Draw All Significant Resonance Structures For The Following Compound

Drawing Significant Resonance Structures: A Comprehensive Guide

Resonance structures are crucial for understanding the behavior of many organic and inorganic molecules. They represent the delocalization of electrons within a molecule, leading to a more stable overall structure. This article provides a comprehensive guide on drawing significant resonance structures, focusing on methodology, identifying key features, and avoiding common pitfalls. We'll delve into several examples, highlighting the nuances of resonance and its implications for molecular properties.

Understanding Resonance: The Basics

Before we start drawing resonance structures, let's solidify our understanding of the concept. Resonance describes a phenomenon where a single Lewis structure is insufficient to accurately represent the true bonding in a molecule. Instead, the molecule is best represented by a combination, or resonance hybrid, of several contributing resonance structures. These structures differ only in the placement of electrons – the positions of atoms remain unchanged.

Key Characteristics of Resonance Structures:

• Electron Delocalization: Electrons are not localized between two specific atoms but are spread out over multiple atoms.
• Equivalent Structures: All contributing structures have the same arrangement of atoms. Only the placement of electrons (and therefore formal charges) differs.
• Resonance Hybrid: The true structure is a weighted average of all contributing resonance structures. This hybrid is more stable than any individual contributing structure.
• Curved Arrows: We use curved arrows to show the movement of electron pairs when transitioning between resonance structures. A single-headed arrow indicates the movement of a single electron, while a double-headed arrow indicates the movement of an electron pair.

Steps for Drawing Resonance Structures

Drawing accurate and significant resonance structures requires a systematic approach. Follow these steps:

1. Draw the Lewis Structure: Begin by drawing the Lewis structure of the molecule, including all lone pairs and formal charges.

2. Identify Electron-Rich and Electron-Poor Centers: Look for atoms with lone pairs (electron-rich) or atoms with incomplete octets (electron-poor). These are the potential sites for electron movement.

3. Move Electron Pairs Using Curved Arrows: Identify possible electron movements. A curved arrow originates from an electron pair (either a lone pair or a pi bond) and points towards the electron-deficient atom or the region where the electron pair can move. Remember, only electrons move; atoms remain fixed.

4. Draw Each Resonance Structure: For each possible electron movement, draw a new resonance structure. Make sure to correctly adjust formal charges in each new structure.

5. Evaluate the Significance of Each Structure: Not all resonance structures contribute equally to the resonance hybrid. More significant structures are those with:

   • Minimized Formal Charges: Structures with fewer and smaller formal charges are generally more stable and contribute more significantly.
   • Octet Rule Satisfaction: Structures where the majority of atoms satisfy the octet rule (or duet rule for hydrogen) are more favored.
   • Negative Charges on More Electronegative Atoms: If negative charges exist, they are more stable on electronegative atoms (like oxygen or nitrogen).
   • Positive Charges on Less Electronegative Atoms: If positive charges exist, they are more stable on less electronegative atoms (like carbon).
   • Separation of Charge: Structures with charges separated over multiple atoms are generally less stable than those with charges localized on single atoms.

6. The Resonance Hybrid: The resonance hybrid represents the average of all significant resonance structures. It doesn't depict the molecule at any particular instant, but rather the overall electron distribution.

Examples of Drawing Resonance Structures

Let's analyze some examples to illustrate the process of drawing and evaluating resonance structures.

Example 1: Nitrate Ion (NO₃⁻)

The nitrate ion, NO₃⁻, is a classic example exhibiting resonance.

1. Lewis Structure: The Lewis structure shows one nitrogen atom singly bonded to two oxygen atoms and doubly bonded to one oxygen atom. The negative charge resides on one of the singly bonded oxygen atoms.

2. Resonance Structures: The double bond can be moved to either of the other two oxygen atoms, creating three equivalent resonance structures. The negative charge will move accordingly.

3. Significance: All three resonance structures are equally significant because they have the same formal charges and octet rule satisfaction.

4. Resonance Hybrid: The resonance hybrid shows that the negative charge is delocalized across all three oxygen atoms, and the nitrogen-oxygen bonds have a bond order of 4/3 (1.33).

Example 2: Benzene (C₆H₆)

Benzene is another quintessential example.

1. Lewis Structure: The Lewis structure shows alternating single and double bonds in a hexagonal ring of carbon atoms.

2. Resonance Structures: The pi electrons can be moved around the ring, creating two major resonance structures. These structures show alternating single and double bonds but with different double bond positions.

3. Significance: Both resonance structures are equally significant, leading to a delocalized pi electron system.

4. Resonance Hybrid: The resonance hybrid depicts all carbon-carbon bonds as identical, having a bond order of 1.5. This explains benzene's exceptional stability.

Example 3: Acetate Ion (CH₃COO⁻)

Acetate ion presents a slightly different scenario.

1. Lewis Structure: The Lewis structure shows one carbon atom double-bonded to one oxygen atom and singly bonded to another oxygen atom with a negative charge. There's also a methyl group (CH₃) bonded to the carbon.

2. Resonance Structures: The double bond can move to the other oxygen atom, creating a resonance structure where the negative charge resides on the other oxygen.

3. Significance: Both structures contribute significantly to the resonance hybrid, though the initial structure with the negative charge on the more electronegative oxygen is slightly more significant.

4. Resonance Hybrid: The negative charge is delocalized over both oxygen atoms, leading to a greater stability than either individual resonance structure.

Example 4: A More Complex Example (Illustrative)

Consider a molecule with a conjugated system involving multiple pi bonds and lone pairs. The process would involve identifying all possible electron movements, drawing each resonance structure, and carefully evaluating their relative contributions based on the factors mentioned earlier (minimized formal charges, octet rule, charge distribution). This might lead to several resonance structures, with some being significantly more important than others.

Common Mistakes to Avoid

• Moving Atoms: Remember, only electrons move in resonance structures. Atoms remain in the same position.
• Incorrect Formal Charges: Always calculate and display formal charges correctly in each resonance structure.
• Ignoring Octet Rule (Where Applicable): Although exceptions exist, strive to satisfy the octet rule as much as possible.
• Failing to Evaluate Significance: Don't treat all resonance structures as equally important. Evaluate their contributions based on the factors discussed above.
• Incomplete Electron Movement: Ensure that all possible significant electron movements are considered. Sometimes, multiple steps are required to reach a valid resonance structure.

Conclusion

Understanding and effectively drawing resonance structures is a fundamental skill in chemistry. By following a systematic approach, carefully evaluating the significance of different structures, and avoiding common pitfalls, you can accurately represent the electron distribution within molecules and gain valuable insights into their properties and reactivity. Remember that the resonance hybrid is the best representation of the molecule and accounts for its overall stability and properties. The examples presented here illustrate a range of complexities, highlighting the versatility of resonance theory in explaining molecular behavior. Mastering this concept will significantly enhance your understanding of chemical bonding and reactivity.