Organic - Separation of Charge?

[AmirTimur]

"Resonance contributors that minimize separation of charge are better than those with a large separation of charge".

Q: What do they mean by separation of charge? I'm very confused.

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[mcatcram]

I don't really know how to explain this, it's kind of something that you just know, but I'll explain with example. You want the amount of charge on a molecule to be minimized. So if the option is having 2 negative and 2 positive charges on a structure vs 1- charge an one positive charge you pick the one with the smaller separation of charge (the latter). Also you want to spread out charge as much as possible, you don't want two negatives right next to each other if you can have the two negative charges at the opposite sides of a molecule. Also pay attention to the which atom has a higher electronegativity, if it does it will be more likely to hold a negative charge. if you have the option of putting a negative charge on an oxygen or a sulfur you would put it on the oxygen and that would be the more valid resonance structure. If you have 2 resonance structures and one can be made by having two negatively charged oxygens connected to say a neutral nitrogen, your other option would be to have one of these oxygens double bonded to nitrogen. and nitrogen will be positive and only one oxygen will be negative.

 

[AmirTimur]

I don't really know how to explain this, it's kind of something that you just know, but I'll explain with example. You want the amount of charge on a molecule to be minimized. So if the option is having 2 negative and 2 positive charges on a structure vs 1- charge an one positive charge you pick the one with the smaller separation of charge (the latter). Also you want to spread out charge as much as possible, you don't want two negatives right next to each other if you can have the two negative charges at the opposite sides of a molecule. Also pay attention to the which atom has a higher electronegativity, if it does it will be more likely to hold a negative charge. if you have the option of putting a negative charge on an oxygen or a sulfur you would put it on the oxygen and that would be the more valid resonance structure. If you have 2 resonance structures and one can be made by having two negatively charged oxygens connected to say a neutral nitrogen, your other option would be to have one of these oxygens double bonded to nitrogen. and nitrogen will be positive and only one oxygen will be negative.

Thanks a lot for the input!

However, does it matter if we have two positive charges next to each other, two negative charges next to each other or a positive and negative charges next to each other?

Does it matter or, regardless of charge (+ or - ), charges should always be as far apart as possible?

 

[mcatcram]

It kind of matters. If that's your only option then that's your only option. But if there are resonance structures the resonance structures that separates those charges the most will contribute the most to the conformation of that molecule. So yes it does matter, and molecules with charges close to each other will be much less stable then those with charges separated. This is why Iodine is more acidic than Fluorine, when H-F loses it's Hydrogen it is less stable then H-I when it loses it's hydrogen, because when the hydrogen is lost, Iodine is better able to spread it's charge around because it's bigger and therefore is more stable. If Iodine if more stable with a negative charge on it, it is more likely to give up a hydrogen, and therefore more acidic. So separation of charge does matter. It's also important to note that when there is a positive and negative charge on one molecule, the resonance structure with the negative charge on the more electronegative atom will be more stable and therefore contribute more to the overall structure of the atom.

 

[ND13]

"Resonance contributors that minimize separation of charge are better than those with a large separation of charge".

Q: What do they mean by separation of charge? I'm very confused.

Pretty sure it means dipole moment.

"Resonance contributors that minimize separation of charge are better than those with a large separation of charge".

Q: What do they mean by separation of charge? I'm very confused.

Pretty sure it just means the dipole moment. Had the same question and I went few pages into the google search and the consensus was dipole.

 

[aldol16]

Pretty sure it means dipole moment.

Pretty sure it just means the dipole moment. Had the same question and I went few pages into the google search and the consensus was dipole.

Separation of charge is not a dipole moment. Basically, in chemistry, a separation of charge usually refers to the separation of two like charges - that's the only case where you want an actual separation. Think about Coulomb's law. If your like charges are close together, then their repulsion is astronomically large. By increasing their separation, or radius in terms of Coulomb's law, you drastically reduce the force by the square of radius. Less repulsion is more stable, so the farther away from each other you can put the charges, the more stable the molecule. That's all separation of charge is.