What is resonance?

Begin by watching this video:

If we were to draw the structure of an aromatic molecule such as 1,2-dimethylbenzene, there are two ways that we could draw the double bonds:

Which way is correct? There are two simple answers to this question: ‘both’ and ‘neither one’. Both ways of drawing the molecule are equally acceptable approximations of the bonding picture for the molecule, but neither one, by itself, is an accurate picture of the delocalized pi bonds. The two alternative drawings, however, when considered together, give a much more accurate picture than either one on its own.  This is because they imply, together, that the carbon-carbon bonds are not double bonds, not single bonds, but about halfway in between.

These two drawings are an example of what is referred to in organic chemistry as resonance contributors: two or more different Lewis structures depicting the same molecule or ion that, when considered together, do a better job of approximating delocalized pi-bonding than any single structure. By convention, resonance contributors are linked by a double-headed arrow, and are sometimes enclosed by brackets:

In order to make it easier to visualize the difference between two resonance contributors, small, curved arrows are often used.  Each of these arrows depicts the ‘movement’ of two pi electrons. A few chapters from now when we begin to study organic reactions – a process in which electron density shifts and covalent bonds between atoms break and form – this ‘curved arrow notation’ will become extremely important in depicting electron movement. In the drawing of resonance contributors, however, this electron ‘movement’ occurs only in our minds, as we try to visualize delocalized pi bonds.  Nevertheless, use of the curved arrow notation is an essential skill that you will need to develop in drawing resonance contributors.

The depiction of benzene using the two resonance contributors A and B in the figure above does not imply that the molecule at one moment looks like structure A, then at  the next moment shifts to look like structure B.  Rather, at all moments, the molecule is a combination, or resonance hybrid of both A and B.

Usually, derivatives of benzene (and phenyl groups, when the benzene ring is incorporated into a larger organic structure) are depicted with only one resonance contributor, and it is assumed that the reader understands that resonance hybridization is implied.  This is the convention that will be used for the most part in this book.  In other books or articles, you may sometimes see benzene or a phenyl group drawn with a circle inside the hexagon, either solid or dashed, as a way of drawing a resonance hybrid.

Resonance contributors for the carboxylate group

The convention of drawing two or more resonance contributors to approximate a single structure may seem a bit clumsy to you at this point, but as you gain experience you will see that the practice is actually very useful when discussing the manner in which many functional groups react. Let’s next consider the carboxylate ion (the conjugate base of a carboxylic acid).  As our example, we will use formate, the simplest possible carboxylate-containing molecule. The conjugate acid of formate is formic acid, which causes the painful sting you felt if you have ever been bitten by an ant.

Usually, you will see carboxylate groups drawn with one carbon-oxygen double bond and one carbon-oxygen single bond, with a negative formal charge located on the single-bonded oxygen.  In actuality, however, the two carbon-oxygen bonds are the same length, and although there is indeed an overall negative formal charge on the group, it is shared equally between the two oxygens.  Therefore, the carboxylate can be more accurately depicted by a pair of resonance contributors. Alternatively, a single structure can be used, with a dashed line depicting the resonance-delocalized pi bond and the negative charge located in between the two oxygens.

Let’s see if we can correlate these drawing conventions to a valence bond theory picture of the bonding in a carboxylate group. We know that the carbon must be sp²-hybridized, (the bond angles are close to 120˚, and the molecule is planar), and we will treat both oxygens as being sp²-hybridized as well. Both carbon-oxygen sigma bonds, then, are formed from the overlap of carbon sp² orbitals and oxygen sp² orbitals.

In addition, the carbon and both oxygens each have an unhybridized 2p_z orbital situated perpendicular to the plane of the sigma bonds.  These three 2p_z orbitals are parallel to each other, and can overlap in a side-by-side fashion to form a delocalized pi bond.

Resonance contributor A shows oxygen #1 sharing a pair of electrons with carbon in a pi bond, and oxygen #2 holding a lone pair of electrons in its 2p_z orbital.  Resonance contributor B, on the other hand, shows oxygen #2 participating in the pi bond with carbon, and oxygen #1 holding a lone pair in its 2p_z orbital.  Overall, the situation is one of three parallel, overlapping 2p_z orbitals sharing four delocalized pi electrons. Because there is one more electron than there are 2p_z orbitals, the system has an overall charge of –1.  This is the kind of 3D picture that resonance contributors are used to approximate, and once you get some practice you should be able to quickly visualize overlapping 2p_z orbitals and delocalized pi electrons whenever you see resonance structures being used. In this text, carboxylate groups will usually be drawn showing only one resonance contributor for the sake of simplicity, but you should always keep in mind that the two C-O bonds are equal, and that the negative charge is delocalized to both oxygens.

Here’s another example, this time with a carbocation.  Recall from section 2.1. that carbocations are sp²-hybridized, with an empty 2p orbital oriented perpendicular to the plane formed by three sigma bonds.  If a carbocation is adjacent to a double bond, then three 2p orbitals can overlap and share the two pi electrons – another kind of conjugated pi system in which the positive charge is shared over two carbons.

