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I would like to compare the basicity of ortho and para isomers of toluidine.After comparing the pKb values online the order of basic strength is:

p—toluidine> m—toluidine> aniline> o—toluidine

My reasoning: according to me ortho form should have been the most basic because while comparing there is no SIR, SIP, localised lone pair, proton sponge or mesomeric effect and in case of ortho and para forms the hyperconjugation is till same extent so we finally come down to inductive effect due to which the clearly the ortho form should be more basic. But it is not so, Why?

I saw a similar question before posting my question on stackexchange but it talked about SIR and SIP effect but I was thought that in case of small groups both these effects are not applicable and that was the sole purpose of posting this question.

Thanks!!

TeslaBolt
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  • Ever heard of the ortho effect? https://en.wikipedia.org/wiki/Ortho_effect – Waylander Aug 09 '22 at 20:23
  • ya but it is due to SIR and here as NH2 is not very big it won't be sterically hindered – TeslaBolt Aug 09 '22 at 20:26
  • The article I linked specifically discusses the toluidines case as an example of the ortho effect. Do you think you know better? – Waylander Aug 09 '22 at 22:07
  • @Waylander does that article mean that irrespective of the fact that group is small or big, any group on the ortho position increases the acidity for a substituted benzoic acid and decreases the basicity of substituted aniline. And also do we consider SIR applicable in presence of small groups like (-OH,-CN,-NH2)? cuz my professor told us that ortho effect is due to SIR which is not applicable when small groups are present. – TeslaBolt Aug 10 '22 at 06:45
  • Read the answers to this question https://chemistry.stackexchange.com/questions/87461/is-steric-inhibition-of-resonance-or-steric-inhibition-of-protonation-dominant-i – Waylander Aug 10 '22 at 07:41
  • Any group in the ortho position is to have SIP. In the cases of OH,CN & NH2 you also have H-bonding complicating the situation. I think your professor's understanding of this is flawed. – Waylander Aug 10 '22 at 07:42
  • @Waylander Thankyou after your article and jan's answer I am clear with this question. – TeslaBolt Aug 10 '22 at 07:50

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