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I'm learning chemistry with Klein's Organic Chemistry as a second language right now:

From all of the cases above (oxgen, nitrogen, and carbon), you can see why you have to know how many lone pairs there are on an atom in order to figure out the formal charge on that atom. Similarly, you have to know the formal charge to figure out how many lone pairs there are on an atom. Take the case below with the nitrogen atom shown:

no lone pairs or charges could either be formal charge minus or formal charge plus

If the lone pairs were drawn, then we would be able to figure out the charge (two lone pairs would mean a negative charge and one lone pair would mean a positive charge). Similarly, if the formal charge was drawn, we would be able to figure out how many lone pairs there are (a negative charge would mean two lone pairs and a positive charge would mean one lone pair). So you can see that drawings must include either lone pairs or formal charges. The convention is to always show formal charges and to leave out the lone pairs. This is much easier to draw, because you usually won't have more than one charge on a drawing (if even that), so you get to save time by not drawing every lone pair on every atom.

I find it confusing to state that if information is "missing" (actually it's not, the information would simply be no formal charge), it's assumed the atom has either one or two lone pairs. Looking at the structure on the far left, where no formal charge is indicated, why wouldn't it be reasonable to assume the atom has one lone pair and a single electron (making it a radical)?

While I understand that such a nitrogen would be highly unstable and reactive, it theoretically aligns with the representation of nitrogen without formal charges. Is this simply a matter of convention, as is often the case in chemistry?

Am I misinterpreting "a could either be b or c" as "a equals either b or c" when in fact "could" suggests two possible forms, rather than stating that the left structure must be exactly one of those two? Because right now, it really states: (no formal charge) => (formal charge [±1])??

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    $\begingroup$ I believe Klein's point here is: in isolation, you have no idea about the electronic structure of that imide nitrogen. The imide nitrogen is always bonded to something else in a neutral imide molecule. Without the bond, the imide ion could either be negatively charged (if there was a group that transferred the bond pair onto the nitrogen), or it could be positively charged (if there was a group that took the bond pair with it while leaving). In context, this is evident as you can track the number of lone pairs, but you must always know how many there are to arrive at an FC. $\endgroup$
    – Sonder
    Commented Oct 16 at 4:31
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    $\begingroup$ Nitrogen with two bonds can be either anion, cation, or radical. It could be even dication in principle. If you don't put there at least a dot or charge, how would you know which it is? I frankly don't see a point to this question - author didn't bother mentioning radical because of associate point about formal charge. $\endgroup$
    – Mithoron
    Commented Oct 16 at 21:37
  • $\begingroup$ @Mithoron "If you don't put there at least a dot or charge, how would you know which it is?" exactly, there's no charge, so it can only be the radical variant, because it's the only way to be neutral. That's the reason I'm so confused. $\endgroup$
    – David222
    Commented Oct 17 at 5:41
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    $\begingroup$ There is no charge because author omitted it. You could just as well say it couldn't be a radical, because there's no dot. $\endgroup$
    – Mithoron
    Commented Oct 17 at 9:26

2 Answers 2

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Ambiguity is the enemy of scientific communication. If things are up to interpretation, misinterpretation will happen, which will likely turn into errors down the line.

The chemistry teacher of a friend of mine has a saying:

There is a difference if you smear honeybees or bee honey on your toast.

When writing in science, you should be as precise as possible. Using ambiguous structural formulae is not that.

If something is ambiguous one could argue that there is not enough information to precisely communicate the intended meaning, therefore some information is indeed missing. I think we can agree, that the structural representation is incomplete: It is either missing charges or electrons (or both). You are perfectly reasonable in assuming that this could be a radical species, but other interpretations are equally as valid.

I have no idea, why the author of that passage did not include this possibility as it would have strengthened their argument. This, however, is a question that will likely remain unanswered forever.

There are plenty of conventions in chemistry. In most cases these are applied to reduce ambiguity and allow for a more streamlined communication.

The author hints at this in their text. For structural representations it is common to omit electron pairs as these could be inferred from charge. Single electrons usually are not omitted since these are often crucial to reactivity. So in this special case, a dot for the radical should have been included.

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Sure, a radical is theoretically possible, but it would be so unstable and so unlikely to form that you functionally don't need to worry about it. If there was a radical on nitrogen, that would be clearly indicated with lone electrons since it would be not at all what you would expect. "Could" does suggest two possible forms, but those two forms are by far the most likely forms (and the form in the middle is much more likely than the form on the right, since it has a complete octet). I think you'll find that in the rest of the textbook, you're not going to find any radicals on nitrogen (not in any common organic mechanisms, at least).

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  • $\begingroup$ N radical is more likely then cation. Why author didn't bother mentioning it isn't even a chemistry question. Likely because of making the connection between lone pairs and charge, but really... whatever. $\endgroup$
    – Mithoron
    Commented Oct 16 at 21:42

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