Table 1: Requirements for Canonical Forms of Molecules with Resonance

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1 rganic Chemistry esonance Lecture eview esonance theory is a simplified alternative to rigorous mathematical descriptions of molecular structure and when used in qualitatively is a convenient method for depicting electron delocalization in molecules. It is among the most useful concepts for the introductory organic chemistry student. Practical application of resonance theory can help students estimate electron distribution within molecules and allows the prediction of chemical reactivity and relative stability of reactants, reaction intermediates and products. esonance and resonance hybrids are defined and descriptive requirements for canonical forms, all of which contribute to the molecular structure, are frequently published in both introductory and advanced organic chemistry texts (See table ). These requirements accurately define criteria for canonical forms, however they are inadequate in defining clear strategies to identify molecules with resonance, and more importantly, do not clearly direct students through a methodical process to draw all or the most important canonical forms of a molecule with resonance. All canonical forms must be valid Lewis structures. All atoms involved in delocalization of electrons must lie approximately in the same plane to allow for maximum overlap of p-orbitals. All canonical forms do not contribute equally to the molecular structure. The greatest contributors have a maximum number of covalent bonds, minimum charge separation, negative charges on the most electronegative atoms possible, and positive charges on the most electropositive atoms possible. Position of atomic nuclei must be the same in all canonical forms All canonical forms must have the same number of unpaired electrons and the same net charge. The greater the number of significant structures that can be written, and the more equal they are, the greater the overall stability. Table : equirements for s of Molecules with esonance A simple approach to teaching organic chemistry students how to identify molecules with resonance, and to draw proper canonical forms using a rational strategy incorporating the curved arrow convention has been developed. The approach presented here is confined to two-electron processes, however it could be extrapolated to include one-electron processes as well. Identifying rganic Molecules with esonance In order to utilize resonance theory effectively, students must first recognize what kinds of organic molecules have resonance. The following criteria can be used to describe the minimum structural requirements that a molecule must possess to have resonance. Specifically, molecules with resonance must meet two criteria, given below: Criterion : All molecules with resonance must have at least one pi bond. Criterion : All molecules with resonance must also have at least one of the following a) a second, conjugated pi bond, or b) an allylic or α-atom with at least one lone pair of electrons, or c) an allylic atom with a vacant p-orbital.

2 rganic Chemistry esonance Lecture eview Most molecules with resonance can be divided into three general categories defined by the criteria above; molecules that meet criteria and a (category A), molecules that meet criteria and b (category B), and molecules that meet criteria and c (category C). Typically, organic chemistry students can easily find pi bonds in a molecule, even complex molecules, to quickly determine if criterion is met. Students must also possess the ability to identify conjugation and allylic or α-atoms in a molecule to determine if criteria a-c are met. An algorithm that helps students to develop the ability to identify molecules with resonance using the criteria above is outlined in the seven steps, listed in Table. Example uses this seven-step process to illustrate its effectiveness.. Does the molecule have at least one pi bond? If yes, then number the two atoms of the pi bond and and proceed to step. If no, then the molecule does not have resonance.. Circle the atom(s) directly bonded to atom.. Circle the atom(s) directly bonded to atom.. Label the circled atoms,,, and.. Is atom,, or part of another pi bond? If yes, then the molecule has resonance and it meets criteria and a (Category A). Go to step.. Does atom,, or have at least one lone pair of electrons or a single unpaired electron? If yes then the molecule has resonance, and meets criteria and b (Category B). Go to step.. Does atom,, or have a vacant p-orbital (usually a carbocation)? If yes, then the molecule has resonance and meets criteria and c (Category C). If no, then the molecule does not have resonance. Table : Seven-step algorithm to identify organic molecules with resonance Example H C H C H For this molecule, the answer to question in the algorithm is yes. The pi bond is identified between atoms and. The atoms of the pi bond are labeled and. All of the atoms bonded to C and C are circled and numbered, -. There is no second, conjugated pi bond involving atoms,, or, nor is a vacant p-orbital present in atoms -, so the answer to questions and is no. The answer to question for atoms, and are no, however, atom (oxygen) has a lone pair and meets criteria b. The molecule has resonance and is defined as a category B molecule. It is important for students to recognize that within the same molecule, multiple structural elements may have resonance, and may fall into more than one of the three defined categories. Thus the proposed algorithm requires that students continue through steps and even if resonance is detected in step.

