol is more acidic than p-cresol. Wouldn't that imply that methoxy stabilizes phenoxide more than a methyl group? – Binary Geek Mar 28 '15 at 16:23
  • In the earlier question, I failed to include an analysis of the starting phenols. I will rework the earlier answer. – ron Mar 28 '15 at 16:33
  • What exactly do you mean by "starting phenols"? – Binary Geek Mar 28 '15 at 16:36
  • When you are comparing the acidity of compounds you must compare the effect of the substituent on both the neutral (reactant) phenol and the ionized phenol (product). – ron Mar 28 '15 at 16:38
  • So this answer is correct and the other one is wrong? – Binary Geek Mar 28 '15 at 16:39
  • Yes, the other answer is incomplete. I cannot delete it unless you remove the acceptance. If you do that, I will delete it. – ron Mar 28 '15 at 16:41
  • Let us continue this discussion in chat. – Binary Geek Mar 28 '15 at 16:42