I know that when there are like charges on a resonating structure, the farther apart they are, more stable is the structure due to reduction in repulsive forces.Is the same true for opposite charges? which structure is more stable -
1)with positive charge on one C and negative on adjacent C
2)with positive charge on one C and negative charge on C separated by 4 carbons between them?
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$\begingroup$ I haven't seen this before but i think a hydrogen will shift from the positively charged carbon to the negative carbon... Thus neutralising both charges $\endgroup$– ULTIMATEGAMER07Commented Dec 17, 2017 at 8:35
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$\begingroup$ In general, the stability of resonating structure is of important within the hybrid of resonance. From a purely electrostatic point of view distance improves stability. A very long dicarboxyloc acid should have just a pKa value, to give you an idea. As far as I know the importance of the zwitterions you described should be negligible anyway.... $\endgroup$– AlchimistaCommented Dec 17, 2017 at 13:13
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$\begingroup$ A carbon with a negative charge adjacent to carbon with a positive charge will form a pi bond. $\endgroup$– SmapCommented Oct 9, 2020 at 17:50
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$\begingroup$ I thought about this as well, since e.g. the most stable canonical for the sulfate anion is at least yiilidic(formally doubly positively charged sulfur directly connected to formally singly negatively charged oxygens). $\endgroup$– Kanghun KimCommented Nov 4, 2021 at 1:49
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2 Answers
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Let's take the example of 1,3-butadiene:
In order to talk about the energies and stabilities, we need to look at the molecular orbitals of the compound. Look at this diagram from ChemLibreTexts
The two filled ground states, both have $\ce{\pi}$ bonds between $\ce{C_1}$ and $\ce{C_2}$, and $\ce{C_3}$ and $\ce{C_4}$ suggesting that separation of charges is not favorable.
The charged resonating structures with charges on $\ce{C_1}$ and $\ce{C_4}$ corresponds to $\ce{\Psi_3}$ in the MO-diagrams where the $\ce{\pi}$ bond is present between $\ce{C_2}$ and $\ce{C_3}$.
This observation questions which structure $\ce{\Psi_4}$ corresponds to. Since no $\ce{\pi}$ bonds are present, I reckon the corresponding resonating structure would be one with charges on each of the four carbons:
Conclusion: At least for the case of but-1,3-diene, structures with both charges placed on the $\ce{\alpha}$-Carbon (to the $\ce{\pi}$ bond) are more stable.
I also want to briefly talk about the MO-diagram of 1,3,5-hexatriene:
Here too, keeping the charges $\ce{\alpha}$ to a $\ce{\pi}$ bond seems more important than the separation between the charges. I wonder if the "transition" charged states even exist under normal conditions.
Reference
Francis A. Carey, Richard J. Sundberg - Advanced Organic Chemistry Part A. Structure and Mechanisms-Springer (2007)
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Criteria to determine stability of resonating structures in decreasing priority are listed below :
- Neutral resonating structures with complete octets are most stable
- Ionic resonating structures with complete octets of all atoms is the next criteria to determine stability of resonating structures
- Ionic structures in which carbocations are present (with incomplete octets) having unlike charges closer, is ranked at the third priority to determine stability.
- Finally, anions at more electronegative element are more stable.
So, according to the information you have provided, the resonating structure which corresponds to the first case is more stable (following the third point)
