Understanding Electron Delocalization and Resonance through Curved Arrow Formalism, Lecture notes of Chemistry

Download Understanding Electron Delocalization and Resonance through Curved Arrow Formalism and more Lecture notes Chemistry in PDF only on Docsity! 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 CO.. .pi bonds The dynamic nature of pi electrons can be further illustrated with the use of arrows, as indicated below for the polar C=O bond: δ + C C O C O Cor or O Cδ − nonpolar pi bond polar pi bond representations O C CO 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 CO or I II OCC O III 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 sp2 hybridized (trigonal planar) in structure I, but sp3 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 sp2 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. 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 CH3 CH3 H N H CH3 CH3 (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 CH3 1 2 H N H O CH3 (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 H3C CH3 O H3C CH3 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 3 benzene DELOCALIZATION, CONJUGATED SYSTEMS, AND RESONANCE ENERGY The presence of alternating pi and sigma bonds in a molecule such as benzene is known as a conjugated system, or conjugated pi bonds. Conjugated systems can extend across the entire molecule, as in benzene, or they can comprise only part of a molecule. A conjugated system always starts and ends with a pi bond (i.e. an sp2 or an sp-hybridized atom), or sometimes with a charge. The atoms that form part of a conjugated system in the examples below are shown in blue, and the ones that do not are shown in red. Most of the times it is sp3 hybridized atoms that break a conjugated system. Practically every time there are pi bonds in a molecule, especially if they form part of a conjugated system, there is a possibility for having resonance structures, that is, several valid Lewis formulas for the same compound. What resonance forms show is that there is electron delocalization, and sometimes charge delocalization. All the examples we have seen so far show that electrons move around and are not static, that is, they are delocalized. Charge delocalization is a stabilizing force because it spreads energy over a larger area rather than keeping it confined to a small area. Since electrons are charges, the presence of delocalized electrons brings extra stability to a system compared to a similar system where electrons are localized. The stabilizing effect of charge and electron delocalization is known as resonance energy. Since conjugation brings up electron delocalization, it follows that the more extensive the conjugated system, the more stable the molecule (i.e. the lower its potential energy). If there are positive or negative charges, they also spread out as a result of resonance.The corollary is that the more resonance forms one can write for a given system, the more stable it is. That is, the greater its resonance energy. Examine the following examples and write as many resonance structures as you can for each to further explore these points: more stable than more stable than or or or more stable than more stable than Let’s look for a moment at the three structures in the last row above. In the first structure, delocalization of the positive charge and the pi bonds occurs over the entire ring. This becomes apparent when we look at all the possible resonance structures as shown below. 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. H3C CH3 OH H3C CH3 OH H3C C O H3C C O N N O N N O