Resonance Structures, Electron Mobility & Delocalization, Schemes and Mind Maps of Structures and Materials

Download Resonance Structures, Electron Mobility & Delocalization and more Schemes and Mind Maps Structures and Materials in PDF only on Docsity! DO utes OF RESONANCE STRUCTURES, ELECTRON MOBILITY & DELOCALIZATION By: Dr. Mohamed Yousri Ayad  Dr. Mohamed Yousri Ayad [1] Lewis formulas are misleading in the sense that atoms and electrons are shown as being static. (2) We know that a given compound can have several valid Lewis formulas. For example CH;CNO can be represented by at least three different Lewis structures called resonance forms, or resonance structures: 4 @.6 He e Hoe. H-C-C=N-O0 H-C-C=N-O H-C-C=N=O H H H I II Ill [3] A stable compound such as the above does not exist in multiple states represented by structures |, or Il, or Ill. The compound exists in a single state called a hybrid of all three structures. That is, it contains contributions of all three resonance forms, much ike a person might have physical features inherited from each parent to varying degrees. [4] In the resonance forms shown above the atoms remain in one place, but some electrons have changed locations. [5] The basic bonding pattern that is unique to a specific compound is made up of sigma bonds. This is called the connectivity. [6] The connectivity Is the same in all the resonance structures. [7] Electrons, on the other hand, can be moved around. That is to say, they possess a certain degree of mobility. (3) Sigma bonds don’t move, as this would destroy the connectivity and therefore the molecule.  Dr. Mohamed Yousri Ayad @This move would result in a nitrogen atom with 5 bonds, which is impossible. Therefore, we must move one of the tr- bonds between C and N to the carbon atom in order to preserve the octet rule. @ As carbon gains one extra electron, it also acquires a negative charge. @ The nitrogen atom gained one electron from oxygen but lost one to carbon, so it retains the same charge. @ Oxygen lost one electron to make the new bond to nitrogen, so it goes from having a negative charge to being neutral. [20] There is no particular order in which resonance structures must be written. Technically, one should be able to go from any resonance structure to any other by pushing mobile electrons using curved arrows. However this can be sometimes trickier than others. [21] There are two basic rules that must be observed when moving electron pairs using curved arrows: @ Electron pairs can only move to adjacent positions. Adjacent positions means neighboring atoms and/or bonds. @ The Lewis structures that result from moving electrons must be valid and must contain the same net charge as the original structure. This example illustrates how a lone pair of electrons (from carbon) can be moved to make a new tr-bond (between carbons), and how a m-bond (between carbon and oxygen) can be moved to make a new lone pair (on oxygen): 2 «. 2 ‘ot ) 70; I | oc Hy H~E-7d ~cHs ca CH  Dr. Mohamed Yousri Ayad We have observed the two rules for moving electrons in resonance structures. We only moved electron pairs to adjacent or neighboring positions. The Lewis structure that resulted from steps 1 and 2 is valid, and the net charge in both structures is -1. 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. 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. ® He} 70 gan a. mee MES TA NES - H In the example below; First, electrons are moving towards an area of high electron density (a negative charge). Second, the octet rule is violated for carbon in the resulting structure. | be Otc. \ Sacha. _ H~,~7— H 7S. CHs \ ve CH3 H H  Dr. Mohamed Yousri Ayad [22] Unshared electron pairs (lone pairs) located on a given atom can only move to an adjacent position to make a new tr-bond to the next atom. 1 ” eo) H os2 H “oO, et om @O : N = N / / H CH3 H CH3 [23] Unless there is a positive charge on the next atom, other electrons will have to be displaced to preserve the octet rule. In resonance structures these are almost always tr-electrons. 9 tor) 20: => CH3 H3C° © ~CH3 H3C We can see electrons from the nitrogen lone pair move towards the neighboring carbon to make a new tr-bond, the tr-electrons making up the C=O bond must be displaced towards the oxygen to avoid ending up with five bonds to the central carbon. [24] 1-electrons can also move to an adjacent position to make new tr-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.