Hi researchers,
I have a series of isomeric aromatic compound(ortho, meta and para) where both substituent are electro donating group. experimental test shows that ortho give high acidity strength follow by meta and lastly para position. The question is, how can explain this result? Does dipole moment give influence on the their acidity behavior?
thanks.
Are you at liberty to give more information on the structure? Ultimately, the acidity of a compound relates to the stability of the anion. The more stable the anion, the more easily charge is distributed (through resonance, typically), the more acidic the compound. Given that the substituent you discuss is electron donating, the fact that the ortho isomer is the most acidic suggests that some structural feature contributes to the anion stability.
I suggest it might be worthwhile for you to look up the subject of Hammett constants. These are values assigned to functional groups that can be used to explain and predict the acidity of substituted benzoic acids. This approach has been applied to many other structure-activity relationships too and works reasonably well for functional groups meta or para to the site of interest. However, it tends to break down for ortho substituents owing to steric effects, e.g. hindrance, and/or electronic effects, e.g. intramolecular hydrogen bonding, although one cannot make specific comments about your case without knowing the identity of the series of compounds.
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