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Resonance
Resonance 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.
Resonance 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 1). 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. Position of atomic nuclei must be the same in all canonical forms 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 must have the same number of unpaired electrons and the same net charge.
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.
The greater the number of significant structures that can be written, and the more equal they are, the greater the overall stability.
Table 1: Requirements for Canonical Forms of Molecules with Resonance
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 Organic Molecules with Resonance 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 1: All molecules with resonance must have at least one pi bond. Criterion 2: 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.
Resonance
Most molecules with resonance can be divided into three general categories defined by the criteria above; molecules that meet criteria 1 and 2a (category A), molecules that meet criteria 1 and 2b (category B), and molecules that meet criteria 1 and 2c (category C). Typically, organic chemistry students can easily find pi bonds in a molecule, even complex molecules, to quickly determine if criterion 1 is met. Students must also possess the ability to identify conjugation and allylic or α-atoms in a molecule to determine if criteria 2a-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 2. Example 1 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 “1” and “2” and proceed to step 2. If no, then the molecule does not have resonance.
- Circle the atom(s) directly bonded to atom “1”.
- Circle the atom(s) directly bonded to atom “2”.
- Label the circled atoms “3”, “4”, “5”, and “6”.
- Is atom 3, 4, 5 or 6 part of another pi bond? If yes, then the molecule has resonance and it meets criteria 1 and 2a ” (Category A). Go to step 6.
- Does atom 3, 4, 5 or 6 have at least one lone pair of electrons or a single unpaired electron? If yes then the molecule has resonance, and meets criteria 1 and 2b (Category B). Go to step 7.
- Does atom “3, 4, 5 or 6 have a vacant p-orbital (usually a carbocation)? If yes, then the molecule has resonance and meets criteria 1 and 2c (Category C). If no, then the molecule does not have resonance.
Table 2 : Seven-step algorithm to identify organic molecules with resonance
Example 1
O
OH
H 3 C
H 3 C
1
2
3
4 5
6
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 6 and 7 even if resonance is detected in step 5.
For this molecule, the answer to question 1in the algorithm is yes. The pi bond is identified between atoms 1 and 2. The atoms of the pi bond are labeled 1 and 2. All of the atoms bonded to C 1 and C 2 are circled and numbered, 3-6. There is no second, conjugated pi bond involving atoms 4,5,6 or 7, nor is a vacant p-orbital present in atoms 4-7, so the answer to questions 5 and 7 is no. The answer to question 6 for atoms 3, 5 and 6 are no, however, atom 4 (oxygen) has a lone pair and meets criteria b. The molecule has resonance and is defined as a category B molecule.
Resonance
1
2
3
4
1
2
3
4
Figure 3 : Canonical forms of 1, 3-butadiene Figure 4 : Canonical forms of α-phellandrene
Electrons move from the 1-2 pi bond to the 2-3 pi bond and the electrons of the 3-4 pi bond become a lone pair on atom 4. A curved arrow with the tail of the arrow originating at the pi bond between carbons 1 and 2 and the head of the arrow directed at the bond between carbons 2 and 3 is drawn, indicating the movement of the pi electrons from the C 1 -C 2 pi bond to the C 2 -C 3 bond. In the same structure, a second curved arrow is drawn, with the tail of the arrow positioned near the C 3 -C 4 pi bond and the head of the arrow directed at C 4. 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. Note that for 1,3-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.
Other 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 1, 3-butadiene and α-phellandrene. In these cases, the conjugation extends beyond four atoms.
1 2
3
4
5
6
1 2
3
4
5
6
Figure 5: Canonical forms of benzene
N O
1 2
3
4 N O
1 2
3
4
Figure 6: Canonical forms of isoxazole
1
2
3 4
1
2
3
4
For example, the six carbon atoms of benzene make up the continuous pi system and are labeled 1-6 (Figure 5). Canonical forms of benzene are generated when the C 1 -C 2 pi electrons are moved to reside between C 2 -C 3. The electrons of the second pi bond (C 3 -C 4 ) are then “pushed” out to reside between C 4 -C 5. Finally, the pi electrons of the C 5 -C 6 pi bond move to the C 6 -C 1 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
Resonance
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 (4) as shown in Figure
- Electrons move from the 1-2 pi bond to the 2-3 pi bond and the electrons of the 3-4 pi bond become a lone pair on atom 4. Formal charges and lone pairs are inserted to generate the canonical form
O
1
2
4
3 5
6 O
1
2 4
3 5
6
O
1
2 4
3 5
6 O
1
2
4
3 5
6
Figure 7: Canonical forms of β-ionone
Drawing Canonical Forms of Category B Molecules The generic structure in Figure 8 can be used to represent category B molecules. X 1 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 1 , and numbering through the two atoms of the pi bond. Electrons are moved from the lone pair on atom 1, to a pi bond between atom 1 and 2. The electrons of the 2-3 pi bond move to atom 3 as a lone pair. Formal charges are inserted appropriately.
X 1
X 1 = carbanion, O, N, S, halogen
X 1
Figure 8: Generic Structure of Category B Molecules
O O
1
2
3
1
2
3
Figure 9: Enolate Carbanion
Similarly, the six-atom conjugated system of β-ionone (Figure 7) 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 1-2 pi bond to the 2-3 pi bond, electrons from the 3-4 pi bond to the 4- bond, and electrons from the 5-6 pi bond to a lone pair on oxygen (atom 6) and inserting the appropriate formal charges and lone pairs generates a new canonical form. Additional canonical forms can be drawn involving only atoms 1-4 and only atoms 3-6 using the same general rules.
Consider the example of the enolate (carbanion) of cyclohexanone, given in Figure 9. The lone pair electrons on the α-carbon atom (1) move toward the pi bond (2-3) as indicated by the curved arrow with its tail originating at the lone pair and its head oriented to the C 1 -C 2 bond. In the same structure a second curved arrow is draw to indicate the repositioning of the pi electrons located between the C 2 -C 3 bond. These pi electrons are “pushed” to reside on the oxygen atom (3), indicated by the second curved arrow, to generate the new canonical form.
Resonance
N
N
NH 2
O
OH
O P
OH
OH
O
1 2 3
4
5
6
7
8
Figure 12: 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 N 7 through the pi system involving atoms C 6 and N 1. The conjugated pi system defined by atoms C 4 , C 5 , C 6 and N 7 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. Notice that this secondary canonical form is identical to the tertiary canonical, IV form derived in Figure
- Lastly, the pi electrons of the C 5 -C 6 bond can be delocalized to the vacant p-orbital of the C 4 carbocation of the secondary canonical form, VI, to give the tertiary canonical form VII.
N
N
NH 2
O R
N
N
NH 2
O R
N
N
NH 2
O R
N
N
NH (^2)
O R
1 Category A
(^2 )
4
5
6
7
8
1
(^2 )
4
5
6
7
8
1
(^2 ) 4
5
6
7
8
1
(^2 ) 4
5
6
7
8
Category C Category B
Original Structure I
Primary Canonical Form II
Secondary Canonical Form III
Tertiary Canonical Form IV
Figure 13: Canonical Forms of Cidofovir
N
N
NH 2
O R
N
N
NH 2
O R
N
N
NH 2
O R
N
N
NH 2
O R
1 Category B
(^2 )
4
5
6
7
8
1
(^2 )
4
5
6
7
8
1
(^2 ) 4
5
6
7
8
1
(^2 ) 4
5
6
7
8
Category A Category C
Original Structure
Primary Canonical Form V
Secondary Canonical Form VI
Tertiary Canonical Form VII
Figure 14: Canonical Forms of Cidofovir