3 rganic Chemistry esonance Lecture eview Example below illustrates this point. Example H H H H H For structure A, there are two pi bonds, a carbon-carbon pi bond and a carbon-oxygen pi bond. Either one of these could be defined as the pi bond necessary to meet criterion. If we label the C-C pi bond as and, all of the atoms bonded to C and C are circled and numbered, -. The answer to question is yes. Carbon is part of a second, conjugated pi bond (the carbon-oxygen pi bond), therefore criteria a is met. The molecule is designated as category A. The answers to questions and are no A H H H H B In structure B, the carbon-oxygen pi bond is identified to meet criterion, and the oxygen and carbon atoms are labeled as and respectively. The two atoms directly bonded to carbon (oxygen has no additional bonds) are circled and labeled as and. The answer to question is yes. Carbon is part of a second, conjugated pi bond (the carbon-carbon pi bond), therefore criteria a is met and the molecule has resonance. Furthermore, the oxygen atom labeled as in structure B has lone pair electrons associated with it, thus criteria b is also met. The molecule is designated as category A and category B. Drawing s of Molecules with esonance nce it has been established that a molecule has resonance, pi electrons or lone pair electrons can be moved through the pi system in a manner that is allowed by the curved arrow convention to generate canonical forms. For each of the resonance categories, A, B and C, distinct processes, all utilizing the curved arrow convention, can be applied to the molecule to draw canonical forms. A set of detailed directions for each category of molecule has been developed that are aimed at simplifying the process of drawing canonical forms for students. Drawing s of Category A Molecules The process of drawing canonical forms of category A molecules begins by consecutively numbering the four atoms that make up the conjugated system, giving the most electronegative atom the highest number. In a simple category A molecule like,-butadiene (shown in Figure ), all of the carbon atoms make up the conjugated system and are numbered -. In α-phellandrene (Figure ), the conjugated system is embedded in a larger structure, however only the four atoms of the conjugated system are numbered. It is sometimes useful to circle the conjugated part of a complex structure to clearly separate it from the rest of the molecule.

4 rganic Chemistry esonance Lecture eview Figure : Canonical forms of, -butadiene Figure : Canonical forms of α-phellandrene Electrons move from the - pi bond to the - pi bond and the electrons of the - pi bond become a lone pair on atom. A curved arrow with the tail of the arrow originating at the pi bond between carbons and and the head of the arrow directed at the bond between carbons and is drawn, indicating the movement of the pi electrons from the C -C pi bond to the C -C bond. In the same structure, a second curved arrow is drawn, with the tail of the arrow positioned near the C -C pi bond and the head of the arrow directed at C. The molecule is redrawn incorporating the new positions of the pi electrons, and new formal charges and lone pairs are inserted to generate the canonical form. The same process can be used to derive canonical forms of α-phellandrene. ote that for,-butadiene and α-phellandrene, the resulting canonical forms introduce formal charges or charge separation into the molecule, thus these canonical forms are not significant contributors to the overall structures. ther category A molecules contain a continuous pi system, such as in the case of benzene and other aromatic hydrocarbons. For these molecules, canonical forms without charge separation can be. drawn using the same general strategy as that proposed for, -butadiene and α-phellandrene. In these cases, the conjugation extends beyond four atoms. Figure : Canonical forms of benzene For example, the six carbon atoms of benzene make up the continuous pi system and are labeled - (Figure ). Canonical forms of benzene are generated when the C -C pi electrons are moved to reside between C -C. The electrons of the second pi bond (C -C ) are then pushed out to reside between C -C. Finally, the pi electrons of the C -C pi bond move to the C -C position. The resulting structure that accounts for the pi electron movement is a canonical form of benzene, with no charge separation, equal in stability to the original structure Figure : Canonical forms of isoxazole

