Draw A Second Resonance Form For The Structure Shown Below

Drawing a Second Resonance Form: A Deep Dive into Organic Chemistry

Resonance structures are a crucial concept in organic chemistry, representing the delocalization of electrons within a molecule. They don't represent different molecules, but rather different ways of depicting the same molecule, showing the distribution of electron density. Understanding resonance is key to predicting reactivity, stability, and properties of organic compounds. This article will explore the concept of resonance, focusing on how to draw a second resonance structure for a given molecule, including examples and detailed explanations.

Understanding Resonance Structures

Before diving into drawing a second resonance structure, it's crucial to understand the fundamental principles of resonance. Resonance structures are used when a single Lewis structure cannot accurately represent the bonding in a molecule. This usually happens when there are conjugated pi systems – alternating single and multiple bonds – or when lone pairs of electrons are adjacent to pi bonds.

Key characteristics of resonance structures:

• Same connectivity: The atoms remain connected in the same way in all resonance structures. Only the placement of electrons changes.
• Different electron placement: The position of electrons (both bonding and non-bonding) differs between resonance structures.
• Formal charges: Formal charges may differ between resonance structures.
• Contributing structures: No single resonance structure accurately represents the true molecule; instead, the actual molecule is a hybrid of all contributing resonance structures. The hybrid is more stable than any individual contributor.
• Curved arrows: Curved arrows are used to show the movement of electron pairs in transforming one resonance structure into another. The arrow's tail starts at the electron source (lone pair or bond), and the arrowhead points to where the electrons move.

Steps to Draw a Second Resonance Structure

Let's outline the steps involved in drawing a second (or subsequent) resonance structure:

1. Identify the conjugated system: Look for alternating single and multiple bonds, or lone pairs adjacent to pi bonds. This is the region where electron delocalization will occur.

2. Identify potential electron movement: Look for lone pairs that can be donated into a pi system, or pi bonds that can be moved to create a new pi bond elsewhere.

3. Use curved arrows to show electron movement: Draw curved arrows to indicate the movement of electron pairs. Remember, arrows always start at the electron source and point to where the electrons are moving. Only move one pair of electrons at a time with each arrow.

4. Draw the new structure: Based on the electron movement indicated by the arrows, draw the new resonance structure. Remember to maintain the same connectivity of atoms.

5. Check formal charges: Calculate the formal charge on each atom in the new resonance structure. The sum of formal charges should equal the overall charge of the molecule (or ion).

6. Assess the relative stability of resonance structures: Resonance structures with fewer formal charges and charges on more electronegative atoms are generally more stable.

Example: Drawing a Second Resonance Structure for a Carboxylate Ion

Let's consider the carboxylate ion (RCOO⁻), a common functional group in organic chemistry. It's a great example to illustrate the concept of resonance.

Structure 1:

      O⁻
      ||
     C
    /  \
   R    O

In this structure, the oxygen atom bearing the negative charge has a formal charge of -1, and the other oxygen has a formal charge of 0.

Drawing the second resonance structure:

1. Identify the conjugated system: The C=O double bond and the C-O single bond are conjugated with the lone pair on the negatively charged oxygen.

2. Identify potential electron movement: The lone pair on the negatively charged oxygen can move to form a new pi bond with the carbon atom. Simultaneously, the pi electrons in the C=O double bond can move to the other oxygen atom.

3. Use curved arrows to show electron movement: Draw a curved arrow from the lone pair on the negatively charged oxygen toward the carbon atom, and another curved arrow from the C=O double bond toward the other oxygen atom.

Structure 2:

      O
      |
     C
    /  \
   R    O⁻

In this structure, the first oxygen atom has a formal charge of 0, and the second oxygen atom now bears the negative charge, resulting in a formal charge of -1.

Resonance Hybrid:

The actual carboxylate ion is a resonance hybrid of both structures. The negative charge is delocalized over both oxygen atoms, leading to increased stability.

More Complex Examples and Considerations

Let's explore more intricate scenarios and factors influencing resonance structure stability.

Example: Benzene

Benzene (C₆H₆) is a classic example of resonance. Its six carbon atoms form a ring with alternating single and double bonds. However, benzene doesn't actually have alternating single and double bonds; instead, it has a delocalized pi system where the electrons are spread across all six carbon atoms. Multiple resonance structures can be drawn for benzene, each showing a different arrangement of double bonds. All resonance structures of benzene contribute equally to the hybrid structure.

Factors Affecting Resonance Stability:

• Minimizing formal charges: Structures with fewer formal charges are generally more stable.
• Placing negative charges on more electronegative atoms: Negative charges are more stable on electronegative atoms (like oxygen or nitrogen) because they can better accommodate the extra electron density.
• Placing positive charges on less electronegative atoms: Positive charges are more stable on less electronegative atoms (like carbon).
• Avoiding large separation of charges: Structures with charges that are far apart are generally less stable.
• Maintaining octet rule (where applicable): Whenever possible, resonance structures should follow the octet rule for main group elements.

By understanding these factors, you can assess the relative contributions of different resonance structures to the overall stability of a molecule.

Beyond the Second Resonance Structure: Exploring Multiple Contributors

Often, more than two resonance structures can contribute to the overall description of a molecule. Consider a molecule with extended conjugation; many different resonance forms are possible. The relative contribution of each structure depends on the factors outlined above. The most stable resonance contributors contribute the most to the hybrid structure.

Conclusion: Mastering Resonance Structures

Drawing resonance structures is a fundamental skill in organic chemistry. By systematically following the steps outlined and considering factors affecting resonance stability, you can accurately depict the electron delocalization within molecules. This understanding is crucial for predicting reactivity, stability, and properties of organic compounds. Practice is key to mastering this skill; continue working through diverse examples to build your proficiency in drawing and interpreting resonance structures. Remember, resonance structures are not different molecules but alternative representations of the same molecule showing the distribution of electron density, contributing to a more stable hybrid structure. The ability to identify and draw multiple resonance forms will deepen your comprehension of organic chemistry.