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ELECTRON DELOCALIZATION AND RESONANCE
LEARNING OBJECTIVES
To introduce the concept of electron delocalization from the perspective of molecular orbitals, to understand the relationship between electron delocalization and resonance, and to learn the principles of electron movement used in writing resonance structures in Lewis notation, known as the curved arrow formalism.
MOBILITY OF Pi ELECTRONS AND UNSHARED ELECTRON PAIRS
Now that we understand the difference between sigma and pi electrons, we remember that the pi bond is made up of loosely held electrons that form a diffuse cloud which can be easily distorted. This can be illustrated by comparing two types of double bonds, one polar and one nonpolar. The C=C double bond on the left below is nonpolar. Therefore the pi electrons occupy a relatively symmetric molecular orbital that’s evenly distributed (shared) over the two carbon atoms. The C=O double bond, on the other hand, is polar due to the higher electronegativity of oxygen. The pi cloud is distorted in a way that results in higher electron density around oxygen compared to carbon. Both atoms still share electrons, but the electrons spend more time around oxygen. The drawing on the right tries to illustrate that concept.
Using simple Lewis formulas, or even line-angle formulas, we can also draw some representations of the two cases above, as follows.
C C pi^ bonds .. O C.
The dynamic nature of pi electrons can be further illustrated with the use of arrows, as indicated below for the polar C=O bond:
C C (^) O C or O C or (^) δ − O C^ δ^ +
nonpolar pi bond (^) polar pi bond representations
O C O C
The CURVED ARROW FORMALISM is a convention used to represent the movement of electrons in molecules and reactions according to certain rules. We’ll study those rules in some detail. For now, we keep a few things in mind:
a) Curved arrows always represent the movement of electrons, not atoms.
b) Electrons always move towards more electronegative atoms or towards positive charges.
We notice that the two structures shown above as a result of “ pushing electrons ” towards the oxygen are RESONANCE STRUCTURES. That is to say, they are both valid Lewis representations of the same species. The actual species is therefore a hybrid of the two structures. We conclude that:
Curved arrows can be used to arrive from one resonance structure to another by following certain rules.
Just like pi electrons have a certain degree of mobility due to the diffuse nature of pi molecular orbitals, unshared electron pairs can also be moved with relative ease because they are not engaged in bonding. No bonds have to be broken to move those electrons. As a result, we keep in mind the following principle:
Curved arrows usually originate with pi electrons or unshared electron pairs, and point towards more electronegative atoms, or towards partial or full positive charges.
Going back to the two resonance structures shown before, we can use the curved arrow formalism either to arrive from structure I to structure II, or vice versa.
O C O C or
I II
C O C O
II I
A B
In case A , the arrow originates with pi electrons, which move towards the more electronegative oxygen. In case B , the arrow originates with one of the unshared electron pairs, which moves towards the positive charge on carbon. We further notice that pi electrons from one structure can become unshared electrons in another, and vice versa. We’ll look at additional guidelines for how to use mobile electrons later.
Finally, in addition to the above, we notice that the oxygen atom, for example, is sp^2 hybridized (trigonal planar) in structure I, but sp^3 hybridized (tetrahedral) in structure II. So, which one is it? Again, what we are talking about is the real species. The real species is a hybrid that contains contributions from both resonance structures. In this particular case, the best we can do for now is issue a qualitative statement: since structure I is the major contributor to the hybrid, we can say that the oxygen atom in the actual species is mostly trigonal planar because it has greater sp^2 character, but it still has some tetrahedral character due to the minor contribution from structure II. We’ll explore and expand on this concept in a variety of contexts throughout the course.
What about sigma electrons, that is to say those forming part of single bonds? These bonds represent the “glue” that holds the atoms together and are a lot more difficult to disrupt. As a result, they are not as mobile as pi electrons or unshared electrons, and are therefore rarely moved. There are however some exceptions, notably with highly polar bonds, such as in the case of HCl illustrated below. We will not encounter such situations very frequently.
H Cl H^ Cl
This representation better conveys the idea that the H–Cl bond is highly polar.
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.
C
C CH 3
O
H H
C
C CH 3
O
H
H
1
2
C
C CH 3
O
H
H
C
C CH 3
O
H
H
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.
C
C CH 3
O
H H
C
C CH 3
O
H H
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.
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.
H N H
CH 3
CH 3
H N H
CH 3
CH 3
(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.
H N H
O
CH 3
1 (^2) H
N H
O
CH 3
(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
H 3 C CH 3
O
H 3 C CH 3
O
(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.
1
(^2 )
benzene
I (^) II III IV V
I
I II
In the second structure, delocalization is only possible over three carbon atoms. This is demonstrated by writing all the possible resonance forms below, which now number only two.
Finally, the third structure has no delocalization of charge or electrons because no resonance forms are possible. Therefore, it is the least stable of the three. This brings us to the last topic. How do we recognize when delocalization is possible? Let’s look at some delocalization setups, that is to say, structural features that result in delocalization of electrons.
DELOCALIZATION SETUPS
There are specific structural features that bring up electron or charge delocalization. The presence of a conjugated system is one of them. Other common arrangements are:
(a) The presence of a positive charge next to a pi bond. The positive charge can be on one of the atoms that make up the pi bond, or on an adjacent atom.
(b) The presence of a positive charge next to an atom bearing lone pairs of electrons.
H 3 C CH 3
OH
H 3 C CH 3
OH
H 3 C C O H 3 C C O
N N O N^ N^ O
(c) The presence of a pi bond next to an atom bearing lone pairs of electrons.
H N
O
CH 3
CH 3
1
2
H (^) N
O
CH 3
CH 3
(^2) O 1 O
ORBITAL PICTURE OF DELOCALIZATION
The orbital view of delocalization can get somewhat complicated. For now we’re going to keep it at a basic level. We start by noting that sp^2 carbons actually come in several varieties. Two of the most important and common are neutral sp^2 carbons and positively charged sp^2 carbons. Substances containing neutral sp^2 carbons are regular alkenes. Species containing positively charged sp^2 carbons are called carbocations. The central carbon in a carbocation has trigonal planar geometry, and the unhybridized p orbital is empty. The following representations convey these concepts.
Top and side view of a carbocation in Lewis and 3-D notation
H 3 C
C CH 3
CH 3
C
H 3 C
H 3 C CH 3^ C^ sp
(^2) orbitals
p
Orbital view of a carbocation. The unhybridized p orbital is empty
A combination of orbital and Lewis or 3-D formulas is a popular means of representing certain features that we may want to highlight. For example, if we’re not interested in the sp^2 orbitals and we just want to focus on what the p orbitals are doing we can use the following notation.
Let’s now focus on two simple systems where we know delocalization of pi electrons exists. One is a system containing two pi bonds in conjugation, and the other has a pi bond next to a positively charged carbon. We can represent these systems as follows.
Line angle and orbital picture of a simple conjugated system
CH 3 CH 3
Line angle and orbital picture of a system containing a carbocation next to a pi bond
C
H 3 C
H 3 C CH 3
