[tasnim rahman]

In the diagram attached, the 3 resonance structures of arenium ion bonded to H electrophile are shown.I labelled the carbon atoms in one of the structures for convenience.

From the diagram, the A and B carbons, and D and E carbons, have a double bond between them for two of the resonance structures; whereas the B and C carbons, and C and D carbons, have a double bond between them for only one of the resonance structures. From this can we safely assume that in the resonance hybrid, the bond between the A and B carbons, and D and E carbons have a high percentage of double bond character; compared to the bond between the B and C carbons, and C and D carbons, which have a higher percentage of single bond character?

Also since the bond between A and B carbons (also between C and D carbons) are double bonds for 2 of the 3 resonance structures can we say that in the resonance hybrid, the charge between the A and B carbons is (2/3)(2) (as the single bond remains fixed and the 2 electrons of double bond become delocalized)? And that each of the carbon atoms has a charge of -(2/3).

[fledarmus]

From the diagram, the A and B carbons, and D and E carbons, have a double bond between them for two of the resonance structures; whereas the B and C carbons, and C and D carbons, have a double bond between them for only one of the resonance structures. From this can we safely assume that in the resonance hybrid, the bond between the A and B carbons, and D and E carbons have a high percentage of double bond character; compared to the bond between the B and C carbons, and C and D carbons, which have a higher percentage of single bond character?

That's a fair starting point, although you are making the assumption that all three of the resonance structures are equal in energy. In fact, the conjugation in your structures 1 and 3 should make them slightly more stable than your structure 2, so they would contribute slightly more to the actual structure.

Also since the bond between A and B carbons (also between C and D carbons) are double bonds for 2 of the 3 resonance structures can we say that in the resonance hybrid, the charge between the A and B carbons is (2/3)(2) (as the single bond remains fixed and the 2 electrons of double bond become delocalized)? And that each of the carbon atoms has a charge of -(2/3).

I'm not sure what you are trying to say here. Are you saying that all five of the carbon atoms have a negative charge?

The arenium has a single positive charge, which can appear on any of three carbon atoms (as you have drawn your resonance structures). If each of the structures contributes equally, those three carbons will each show about 1/3 partial positive charge. Depending how much the conjugation in 1 and 3 contribute to stability, the charge on carbon C will be slightly lower than 1/3, and on A and E slightly higher than 1/3. Carbons B and D are neutral in all of your resonance structures and should be neutral in the true structure.

[tasnim rahman]

Thanks a lot, fledarmus.

For the first one, why should structure structure 2 be less stable than the other two? Sorry, I did not get this part?

For the second one, I was trying to calculate the negative charge each carbon atom would possess, if we consider the 4 pi-bond electrons to be delocalized, along with the delocalization of the positive charge, based on the resonance structures. From there we calculate the net gain of positive charge for the three carbon atoms, by subtracting the negative charge they possess from the negative charge they should possess to be neutral.

Thanks again.

[orgopete]

After I got raked over arguing inductive effects over resonance, I am waiting to see how this is argued. If resonance predominates, you would think they would be equal. If sp2 carbons are electron withdrawing, then one might think C would be less stable?

[fledarmus]

I am thinking purely in terms of conjugation - forms 1 and 3 have conjugated dienes, which are slightly more stable in general than isolated dienes.

[tasnim rahman]

I am thinking purely in terms of conjugation - forms 1 and 3 have conjugated dienes, which are slightly more stable in general than isolated dienes.

..er...Why are conjugated di-enes more stable than isolated di-enes?