6.2: Using Curved Arrows--Additional Concepts
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We now go back to an old friend of ours, CH3CNO, which we introduced when we first talked about resonance structures. We use this compound to further illustrate how mobile electrons are “pushed” to arrive from one resonance structure to another.
The movement of electrons that takes place to arrive at structure II from structure I starts with the triple bond between carbon and nitrogen. We’ll move one of the two pi bonds that form part of the triple bond towards the positive charge on nitrogen, as shown:
When we do this, we pay close attention to the new status of the affected atoms and make any necessary adjustments to the charges, bonds, and unshared electrons to preserve the validity of the resulting formulas. In this case, for example, the carbon that forms part of the triple bond in structure I has to acquire a positive charge in structure II because it’s lost one electron. The nitrogen, on the other hand, is now neutral because it gained one electron and it’s forming three bonds instead of four.
We can also arrive from structure I to structure III by pushing electrons in the following manner. The arrows have been numbered in this example to indicate which movement starts first, but that’s not part of the conventions used in the curved arrow formalism.
As we move a pair of unshared electrons from oxygen towards the nitrogen atom as shown in step 1, we are forced to displace electrons from nitrogen towards carbon as shown in step 2. Otherwise we would end up with a nitrogen with 5 bonds, which is impossible, even if only momentarily. Again, notice that in step 1 the arrow originates with an unshared electron pair from oxygen and moves towards the positive charge on nitrogen. A new pi bond forms between nitrogen and oxygen. At the same time, the pi electrons being displaced towards carbon in step 2 become a pair of unshared electrons in structure III. Finally, the hybridization state of some atoms also changes. For example the carbon atom in structure I is sp hybridized, but in structure III it is sp3 hybridized.
You may want to play around some more and see if you can arrive from structure II to structure III, etc. However, be warned that sometimes it is trickier than it may seem at first sight.
Here are some additional rules for moving electrons to write resonance structures:
1. Electron pairs can only move to adjacent positions. Adjacent positions means neighboring atoms and/or bonds.
2. The Lewis structures that result from moving electrons must be valid and must contain the same net charge as all the other resonance structures.
The following example illustrates how a lone pair of electrons from carbon can be moved to make a new pi bond to an adjacent carbon, and how the pi electrons between carbon and oxygen can be moved to become a pair of unshared electrons on oxygen. None of the previous rules has been violated in any of these examples.
Now let’s look at some examples of HOW NOT TO MOVE ELECTRONS. Using the same example, but moving electrons in a different way, illustrates how such movement would result in invalid Lewis formulas, and therefore is unacceptable. Not only are we moving electrons in the wrong direction (away from a more electronegative atom), but the resulting structure violates several conventions. First, the central carbon has five bonds and therefore violates the octet rule. Second, the overall charge of the second structure is different from the first. To avoid having a carbon with five bonds we would have to destroy one of the C–C single bonds, destroying the molecular skeleton in the process.
In the example below electrons are being moved towards an area of high electron density (a negative charge), rather than towards a positive charge. In addition, the octet rule is violated for carbon in the resulting structure, where it shares more than eight electrons.
Additional examples further illustrate the rules we’ve been talking about.
(a) Unshared electron pairs (lone pairs) located on a given atom can only move to an adjacent position to make a new pi bond to the next atom.
(b) Unless there is a positive charge on the next atom (carbon above), other electrons will have to be displaced to preserve the octet rule. In resonance structures these are almost always pi electrons, and almost never sigma electrons.
As the electrons from the nitrogen lone pair move towards the neighboring carbon to make a new pi bond, the pi electrons making up the C=O bond must be displaced towards the oxygen to avoid ending up with five bonds to the central carbon.
(c) As can be seen above, pi electrons can move towards one of the two atoms they share to form a new lone pair. In the example above, the pi electrons from the C=O bond moved towards the oxygen to form a new lone pair. Another example is:t
(d) pi electrons can also move to an adjacent position to make new pi bond. Once again, the octet rule must be observed:
One of the most common examples of this feature is observed when writing resonance forms for benzene and similar rings.