Kevin B. answered • 08/29/23
Enthusiastic Orgo Professor Can Help You Unlock Organic Chemistry
In the absence of easily generated images of Lewis structures, it is probably best to clarify first what the structure of "N-vinylcyanamide" actually is. "Cyanamide" is a nitrile (cyanide) derivative where the CN triple bond is connected to a -NH2. So parent, unsubstituted cyanamide would be NC-NH2, where there is a triple bond between the NC. A "vinyl" substituent is a C=C double bond, with three H atoms bonded to the two carbon atoms (leaving one C valence available to bond it to the rest of the molecule). The capital "N" is a "locant" which tells you the location of the substituent, which in this case is kinda unnecessary, but indicates that the vinyl substituent is attached to the nitrogen. As such, "N-vinylcyanamide" would be NC-NH-CH=CH2. Proceeding to draw resonance structures for this requires that you can draw it out as a full Lewis structure, showing all lone pair electrons as well. If that is difficult, then you probably need to review Lewis structures before moving on to resonance.
When considering resonance, you are generally looking at what I call "pushable" electron pairs -- usually, lone pairs and/or pi bonds. In this case, there is a lone pair on the central nitrogen and three pi bonds -- two between the nitrile/cyano NC (a triple bond consists of one sigma bond and two pi bonds), and one between the double bonded C=C carbons. If the terms "sigma" and "pi" are not familiar to you, you should head to your textbook to understand how they differ -- that distinction is assumed in this answer.
When pushing electrons in resonance, you can only push to/from adjacent atoms -- you generally will never see an electron pair be pushed to/from an atom which is not covalently bonded to its original location. A lone pair can be "pushed" toward any one of the covalently bonded neighbors -- assuming there is "space"! You never want to violate the octet rule after all! That "space" COULD be what I call an "octet hole", where an adjacent atom has room to accommodate another electron pair. That is often the case when drawing resonance for cations, but it's not the case here. In this specific example, the "space" on the adjacent atoms are "pushable" pi bonds.
Pi bonds can be "pushed" in either direction, by "piling up" the pi bond on one end or the other. That creates a lone pair on one of the pi-bonded atoms and an "octet hole" on the other end. There are also probably formal charges created -- a negative charge where the lone pair is, and a positive charge where the octet hole is -- but that assumes both atoms had neutral F.C. to start. In cases where one of the two pi-bonded atoms starts out with either a positive or negative charge, you might instead simply move the charge from one end of the pi bond to the other by piling up the pi bonding electrons. But that is NOT the case here, since all atoms have a zero formal charge in the starting Lewis structure, so piling up the a pi bond WILL create charges.
Now simply piling up a pi bond one one atom is usually a very minor resonance structure -- it loses a bond (which is bad, energetically), creates an atom without an octet (also bad), and creates/separates charges. That's a "three strikes, yer out!" resonance structure, which can typically (though not always) be ignored.
But when piling up a pi bond occurs in concert with pushing another electron pair, then the resulting resonance structure is often a more significant contributor, and more important to consider. That's the case in this molecule. One of the pi bonds "piles up" at the same time as the nitrogen lone pair flows in -- another way to think about it is that the motion of the N lone pair "pushes" the pi bonding electron pair "out of the way". This electron motion ends up creating a new pi bond to the central nitrogen, at the "expense" of a pi bond to either of the terminal atoms.
This kind of electron motion could conceivably happen in either direction. That is, the Nitrogen lone pair could flow toward the N(triple)C carbon, and push one of those two pi bonds "out of the way" and make it into a lone pair on the terminal nitrogen. This would result in a Lewis structure of: N=C=NH-CH=CH2, with a negative charge (and two lone pairs) on the left-most nitrogen, and a positive charge on the central nitrogen. OR it could happen to the OTHER side, where the N lone pair flows toward the vinyl -CH=CH2 carbon, pushing the C=C pi bond "out of the way", making it into a lone pair of the terminal carbon. This would result in a Lewis structure of: N(triple)C-NH=CH-CH2, with a positive charge on the central nitrogen, and a negative charge (and one lone pair) on the terminal -CH2. Both of those are acceptable resonance structures for N-vinylcyanamide.
But they are not equivalent, and they do not contribute equally to the "resonance hybrid". First of all, both of these two new RS's are minor, when compared to the original Lewis structure, because they both have non-zero formal charges, where all atoms in the original structure had zero FC. Furthermore, the first one is a more significant contributor than the second, because the negative charge is on a nitrogen, which is more electronegative than carbon. (The positive charge is on nitrogen in both RS, so that's a wash, and we can focus only on the location of the negative in this case.)
The resonance hybrid (which is the best description of the actual structure of the molecule) is a "weighted average" of all contributing resonance structures. In this case, there are only three which "matter". (There are many other RS's which could be drawn, but their contribution to the hybrid is negligible, and they can be ignored.) The first one is by far the major contributor, and the other two both contribute only a small portion (with a slightly greater contribution from the RS with the negative charge on the cyano- nitrogen).
Qualitatively, this resonance analysis makes a VERY important prediction.... Specifically, that N-vinylcyanamide should have a small but non-negligible amount of negative charge on BOTH terminal atoms! Similarly, it predicts that the central nitrogen atom should have a certain degree of positive charge! Both of those predictions may be somewhat counterintuitive -- but that demonstrates the predictive power of resonance analysis! It also predicts that the magnitude of the negative charge should be slightly larger on the cyano, N(triple)C nitrogen than it is on the terminal vinyl carbon.
Hopefully that helps! I know it's a lot of words, but it's good practice to read these words, and then translate them into structures on the page. You should also draw "curved arrows" showing the electron motion corresponding to the formation of each RS.
