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I am facing difficulty in the following problem.

What is the decreasing order of acidity of the following phenol derivatives?

Phenols

In my view benzene-1,2-diol 1 should be most acidic because $-\ce{OH}$ is a strong $-I$ group thereby it will stabilise the negative charge on oxygen formed after removal of a proton.

$\ce{OCH3}$ is a $+R$ and $-I$ group. Near the carbanion $-I$ effect should be more effective. Hence the second most acidic should be 2-methoxyphenol 2. However the answer key gives it as 4-methoxyphenol 3.

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The acidity order should be 1 > 2 > 3 as they have pKa values of 9.48, 9.98, and 10.21 respectively.

+R effect of $\ce{-OCH3}$ is more significant than +R effect of $\ce{-OH}$

In case of 1, -I effect works at -ortho position and also +R effect works at -ortho position.

In case of 2, -I effect works at -ortho position and also +R effect works at -ortho position. Net effect is +R effect as +R effect is dominant than -I effect.

In case of 3, -I effect works at -para position and also +R effect works at -para position. Net effect is +R effect as +R effect is dominant than -I effect. But this +R effect is more dominant than in case 2 because -I effect is more dominant at -ortho position than at -para position but+R effect of equally effective at both -ortho and -para positions.

As a result of which, acidity is reduced to greater extent in the third compound compared to the second compound.

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  • $\begingroup$ How do you know that +R effect is more dominant in case b). $\endgroup$ – Pink Dec 26 '16 at 6:51
  • $\begingroup$ If your assumption is taken as granted then would be the relative acidic strength of o-,m-,p-methoxy benzoic acid.this accordingly will be then m->o- > p-. $\endgroup$ – Pink Dec 26 '16 at 7:00
  • $\begingroup$ Yes. It is not I am saying for granted, it is general organic chemistry. Also +R effect is not meaningful at meso position. $\endgroup$ – Resorcinol Dec 26 '16 at 8:15
  • $\begingroup$ Answer to the first comment, I have very clearly explained in my answer. Please read the answer very carefully $\endgroup$ – Resorcinol Dec 26 '16 at 11:14
  • $\begingroup$ With regards to paragraphing, you need to press Enter twice to make a line break. $\endgroup$ – orthocresol Jan 7 '17 at 8:57

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