According to my school textbook, carbon atom of the carbonyl group benzaldehyde is less electrophilic than that of propanal because polarity of the carbonyl group is reduced in benzaldehyde due to resonance.

I can't understand how. If I draw the resonating structures, the oxygen atom takes the delocalised electron and not the carbon atom. So, resonance shouldn't affect the electrophilicity of the carbon atom. Moreover, benzene being a -I group, induces a partial positive charge on the carbon.

What is the correct reason?

  • $\begingroup$ Observe the correct resonance structures properly. See this i.stack.imgur.com/fK7LM.jpg $\endgroup$ – Soumik Das Mar 8 at 18:23
  • $\begingroup$ @SoumikDas Okay, but doesn't the carbon attached to the carbonyl get a positive charge in the last resonating structure? So, its electrophilicity should increase? Honestly, I don't exactly understand how resonating structures affect electrophilicity or nucleophilicity, so should I change my question to that? $\endgroup$ – Anubhab Das Mar 8 at 19:03
  • $\begingroup$ I also didn't see the last structure exactly,instead of charge seperation like that,it will be a $\ce{C=O}$ bond there. So, forget about increase of electrophilicity, it will be less definitely.Now, see what resonating structures are basically delocalised $\pi$-electron density structures. If you have more resonating structures concerning the carbonyl carbon, in the resonance hybrid the $\pi$-electron cloud will be more towards the carbonyl, and due to such enhanced $\pi$-cloud, it will be difficult for the Nucleophile to approach resulting in decrease in electrophilicity. $\endgroup$ – Soumik Das Mar 8 at 19:19
  • $\begingroup$ @SoumikDas Oh I got it. I didn't know I was supposed to think in terms of $\pi$ electron cloud shift. Btw, is there a way to justify this decrease in electrophilicity just the way increase in electrophilicity in say, phenol, is justified (the lone pair of electrons of oxygen are delocalised in the ortho and para positions, making those positions more electrophilic)? $\endgroup$ – Anubhab Das Mar 8 at 19:49
  • $\begingroup$ Firstly, the ortho and para positions of Phenol become rich in electron density and become more nucleophilic (not electrophilic). So, first clarify this. And the easiest way to look for enhanced or diminished electrophilicity is to observe the structure of the molecule. If there is more $\delta +$ charge on any atom in native state or in resonance hybrid, that centre is electrophilic, and if the charge on some other species is $\delta - $, it tends to become nucleophilic.So, simply, decrease of $\delta +$ charge or increase in $\delta -$ results in decrease in electrophilicity. $\endgroup$ – Soumik Das Mar 9 at 4:50

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