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I was trying to compare acidity of some organic compounds but can't understand some:

Why should 2,6-dimethyl-4-nitrophenol be more acidic than 3,5-dimethyl-4-nitrophenol?

I have been trying to figure it out using various factors like hyperconjugation and induction effect of Me group but its not working. Suppose i take H.C effect 2,6-dimethyl-4-nitrophenol be "less" acidic than 3,5-dimethyl-4-nitrophenol since it destabilizes the negative charge on conjugate base. But this logic is not working.

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In the 3,5-dimethyl isomer, steric effects come into play and influence the acidity.

Resonance structures like B in the figure below, that stabilize the phenoxide anion explain why 4-nitrophenol ($\ce{R=H}$) is more acidic than phenol. Resonance structure B requires that the nitro group be planar with the aromatic ring. If methyl groups are placed adjacent to the nitro group ($\ce{R=CH3}$), the steric interactions between the two methyl groups and the nitro group becomes large and forces the nitro group to rotate out of the aromatic plane, Hence resonance structure B can no longer contribute and the compound is less acidic.

enter image description here

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    $\begingroup$ So that's basically ortho effect? Thanks i got it now :-) $\endgroup$ – user14857 Apr 11 '15 at 19:45
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    $\begingroup$ Yes exactly, in this case it's a double ortho effect. $\endgroup$ – ron Apr 11 '15 at 19:45
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In3,5 dimethyl 4 nitro phenol , the nitro group is between two ortho methyl groups . Therefore due to this steric inhibition of resonance occurs and this nitro group is pushed out of plane by two methyl groups. Such that nitro group does not take part in the resonance. And there have no extra stable structure. That is why it is less acidic than2,6 dimethyl 4 nitro phenol.

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