Lesson 8 of 13 · 8 min
Resonance in organic molecules
NCERT § "Resonance Structure"
Ethanol and acetic acid both carry an O–H. Ethanol, with a pKa near 16, is not acidic in everyday terms; acetic acid, pKa 4.76, is sour enough to pickle a mango. The difference is a factor of about 10¹¹, and it lives entirely in what happens after the proton leaves.
The lesson in notes
In short
When one Lewis structure cannot describe a molecule, several contributing structures are drawn; the true molecule is their resonance hybrid.
All six C–C bonds in benzene measure 139 pm, a value lying between a C–C single bond (154 pm) and a C=C double bond (134 pm), which shows the π electrons are delocalised.
Contributing structures must have the same positions of nuclei and the same number of unpaired electrons.
The more stable a contributing structure, the more it contributes: structures with more covalent bonds, octets on all atoms, less charge separation and negative charge on the more electronegative atom are favoured.
Resonance energy is the difference between the energy of the real molecule and that of the most stable contributing structure; resonance always lowers the energy.
In the carboxylate ion (e.g. acetate), the negative charge is spread equally over both oxygens and the two C–O bonds are identical.
In the allyl cation the two contributing structures are equivalent, so they contribute equally and the charge is shared by the two end carbons.