5 rganic Chemistry esonance Lecture eview In the molecule isoxazole, the conjugated system contains three carbon atoms and one nitrogen atom. The atoms are numbered so that the nitrogen atom has the highest number () as shown in Figure. Electrons move from the - pi bond to the - pi bond and the electrons of the - pi bond become a lone pair on atom. Formal charges and lone pairs are inserted to generate the canonical form Figure : Canonical forms of β-ionone Similarly, the six-atom conjugated system of β-ionone (Figure ) is numbered to give the electronegative oxygen atom the number six position. β-ionone also represents a molecule with extended conjugation, i.e conjugation involving more than four atoms. Moving electrons from the - pi bond to the - pi bond, electrons from the - pi bond to the - bond, and electrons from the - pi bond to a lone pair on oxygen (atom ) and inserting the appropriate formal charges and lone pairs generates a new canonical form. Additional canonical forms can be drawn involving only atoms - and only atoms - using the same general rules. Drawing s of Category B Molecules The generic structure in Figure can be used to represent category B molecules. X represents the allylic or α-atom with at least one lone pair (i.e., a carbanion, a halogen, nitrogen, oxygen or sulfur atom). Drawing canonical forms of these molecules requires consecutively numbering the atoms of the structure, starting with X, and numbering through the two atoms of the pi bond. Electrons are moved from the lone pair on atom, to a pi bond between atom and. The electrons of the - pi bond move to atom as a lone pair. Formal charges are inserted appropriately. X X = carbanion,,, S, halogen Figure : Generic Structure of Category B Molecules X Figure 9: Enolate Carbanion Consider the example of the enolate (carbanion) of cyclohexanone, given in Figure 9. The lone pair electrons on the α-carbon atom () move toward the pi bond (-) as indicated by the curved arrow with its tail originating at the lone pair and its head oriented to the C -C bond. In the same structure a second curved arrow is draw to indicate the repositioning of the pi electrons located between the C -C bond. These pi electrons are pushed to reside on the oxygen atom (), indicated by the second curved arrow, to generate the new canonical form.

6 rganic Chemistry esonance Lecture eview Drawing s of Category C Molecules Category C molecules are typically those that contain allylic and benzylic carbocations. These molecules play dominant roles in the prediction of products of reactions involving carbocation intermediates. In these systems, the allylic atom and the two atoms of the pi bond are numbered -. Secondary Allylic carbocation Primary Allylic carbocation Figure 0: s of an Allylic Carbocation The electrons of the pi bond move toward the vacant p-orbital of the allylic atom as indicated by the curved arrow in Figure 0. ote that in this particular example, the structure on the left depicts a secondary carbocation, and the structure on the right depicts a primary carbocation. The structure on the left would be a greater resonance contributor that the structure on the right, since secondary carbocations are more stable than primary carbocations. For benzylic carbocations, the same process can be applied to derive multiple canonical forms. The numbering scheme and the curved arrow convention can be applied to the first canonical structure, from which subsequent canonical forms can be generated. CH CH CH CH Figure : s of Category C Molecules Molecules with Multiple s For some molecules, structural elements may be present that meet the criteria of multiple categories, giving rise to numerous canonical forms. These canonical forms may be generated directly from the original structure (primary), or from canonical forms subsequently derived from the original structure (secondary, tertiary, etc). For example, the molecule cidofovir, shown in Figure, has structural elements that meet the criteria for both category A and category B molecules. The atoms of the sixmembered ring, the oxygen atom of the urea, and the nitrogen atom of the amino substituent have been numbered to indicate the various combinations of atoms and bonding that fit these categories. The bonding pattern among atoms C,, C and of the original structure, I, indicates a category A molecule where pi electrons are moved from the C -C pi bond toward C, as indicated by the curved arrows in Figure. The C - pi electrons are pulled toward the electronegative oxygen atom to generate a primary canonical form, II. The category C designation can then be applied to atoms C, C and C of the primary canonical form to generate a secondary canonical form, III. Finally, atoms, C and of the secondary canonical form, III meet the requirements for a category B molecule, from which the tertiary canonical form, III can be generated by moving the lone pair electrons on toward the pi bonds, while the pi electrons shifty out to. The percent contribution of each of these canonical forms to the overall molecular structure, regardless of its designation as primary, secondary or tertiary, is dependent on its stability.

7 rganic Chemistry esonance Lecture eview H P H H H Figure : Structure of Cidofovir Various combinations of atoms and bonding also could be identified in cidofovir to meet the criteria for a category B molecule. A primary canonical form (V) can be derived from the original structure by delocalizing the lone pair electrons of through the pi system involving atoms C and. The conjugated pi system defined by atoms C, C, C and of the primary canonical form V, then meets the criteria for a category A molecule from which the secondary canonical form, VI, can be derived. otice that this secondary canonical form is identical to the tertiary canonical, IV form derived in Figure. Lastly, the pi electrons of the C -C bond can be delocalized to the vacant p-orbital of the C carbocation of the secondary canonical form, VI, to give the tertiary canonical form VII. H Category A H Category C H Category B H riginal Structure I Primary II Secondary III Tertiary IV Figure : s of Cidofovir H Category B H Category A H Category C H riginal Structure Primary V Secondary VI Tertiary VII Figure : s of Cidofovir

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