Draw A Resonance Structure That Places A Pi Bond

Drawing Resonance Structures with Pi Bonds: A Comprehensive Guide

Resonance structures are crucial for understanding the delocalization of electrons in molecules, particularly those containing pi bonds. This concept is fundamental in organic chemistry and beyond, impacting reactivity, stability, and physical properties. Mastering the ability to draw and interpret resonance structures is essential for any chemistry student or professional. This article will provide a comprehensive guide to drawing resonance structures, focusing specifically on the placement of pi bonds and exploring various examples.

Understanding Resonance and Pi Bonds

Before diving into drawing resonance structures, let's solidify our understanding of the underlying concepts.

What is Resonance?

Resonance describes a phenomenon where a single Lewis structure is insufficient to represent the true bonding in a molecule. Instead, the actual structure is a hybrid – a weighted average – of multiple contributing resonance structures. These structures differ only in the placement of electrons, not the arrangement of atoms. Think of it like a chameleon changing its color; the underlying structure (the chameleon) remains the same, but its appearance (electron distribution) changes. Importantly, resonance structures are not real structures; they are theoretical representations used to understand the true electron distribution.

Pi Bonds: The Key Players in Resonance

Pi (π) bonds are formed by the sideways overlap of p orbitals. Unlike sigma (σ) bonds, which are stronger and directly between atoms, pi bonds are weaker and exist above and below the plane of the sigma bond. This weaker, less localized nature of pi electrons is what allows for their delocalization and the creation of resonance structures. Pi bonds are essential for resonance because the electrons involved are more easily moved around within the molecule, creating multiple possible representations.

Identifying Molecules with Resonance

Many molecules exhibit resonance. Common structural features that suggest the possibility of resonance include:

• Conjugated systems: These involve alternating single and multiple bonds, allowing for electron delocalization across multiple atoms.
• Presence of lone pairs on atoms adjacent to multiple bonds: These lone pairs can participate in resonance by delocalizing into the pi system.
• Presence of charged atoms adjacent to multiple bonds: Charges can participate in resonance by shifting electron density.

Steps to Draw Resonance Structures

Drawing accurate resonance structures involves a systematic approach. Here's a step-by-step guide:

1. Draw the Lewis Structure: Begin by constructing a valid Lewis structure for the molecule, ensuring all atoms have a full octet (except for hydrogen).

2. Identify Pi Bonds and Lone Pairs: Locate all pi bonds and lone pairs on atoms adjacent to pi bonds. These are the electrons that will participate in resonance.

3. Move Pi Electrons: Systematically move pairs of pi electrons. You can move a pair of pi electrons from a pi bond to form a lone pair on an adjacent atom, or move a lone pair into an adjacent pi bond. Remember, only electrons can move, not atoms.

4. Maintain Formal Charges: As you move electrons, formal charges on atoms will change. Carefully track these changes. The sum of formal charges in all resonance structures must be equal to the overall charge of the molecule (or ion).

5. Draw Multiple Resonance Structures: Explore all possible valid resonance structures. Some resonance structures will be more significant contributors to the resonance hybrid than others (more on this later).

6. Use Curved Arrows: Use curved arrows to show the movement of electrons. The arrow's tail should originate from the electron pair being moved, and the arrowhead points to where the electron pair moves to. This visually represents the electron flow.

Examples of Drawing Resonance Structures with Pi Bonds

Let's illustrate the process with some examples.

Example 1: Nitrate Ion (NO₃⁻)

The nitrate ion is a classic example exhibiting resonance.

1. Lewis Structure: A central nitrogen atom is bonded to three oxygen atoms. One oxygen has a double bond, while the other two have single bonds and carry a negative formal charge.

2. Identifying Pi Bonds and Lone Pairs: We have one pi bond and two lone pairs on oxygen atoms adjacent to the pi bond.

3. Moving Pi Electrons: We can move the pi electrons from the double bond to one of the oxygen atoms with a single bond and a negative charge, simultaneously creating a double bond on that oxygen and removing the negative charge.

4. Result: This generates three equivalent resonance structures, each differing only in the placement of the double bond.

5. Curved Arrows: The curved arrows clearly show the electron movement in the resonance process.

Example 2: Benzene (C₆H₆)

Benzene is an excellent example of resonance involving a cyclic conjugated system.

1. Lewis Structure: A six-membered ring with alternating single and double bonds.

2. Identifying Pi Bonds: All six pi electrons are delocalized across the ring.

3. Moving Pi Electrons: We can move the pi electrons in a cyclic manner, creating two major resonance structures where the double bonds are essentially 'shifted' around the ring.

4. Result: These two structures, together with other possible minor contributors, contribute to the highly stable aromatic system of benzene.

5. Curved Arrows: Again, curved arrows illustrate the cyclic electron movement.

Example 3: Carboxylate Ion (RCOO⁻)

Carboxylate ions, such as acetate (CH₃COO⁻), demonstrate resonance involving a carboxyl group.

1. Lewis Structure: The carboxyl group has a carbon atom double-bonded to one oxygen and single-bonded to another oxygen carrying a negative charge.

2. Identifying Pi Bonds and Lone Pairs: The pi bond in the carbonyl group and the lone pairs on the negatively charged oxygen are involved.

3. Moving Pi Electrons: The lone pair on the negatively charged oxygen can be moved to form a double bond with the carbon atom. Concurrently, the pi electrons from the carbonyl bond can shift onto the other oxygen, creating a new negative charge there.

4. Result: Two resonance structures are generated, with the negative charge distributed equally between the two oxygen atoms. This creates a very stable anion.

5. Curved Arrows: Curved arrows clearly show the movement of electrons between oxygen atoms and carbon.

Evaluating Resonance Structures: Major and Minor Contributors

Not all resonance structures contribute equally to the resonance hybrid. Several factors influence the relative importance of a resonance structure:

• Octet Rule: Structures with complete octets on all atoms are generally more stable and contribute more significantly.
• Formal Charges: Structures with minimal formal charges are preferred. Structures with like charges on adjacent atoms are less stable.
• Electronegativity: Structures where negative charges reside on more electronegative atoms (like oxygen) are favored.

Structures closer to the true electron distribution of the molecule are considered major contributors, while those less representative are minor contributors. The actual structure is a weighted average of all contributing structures, with major contributors having a greater influence.

Resonance and Molecular Properties

Understanding resonance is crucial for predicting molecular properties. Delocalization of electrons through resonance generally leads to:

• Increased stability: The delocalization of electrons lowers the overall energy of the molecule, enhancing its stability.
• Altered reactivity: The distribution of electron density influences the molecule’s susceptibility to react with electrophiles or nucleophiles.
• Changes in bond lengths: Bond lengths in resonance structures are often intermediate between single and double bonds, reflecting the delocalization of electrons.

Conclusion

Drawing resonance structures, particularly those involving pi bonds, is a fundamental skill in chemistry. By mastering the steps outlined here and practicing with various examples, you can gain a deeper understanding of electron delocalization, molecular stability, and reactivity. Remember that resonance structures are tools; the true structure is the resonance hybrid, a blend of all contributing forms. The ability to correctly draw and interpret these structures is vital for success in many areas of chemistry. Continual practice and a thorough understanding of underlying principles are key to achieving proficiency.