6
$\begingroup$

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.

$\endgroup$
8
$\begingroup$

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

$\endgroup$
  • 1
    $\begingroup$ So that's basically ortho effect? Thanks i got it now :-) $\endgroup$ – user14857 Apr 11 '15 at 19:45
  • 1
    $\begingroup$ Yes exactly, in this case it's a double ortho effect. $\endgroup$ – ron Apr 11 '15 at 19:45

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy