The Structure Is An Anion With Three Possible Resonance Contributors

Let's delve into the fascinating world of anions exhibiting resonance, specifically focusing on those with three possible resonance contributors. These structures are common in organic chemistry and play a crucial role in determining a molecule's stability, reactivity, and properties. Understanding the principles governing resonance and its application to anions with three resonance forms is essential for any aspiring chemist.

Understanding Resonance

Resonance, also known as mesomerism, is a way of describing delocalized electrons in certain molecules or polyatomic ions where the bonding cannot be expressed by a single Lewis formula. Instead, a combination of several Lewis structures, known as resonance structures or resonance contributors, is used to represent the actual electronic structure. The actual structure is understood to be an intermediate or hybrid of these contributing structures.

Key Principles of Resonance:

• Only electrons move: Atoms never change position between resonance structures. The only difference lies in the arrangement of pi electrons and lone pairs.
• Resonance structures are not isomers: Isomers have different arrangements of atoms, while resonance structures represent different arrangements of electrons within the same atomic framework.
• The true structure is a hybrid: The actual molecule is not constantly flipping between resonance structures. It exists as a resonance hybrid, which is a weighted average of all contributing structures.
• Resonance stabilizes the molecule: Delocalization of electrons lowers the overall energy of the molecule, making it more stable than any single resonance contributor would suggest.
• Equivalent resonance structures contribute equally: If all resonance structures are identical in terms of bonding and charge distribution, they contribute equally to the resonance hybrid.
• Non-equivalent resonance structures contribute unequally: If resonance structures differ in stability, the more stable structures contribute more to the resonance hybrid. Factors influencing stability include:
  • Octet rule: Structures where all atoms have a complete octet are generally more stable.
  • Charge separation: Structures with minimal charge separation are more stable.
  • Electronegativity: Negative charge should reside on the more electronegative atom(s). Positive charge should reside on the more electropositive atom(s).
  • Bonding: Structures with more bonds are generally more stable.

Anions and Resonance

Anions, being negatively charged species, are often stabilized by resonance. The negative charge can be delocalized over multiple atoms, reducing the charge density on any single atom and thus increasing stability. When dealing with anions exhibiting three resonance contributors, the principles of resonance described above become even more critical in evaluating the relative contribution of each structure.

Common Examples of Anions with Three Resonance Contributors

Several important classes of organic anions exhibit three resonance contributors. Here are a few prominent examples:

• Enolates: Enolates are anions formed by the deprotonation of a carbon atom adjacent to a carbonyl group (C=O). The negative charge can be delocalized between the carbon and the oxygen atom of the carbonyl group, resulting in three resonance contributors.
• Allylic Anions: Allylic anions are formed by the deprotonation of a carbon atom adjacent to a double bond (C=C). The negative charge can be delocalized over the allylic system, creating three resonance contributors.
• Phenoxide Ions: Phenols, compounds with a hydroxyl group (OH) attached to a benzene ring, can be deprotonated to form phenoxide ions. The negative charge on the oxygen atom can be delocalized into the benzene ring via resonance, resulting in multiple (more than three, but key intermediate steps involve three contributor-like structures) resonance contributors. We'll focus on the core concepts applicable to three contributors even in this context.
• Carboxylate Ions (R-COO-): While often depicted with two main resonance structures, understanding the reaction mechanisms involving carboxylate ions sometimes requires considering a third, less significant, contributor to fully explain certain aspects of their reactivity.

We'll now analyze each of these examples in more detail, highlighting the resonance structures and discussing their relative contributions to the resonance hybrid.

Case Study 1: Enolates

Enolates are crucial intermediates in many organic reactions, including aldol condensations, Claisen ester condensations, and alkylation reactions. The formation of an enolate requires the removal of a proton from an α-carbon (a carbon adjacent to the carbonyl group) by a base.

Resonance Structures of an Enolate:

Consider the enolate derived from acetaldehyde (CH3CHO). The three possible resonance structures are:

1. Structure 1: CH2=CH-O- (Negative charge on oxygen, double bond between Cα and Cβ)
2. Structure 2: -CH2-CH=O (Negative charge on the α-carbon, double bond between the carbonyl carbon and oxygen)
3. Structure 3: CH2(-)-CH=O (Carbanion character on alpha carbon, with implied positive charge somewhere else in the molecule for charge neutrality, often not explicitly drawn but important for understanding reactivity)

Relative Contribution:

• Structure 1 is generally considered the major contributor. Oxygen is more electronegative than carbon, and thus better able to stabilize the negative charge. This structure places the negative charge on the oxygen atom, satisfying the electronegativity principle.
• Structure 2 is a significant contributor but less stable than Structure 1. The negative charge resides on the carbon atom, which is less electronegative than oxygen.
• Structure 3 represents the carbanion character of the enolate. Although it's not a dominant contributor in terms of representing the overall electron distribution, it's vital for understanding the reactivity of the enolate. This structure highlights the nucleophilic character of the α-carbon, making it a reactive site for electrophilic attack.

Implications for Reactivity:

The resonance hybrid of the enolate reflects that it is nucleophilic at both the oxygen and the α-carbon atoms. This means that enolates can react with electrophiles at either position, leading to the formation of different products. The regioselectivity (which atom reacts) is influenced by several factors, including the electrophile, the solvent, and the counterion associated with the enolate. Hard electrophiles tend to react at the oxygen (oxygen is "harder" than carbon), while softer electrophiles tend to react at the α-carbon.

Case Study 2: Allylic Anions

Allylic anions are formed by the deprotonation of a carbon adjacent to a carbon-carbon double bond. They are important intermediates in reactions such as allylic alkylations.

Resonance Structures of an Allylic Anion:

Consider the allylic anion derived from propene (CH2=CH-CH3). The three possible resonance structures are:

1. Structure 1: CH2=CH-CH2- (Negative charge on the terminal carbon, double bond between the other two carbons)
2. Structure 2: -CH2-CH=CH2 (Negative charge on the other terminal carbon, double bond between the other two carbons)
3. Structure 3: CH2(-)-CH=CH2 (Carbanion character, implied positive charge elsewhere - highlights carbon reactivity)

Relative Contribution:

• Structures 1 and 2 are equivalent and contribute equally to the resonance hybrid if the substituents on the terminal carbons are identical. This is because the negative charge is equally delocalized across both terminal carbons.
• Structure 3, while not explicitly drawn as frequently, is crucial for depicting the reactivity of the allylic anion as a carbanion capable of nucleophilic attack.

Implications for Reactivity:

The resonance hybrid of the allylic anion indicates that the negative charge is delocalized over the two terminal carbons. Therefore, an electrophile can attack at either of these positions. In cases where the substituents on the terminal carbons are different, the major product will depend on the relative stability of the resulting alkenes.

Case Study 3: Phenoxide Ions

Phenols are weakly acidic due to the ability of the resulting phenoxide ion to be stabilized by resonance. Deprotonation of the hydroxyl group leads to a negative charge on the oxygen, which can be delocalized into the aromatic ring.

Resonance Structures of a Phenoxide Ion:

Consider the phenoxide ion derived from phenol (C6H5OH). While there are more than three resonance structures, let's focus on a key subset that mirrors the behavior of structures with three contributors. We are simplifying the full set of resonance structures to illustrate the principles.

1. Structure 1: O- (Negative charge on the oxygen atom directly attached to the benzene ring)
2. Structure 2: O= (Double bond between the oxygen and a carbon on the benzene ring, negative charge delocalized to an ortho position)
3. Structure 3: O= (Double bond between the oxygen and a carbon on the benzene ring, negative charge delocalized to the para position)

Relative Contribution:

• Structure 1, with the negative charge localized on the electronegative oxygen atom, is a significant contributor.
• Structures 2 and 3, representing the delocalization of the negative charge into the benzene ring to the ortho and para positions respectively, are also important contributors. These demonstrate how the electron density spreads throughout the ring system.
• There are additional resonance structures where the negative charge is delocalized to the other ortho position (equivalent to Structure 2) and para position.

Implications for Reactivity:

The resonance hybrid of the phenoxide ion shows increased electron density at the ortho and para positions of the benzene ring. This makes these positions susceptible to electrophilic attack. Phenols are therefore ortho, para-directing in electrophilic aromatic substitution reactions.

Factors Affecting the Contribution of Resonance Structures

Several factors influence the relative contribution of each resonance structure to the resonance hybrid:

• Electronegativity: Structures with negative charges on more electronegative atoms are more stable and contribute more.
• Charge Separation: Structures with minimal charge separation are generally more stable. Avoid placing like charges close to each other or separating opposite charges over large distances.
• Octet Rule: Structures where all atoms have a complete octet are more stable.
• Bonding: Structures with more bonds are generally more stable.
• Equivalence: If resonance structures are equivalent, they contribute equally.

Identifying and Drawing Resonance Structures

Here's a step-by-step guide to drawing resonance structures:

1. Identify the conjugated system: Look for pi bonds and lone pairs that are adjacent to each other (separated by only one sigma bond). This is where electron delocalization can occur.
2. Move electrons: Use curved arrows to show the movement of electron pairs. Remember that electrons move towards more electronegative atoms or to form new pi bonds.
3. Draw the new structure: Draw the structure that results from the movement of electrons. Be sure to show all lone pairs, formal charges, and pi bonds.
4. Repeat steps 2 and 3: Continue moving electrons to generate all possible resonance structures.
5. Evaluate the structures: Assess the relative stability of each structure based on the factors discussed above.

The Importance of Resonance in Chemistry

Resonance is a fundamental concept in chemistry with broad implications:

• Stability: Resonance stabilizes molecules and ions, making them less reactive.
• Reactivity: Resonance influences the reactivity of molecules and ions, determining where reactions will occur.
• Acidity and Basicity: Resonance can affect the acidity and basicity of compounds by stabilizing the conjugate base or acid.
• Spectroscopy: Resonance affects the spectroscopic properties of molecules, such as UV-Vis absorption.
• Drug Design: Understanding resonance is crucial in drug design, as it can affect how a drug interacts with its target.

Advanced Considerations

While this article focuses on three resonance contributors for clarity, many molecules exhibit more complex resonance systems with multiple contributors. The same principles apply, but the analysis becomes more intricate. Computational chemistry methods can be used to accurately predict the relative contribution of each resonance structure and the overall electronic structure of the molecule. Furthermore, hyperconjugation and other subtle electronic effects can further stabilize anionic species, adding layers of complexity beyond simple resonance depictions.

Conclusion

Understanding resonance, especially in the context of anions with three possible resonance contributors, is crucial for predicting and explaining chemical behavior. By considering the principles of electron delocalization, electronegativity, charge separation, and octet rule satisfaction, chemists can assess the relative contribution of each resonance structure and understand the overall properties of the molecule or ion. This knowledge is essential for designing new reactions, developing new materials, and understanding the complexities of chemical systems. The examples of enolates, allylic anions, and phenoxide ions illustrate the importance of resonance in stabilizing anions and influencing their reactivity. Mastering these concepts will undoubtedly enhance your understanding and appreciation of organic chemistry.