Worksheet on Resonance Structures General Guidelines for Resonance Structures 1 2 3 4 5 6 7 8 Try to draw structures that are as low in energy as possible The best structures tend to have the maximum number of bonds and the most octets When structures are equivalent in terms of bonds and octets minimize formal charge to find the more stable structure All structures must be valid Only electrons may be moved to change between structures bonding sequence of atoms must remain the same Use curved arrows to show the movement of electrons Only move lone pairs and multiple bonds electrons Separate resonance structures by a double headed arrow Resonance stabilization is very important when it delocalizes or spreads a charge over two or more atoms Negative formal charges are more stable on atoms with higher electronegativities 1 Draw all the resonance structures for the following ion Show all electron pushing arrows 2 Draw all the resonance structures for the following ion Show all electron pushing arrows 3 Draw all the resonance structures 4 more for the following ion Show all electron pushing arrows 4 Draw all the resonance structures 4 more for the following ion Show all electron pushing arrows 5 Draw all the resonance structures for the following molecule Show all electron pushing arrows 6 Based on your knowledge of electron delocalization through resonance structures circle the most basic molecule for each of the following pairs of molecules 7 Based on your knowledge of electron delocalization through resonance structures circle the most acidic proton which is in bold in the following molecules Simple Ions Cations One Lewis structure for the allyl cation is This structure contains a pair of pushable electrons namely the pi electrons in the double bond between C2 and C3 The structure also contains a positively charged carbon atom at C1 that can act as a receptor A new resonance structure can be generated by pushing the pair of pi electrons to the receptor This is signified by the curved arrow with two barbs 1 2 By pushing electrons to the receptor the positive charge has been neutralized but a new positive charge has been generated at C1 Structure 1 can be regenerated from 2 by pushing the pi electrons between C1 and C2 toward the positively charged carbon C3 Structures 1 and 2 are equivalent but not identical It is important to recognize opportunities to push electrons and generate new resonance structures It is also important to recognize those structures in which one cannot push electrons In order to be able to generate one or more resonance structure a Lewis structure must have pushable electrons and a receptor Furthermore the receptor must be next to the pushable electrons 1 One Lewis structure for the butenyl cation is be generated by pushing the pi electrons to the receptor A new resonance structure can 2 There are no important resonance structures for the isopropyl cation because there are no in the structure There are no important resonance structures for dimethyl ether because although there are pushable electrons on the oxygen atom there is no There are no important resonance structures for the 5 pentenyl cation because the pushable electrons and the receptor are separated by two methylene groups CH2 Anions One Lewis structure for the acetate ion is This structure contains several pairs of pushable electrons Since the object of writing resonance structures is to show delocalization of charge it is reasonable that electrons should be pushed away from a center of negative charge Pushing a pair of unshared electrons located on the negatively charged oxygen toward the carbonyl carbon would give This however gives the intolerable situation of having ten valence electrons around the carboxyl carbon The situation can be relieved by pushing the pi electrons of the original carbon oxygen double bond toward the oxygen atom which can tolerate the negative charge Thus 3 The cyclohexane carboxylate anion has a Lewis structure Pushing a pair of unshared electrons away from the negatively charged oxygen atom and at the same time pushing a pair of pi electrons toward the other oxygen will generate a second resonance structure Thus Here are several errors and misconceptions that appear frequently in attempts to write resonance structures I Always push electron never push positive charges Remember that the arrows used in generating resonance structures indicate how the electrons are moving A structure such as the one shown below is incorrect II Always push electrons away from the centers of negative charge and toward centers of positive charge The resonance method depends on delocalizing charge Electrons which have negative charge must be pushed away from centers of relatively high electron density and toward centers of relatively low electron density More Complicated Ions Very often more than two resonance structures can be written for an ion One Lewis structure for the phenoxide ion is The structure contains a pair of pushable electrons namely the unshared electrons on the negatively charged oxygen atom The structure also contains a receptor from which a pair of pi electrons can be pushed This is shown as A B Structure B can be used to generate another resonance structure by pushing the unshared electrons at C2 toward C3 as the C3 C4 pi electrons are pushed to C4 A final structure equivalent to but not identical to B can be arrived at by pushing the unshared pair at C4 toward C5 and moving the C5 C6 pi electrons to C6 Try to complete the following without referring to the structures you have just written The complete resonance method designation for phenoxide ion including both Kekule forms of the first structure is Exercises Complete the problems below by drawing as many resonance structures as possible and using curved arrows In instances where no resonance structures can be drawn write NR no resonance A B C D E F G H I J K L M N O P Q R S T U