Exercises

1. Draw the resonance contributors that correspond to the curved, two-electron movement arrows in the resonance expressions below.

2. In each resonance expression, draw curved two-electron movement arrows on the left-side contributor that shows how we get to the right-side contributor. Be sure to include formal charges.

Show Solution

Some common patterns for resonance

If you examine a large number of resonance examples, you will begin to notice that they nearly always match with certain common patterns, of which there are only four.  Of these, two relate to uncharged structures, and two relate to charged ones, as seen in the figure:

These are listed as types I-IV, but these terms are not used outside this textbook!  The most important are types III and IV, where resonance is used to share or “delocalize” a charge around the structure to stabilize the structure.  In more complex cases, both types may occur simultaneously.

Type III resonance is only seen with a + charge, and usually involves a positive charge on oxygen or nitrogen being shared onto a carbon; the carbocation form has only six valence electrons on the carbon, so it is a less stable form than the major form (which has complete octets).

As you work on learning how to draw and interpret resonance structures, there are a few basic rules that you should keep in mind in order to avoid drawing nonsensical structures.  All of these rules make perfect sense as long as you keep in mind that resonance contributors are merely a human-invented convention for depicting the delocalization of pi electrons in conjugated systems.

To expand a bit on rule #4, there are really only three things we can do with curved arrows when drawing resonance structures.  First, we can take the two electrons in a pi bond and shift them to become a lone pair on an adjacent atom (arrow ‘a’ below).

Second, we can take a lone pair on an atom and put those two electrons into a pi bond on the same atom (arrow ‘b’). Third, we can shift a pi bond one position over (arrow c).

Resonance arrows can also be combined – below, we show arrows a and b together:

Notice that we do not exceed the octet rule on any atoms when we move electrons with arrows a, b and c.  The resonance picture below shows an ‘illegal’ movement of electrons, because it would result in a carbon with five bonds, or 10 valence electrons (this would break the octet rule):

Always be very careful when drawing resonance structures that your arrows do only the three types of electron movement described above, and that you never exceed the octet rule on a second-row element. It is often helpful (but optional), to include all lone-pair electrons on oxygen and nitrogen in the drawing in order to keep track of valence electrons, avoid breaking the octet rule, and recognize when atoms have a negative or positive formal charge. Getting the ‘electron accounting’ correct is a big part of working with resonance contributors.

Below are a few more examples of ‘legal’ resonance expressions.  Confirm for yourself that the octet rule is not exceeded for any atoms, and that formal charges are correct.

Major vs minor resonance contributors

Different resonance contributors do not always make the same contribution to the overall structure of the hybrid – rather, in many cases one contributor comes closer to depicting the actual bonding picture than another.  In the case of carboxylates, contributors A and B below are equivalent in terms of their relative contribution to the hybrid structure.  However, there is also a third resonance contributor ‘C,  in which the carbon bears a positive formal charge and both oxygens are single-bonded and bear negative charges.

Structure C makes a less important contribution to the overall bonding picture of the group relative to A and B. How do we know that structure C is the ‘minor’ contributor?  There are four basic rules which you need to learn in order to evaluate the relative importance of different resonance contributors.  We will number them 5-8 so that they may be added to in the ‘rules for resonance’ list earlier on this page.

Why do we worry about a resonance contributor if it is the minor one?  We will see later that very often a minor contributor can still be extremely important to our understanding of how a molecule reacts.

Resonance and peptide bonds

What is the hybridization state of the nitrogen atom in an amide?  At first glance, it would seem logical to say that it is sp³-hybridized, because, like the nitrogen in an amine, the Lewis structure shows three single bonds and a lone pair.  The picture looks quite different, though, if we consider another resonance contributor in which the nitrogen has a double bond to the carbonyl carbon: in this case, we would have to say that applicable hybridization is sp², and the bonding geometry trigonal planar rather than tetrahedral.

In fact, the latter picture is more accurate: the lone pair of electrons on an amide nitrogen are not localized in an sp³ orbital, rather, they are delocalized as part of a conjugated pi system, and the bonding geometry around the nitrogen is trigonal planar as expected for sp² hybridization. This is a good illustration of an important point: conjugation and the corresponding delocalization of electron density is stabilizing, thus if conjugation can occur, it probably will.

One of the most important examples of amide groups in nature is the ‘peptide bond’ that links amino acids to form polypeptides and proteins. 

Critical to the structure of proteins is the fact that, although it is conventionally drawn as a single bond, the C-N bond in a peptide linkage has a significant barrier to rotation, indicating  that to some degree, C-N pi overlap is present – in other words, there is some double bond character, and the nitrogen is sp² hybridized with trigonal planar geometry.

The barrier to rotation in peptide bonds is an integral part of protein structure, introducing more rigidity to the protein’s backbone. If there were no barrier to rotation in a peptide bond, proteins would be much more ‘floppy’ and three dimensional folding would be very different.