# 1 Lecture 5 Resonance 1. Lone pair next to empty 2p orbital

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1 1 Lecture 5 esonance 1. Lone pair next to empty 2p orbital sp 2 + is more common sp + is less common + needs electrons, has to overlap with a. an adjacent 2p lone pair with electrons b. an adjacent pi bond better, more bonds, full octets 3D resonance 2D resonance b. an adjacent 2p lone pair with electrons on a negative atom (neutral overall, delocalization of electrons) 3D resonance =anion with lone pair 3D resonance 2D resonance a. an adjacent 2p lone pair with electrons on a neutral atom (+ overall, delocalization of positive charge) 3D resonance = neutral atom with lone pair better, more bonds, full octets electron pair acceptors - lack of electrons

2 2 Lecture 5 Problem 1 All of the following examples demonstrate delocalization of a lone pair of electrons into an empty 2p orbital. Usually in organic chemistry this is a carbocation site, but not always. There are many variations. Assume full octets on all nonhydrogen atoms below, unless you see a carbon with three bonds and a positive formal charge (this will be a carbocation) or an atom drawn with lone pairs explicitly drawn and a positive formal charge. Add in missing lone pairs and show proper formal charge (a number of examples need negative formal charge added). Use correct resonance arrows (curved and double headed) where appropriate. What is the hybridization of all nonhydrogen atoms? You should be able to draw a 3D picture of any of these molecules. a. b. better, more bonds, full octets, are both sp 2 better, more bonds, full octets, are both sp 2 c. better, more bonds, full octets, and are all sp 2 d. better, more bonds, full octets, no charge, and are all sp 2 e. better, more bonds, full octets, less electronegative atom with positive formal charge (>>),, and are all sp 2

3 f. g. 3 better, more bonds, full octets and are both sp better, more bonds, full octets and are both sp h. i. better, more bonds, full octets, no formal charge, and are both sp better, more bonds, full octets, no formal charge, and are both sp 2 j. k. better, more bonds, full octets, no formal charge, and are both sp 2 better, more bonds, full octets, less electronegative atom with positive formal charge (>), and are all sp 2 l. better, more bonds, full octets, no formal charge,, and are all sp 2

4 m. 4 These 2 and better, more bonds, full octets, oxygen atoms on the ends are partially negative and the oxygen atom in the middle is positive, all is sp 2 n. +2 These 2 and better, more bonds, full octets, oxygen atoms on the ends are partially negative and the nitrogen atom in the middle is positive, all and are sp 2 Problem 2 All of the following examples demonstrate delocalization of a lone pair of electrons into a pi bond. The variations are almost limitless., Y and Z below can be sp or sp 2 hybridized and they can be carbon, nitrogen or oxygen. and Z can also be fluorine. In this problem the sigma skeleton is sp 2 -sp 2 -sp 2 for all parts. Assume full octets below on all nonhydrogen atoms. This means you will have to add in lone pairs, if missing. Include proper formal charge in your created resonance structures (some of the structures need additional positive formal charge). Use correct resonance arrows (curved and double headed) where appropriate. What is the hybridization of all nonhydrogen atoms? represents an organic group or a hydrogen atom. The generalized 3D example provided in this problem that can be modified to fit all of the parts of this problem. Y Z Y Z o formal charge is indicated., Y and Z are sp 2 hybridized in these examples, but there are many other possibilities.

5 5 Lecture 5 a. b. 2 nd structure is better, c. d. 2 nd structure is better, 1 st structure is better, e. f. 2 nd structure is better, 1 st structure is better, no charge g. h. 1 st structure is better, 1 st structure is better, i. j. 2 nd structure is better, k. l. 2 nd structure is better, m. n. o. p. 2 nd structure is better, no charge 2 nd structure is better, no charge

6 6 Lecture 5 q. r. 2 nd structure is better, no charge 2 nd structure is better, s. t. Problem 3 The following examples also demonstrate delocalization of a lone pair of electrons into a pi bond, but with a different sigma skeleton (sp 2 -sp-sp). Again, there are numerous variations. Assume full octets below on all nonhydrogen atoms. This means you will have to add in lone pairs, if missing. Include proper formal charge in your resonance structures (some of the structures need additional positive formal charge). Use correct resonance arrows (curved and double headed) where appropriate. What is the hybridization of all nonhydrogen atoms? A generalized 3D example is provided that can be remade to fit all of the parts of the problem. an you draw your own 3D structures? Y Z Y Z a. b. c. d. e. f.

7 g. h. 7 i. j. k. l. m. n. Problem 4 Azide and the nitronium ion can generate similar looking resonance structures, except for the formal charge. Azide is an anion and nitronium is a cation. irst, draw an acceptable 2D Lewis structure for each of these ions. Draw two additional resonance structures for each and state which are the major and minor resonance contributors. Include all of the details of resonance: lone pairs, formal charge and curved and double headed arrows. Use your 2D resonance structures to determine the hybridization of each of the atoms and draw a 3D structure of the best resonance contributor. A generalized 3D skeleton is provided as a guide. azide = 3 nitronium ion = 2 3D skeleton and resonance structures. Y Z Y Z Y Z 3. Pi bond is donated into an empty 2p orbital (usually a positive carbocation). Problem 5 Two additional resonance structures can be drawn for each of the following carbocations? Draw them using proper resonance arrows and formal charge. The first resonance contributor is formed using pattern three resonance and the second resonance contributor is formed using pattern one. Which carbocation of the three (a, b or c) is more stable? Why?

