In one of my Organic Chemistry textbooks I just finished reading the rules on shorthand. Here they are:
1. Carbon atoms aren't usually shown. Instead, a carbon atom is assumed to be at the intersection of two lines (bonds) and at the open end of each line. Occasionally, a carbon atom might be shown for emphasis or for clarity.
2. Hydrogen atoms bonded to carbon aren't shown. Since carbon always has a valence of four* (astrix and comment inserted by me, not the text), we mentally supply the correct number of hydrogen atoms for each carbon.
3. All other atoms other than carbon and hydrogen are shown. (p. 42)
I have to admit to not knowing if the same rules apply to resonance structures, but if they do the hydrogen should have been shown on the nitrogen atom. They give indole as one of the examples on the next page and the hydrogen atom on that nitrogen atom is represented. Let me read up on resonance stuctures and think about the concept to make sure the same rules apply.
Okay, there should be a hydrogen on the nitrogen atom. The rules are not similar but involve, as the bees above mentioned, shared pi elecrons. This wouldn't apply to nitrogen. In the resonance example given in my text hydrogen is represented on the nitrogen atom. I would do a pretty pour job of explaining what I just read so I won't try to explain the theory. The inner lines represent that the pi electrons move from one carbon to another fairly freely. In other words,
C
// \
C C
| ||
C C
\\ /
C
is a fiction of sorts. It would be some hybride between these two (2) forms:
C C
/ \\ //\
C C C C
|| | <----> | ||
C C C C
\ // \\ /
C C
The idea is to emphasize the moving pi electrons from carbon to carbon.
My textbook's lists five (5) rules for drawing and interpreting resonance structures:
1. Resonance forms are imaginary, not real. The real structure is a composite hybride of the different forms. Substances like allyic carbocation, the carbonate ion, and benzene are no different from any other substance in having single, unchanging structures. The only difference is the way they must be represented on paper.
2. Resonance forms differ from each other only in the placement of the pi electron. Neither the position nor the hybridization of atoms changes from one resonance form to another. In benzene, for example, the pi electrons in the double bonds move, but the six carbon atoms remain in place: By contrast, structures like 1,3-cyclohexadiene and 1,4-cyclohexadiene are not resonance structures because their hydrogen atoms don't occupy the same positions. Instead, the two dienes are constitutional isomers.
(look those up at
http://chemfinder.com/
because I can't draw them).
3. Different resonance forms of a substance don't have to be equivalent. For example, the allylic carbocation obtained by reaction of 1,3-butadiene with H[/sup]+
is unsymmetrical. One, end of the delocalized pi-electron system has a methyl substituent, and the other end is unsubstituted. Even though the two resonance forms aren't equivalent, they both contribute to the overall resonance hybride.
In general, when two resonance forms are not equivalent, the actual structure of the resonance hybride is closer to the more stable form than the less stable form. Thus, we might expect the butenyl carbocation to look more like a secondary carbocation than a primary carbocation.
4. All resonance forms must obay rules of valency. Resonance forms are like any other structure: The octet rule still holds. For example, one of the following structures for the carbonate ion is not a valid resonance form because the carbon atom has five bonds and ten electrons:
O O
|| ||
C C-
/ \ // \
-O O- O O-
Carbonate Not a
ion resonance form
5. The resonance hybride is more stable than any single resonance form. In other words, resonance leads to stability The greater the number of resonance forms possible, the more stable the substance. We've already seen, for example, that an allyic carbocation is more stable than a normal carbocation. In a similar manner, we'll see...that the benzene ring is more stable than a cyclic alkene. (p.110-111)
As for the hydrogen being shown on the nitrogen atom of the MDMA molecule I'd say that a double bond is not close enough to that atom to allow pi electrons to move to it. I could have misinterpreted what is written above but I think the theory is clear about what types of molecules and at what points on them pi electrons are free to move. Even if it were, it doesn't appear as if the rules for showing hydrogen bonded to nitrogen is in anyway different, not to mention that examples clearly showed them. In another resonance structure given as an example in the book I have (the example is morphine) all double bonded carbons do not show a hydrogen atom but all singe bonds show them. Maybe it might be more correct to show a few hydrogen atoms on carbon atoms that do not share a double bond with another carbon atom? I truly can't say as I am no expert. Regardless, it's assumed where a valence may be filled that hydrogen is what goes there. Which is more correct? They are probably both correct or at least accepted but I'm with the bee who advocated the showing of hydrogen on the nitrogen atom of MDMA. (p. 364)
All the rules came from an Organic Chemistry book entitled "Fundamentals of Organic Chemistry, 2nd Edition", John McMurry, Brooks/Cole Publishing Company, 1990. I doubt that text is even locateable so referencing it was probably not necessary.
*Those rules are for Organic Chemistry. The book I have that is not Organic, but more like a General Chemistry Text, notes that oxidation states of -4, +2 and +4 are possible for carbon atoms. At the moment I can only think of carbon monoxide (CO) as an example.
Who wants to play cops and dope fiends?