8 8 a. 2 b. 2 c. 2 Problem 6 Which pi bond is more stabilizing to the carbocation in each part? Draw the additional resonance structure(s) using proper conventions of resonance. Are any of the resonance contributors unacceptable? a b Problem 7 ow many additional resonance structures can be drawn for each of the following carbocations? Draw them using proper resonance arrows and formal charge. Which carbocation is more stable, a or b? Why? a. b. 4. Pi bond electrons are donated into an adjacent pi bond. There can be as few as two adjacent pi bonds, or essentially infinite pi systems, such as graphite. The pi bonds can be = of alkenes, alkynes and aromatics, =, triple bond and many variations of =. ne pi bond will donate pi electrons and the other pi bond will accept the pi electrons. Sometimes formal charge is created and sometimes formal charge is delocalized. In other examples neutral pi electrons are moved to new positions, but remain neutral. In all of these examples pi electrons are delocalized. We can only look at a few representative examples.

9 These can be pi donor systems. These can be pi acceptor systems. 9 donate electrons many variations donate electrons many variations donate electrons many variations

10 10 Lecture 5 a. Electrons are delocalized and no formal charge is created. This is the best kind of resonance. All of the pi bonds in the ring have shifted over by one position. The electrons that were taken away with the first arrow are filled back in with the last arrow. Benzene resonance 2D structures Pi bonds have shifted positions, but no formal charge is created. 3D structures onbenzenoid resonance 2D structures Pi bonds have shifted positions, but no formal charge is created. 3D structures

11 Problem 8 ow many atoms are in a plane in each of the two structures above? 11 b. Electrons are delocalized in a neutral, conjugated pi system and formal charge is created. The additional resonance structures are minor resonance contributors, but are often informative about the chemistry of the functional group. Typically, an electronegative atom (nitrogen or oxygen) will be at the end receiving electron density. i. ii. The pi bond shifts can be shown step wise or both resonance arrows can be used together to go from the first structure to the last structure. iii. etc. ive hydrogen atoms are not shown. iv. Symbolically, this looks the same as "i" above. owever, there is no electronegative atom to pull electrons to itself and the dipolar minor resonance contributors have much less importance than when an electronegative atom like oxygen or nitrogen is present. We don't usually write these, although they are valid contributors according to our rules. c. Electrons are delocalized in a positively charged, conjugated pi system and the positive formal charge is delocalized. The additional resonance structures can be minor or major resonance contributors, and are often informative about the chemistry of the functional group. Each of the structures could have been formed by protonation of the neutral heteroatom in strong acid. i. ii.

12 12 iii. etc. Problem 9 What is the hybridization of all nonhydrogen atoms in parts b and c above? Problem 10 Which carbonyl bond (=) is able to get more electron density from the pi bonds of the aromatic ring? The nitrogen lone pair in the second ring does not participate in resonance. Why not? The nitrogen atom off the side of the third ring powerfully donates its electrons. Why? Use 2D and/or 3D resonance structures that explain your reasoning. Problem 11 Write three additional resonance structures for the following cation. rder the resonance contributors from best (=1) to poorest. Where do you begin your arrow pushing? Look at the charge on the structure. emember, electrons are negatively charged. What is the hybridization of the nitrogen atom? A B D

13 13 Lecture 5 Problem 12 Draw a 3D structure for each given representation below. Identify the best resonance structure among any other resonance structures. Drawing the 2D possibilities first may help you do this. Show sigma bonds as lines, wedges and dashes and the p orbitals in pi bonds, as well as any orbitals holding lone pairs of electrons. Draw two dots for pi bond electrons and two dots inside a circle for lone pair electrons. Indicate any formal charge present and give the hybridization, bond angles and shape of each nonhydrogen atom. Assume full octets, unless a carbocation is indicated. Include proper resonance arrow conventions. ank your resonance structures from best (= 1) to poorest. It is helpful for drawing 3D structures to begin your drawing with a pi bond in the plane of the page and to include as many pi bonds and other atoms in the plane of the page or parallel to the page as possible. a. b. c. 3 d e. 3 3 If you begin drawing where an arrow is pointing, you can put more atoms in the plane of the paper. Problem 13 Poorer resonance structures are sometimes used to reinforce obvious polar effects and indicate typical reactivity of a functional group. This is especially true for carbonyl bonds (=). The carbonyl functional feature shows up in aldehydes, ketones, acids, esters, amides, acid chlorides, anhydrides, ureas, carbonates, urethanes and more. Indicate in which polar pi bonds, below, such a resonance structure would be reasonable and show this resonance structure. 2D structures are fine. If not expected, point out why this is the case. arbon-nitrogen pi bonds can be explained in a similar manner to the carbon-oxygen pi bonds. 2D resonance structures 3D resonance structures - carbonyl resonance Additional resonance is possible with the positively charged carbon, if it is connected to an atom with a lone pair of electrons or another pi bond. a. b. c. d. e f, g. h. i

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