Question

(a) What is the difference between a localized \(\pi\) bond and a delocalized one? (b) How can you determine whether a molecule or ion will exhibit delocalized \(\pi\) bonding? (c) Is the \(\pi\) bond in \(\mathrm{NO}_{2}^{-}\) localized or delocalized?

Short answer

(a) A localized π bond is a covalent bond formed between two adjacent atoms, while a delocalized π bond involves π electrons shared across three or more atoms within a molecule, often seen in molecules with resonance structures. (b) To determine if a molecule or ion exhibits delocalized π bonding, look for resonance structures, conjugated bonds, and increased stability. (c) The π bond in the NO2- ion is delocalized due to the presence of resonance structures, a conjugated system, and increased stability.

Step-by-step solution

(a) Definition of localized and delocalized π bonds

A localized π bond is a covalent bond formed between two adjacent atoms, resulting from the side-to-side overlap of the adjacent p orbitals. These π bonds exist exclusively between the two atoms and are not shared with other atoms in the molecule. In contrast, a delocalized π bond is a bond where π electrons are shared and distributed across three or more atoms within a molecule. These electrons are not confined to just two atoms, and contribute to the bonding structure of multiple atoms, resulting in a more stable molecular structure. This phenomenon is often seen in molecules or ions with resonance structures.

(b) Determining delocalized π bonds in a molecule or ion

To determine whether a molecule or ion exhibits delocalized π bonding, consider the following points: 1. Identify if the molecule/ion has any resonance structures. If there are multiple valid Lewis structures where double bonds are located in different positions, it can indicate delocalized π bonding is present. 2. Look for conjugated (alternating single and double) bonds within the molecule or ion. In a conjugated system, the π electrons can delocalize across the entire system, forming delocalized π bonds. 3. Assess the molecule/ion's stability. Delocalized π bonding contributes to the stability of a molecule or ion due to the distribution of bonding electrons over multiple atoms. If a molecule/ion is relatively more stable than expected, it could be due to delocalized π bonding.

(c) Determining whether the π bond in NO2- is localized or delocalized

To determine if the π bond in NO2- ion is localized or delocalized, let's follow the points discussed in step (b). 1. The Lewis structure of the NO2- ion exhibits resonance, meaning there are multiple valid ways to represent its bonding structure with double bonds in different positions. In this case, the double bond can be either between the nitrogen atom and one of the oxygen atoms, or between the nitrogen atom and the other oxygen atom. 2. There is a conjugated system in NO2- ion, due to the presence of alternating single and double bonds in the resonance structures. 3. A delocalized π bond allows for better distribution of electrons and greater stability in the overall ion. Given these reasons, we can conclude that the π bond in the NO2- ion is delocalized.

Key concepts

Localized Pi Bond

Localized pi bonds are a fundamental aspect of molecular chemistry. In a localized pi bond, the pi electrons are confined to two specific atoms. This type of bond forms by the side-to-side overlap of adjacent p orbitals. The electrons involved do not participate in bonding with other atoms, which means they stay within the orbit of these paired atoms.

This confinement occurs because the pi electrons are restricted between two adjacent atoms, forming a covalent bond. For example, in ethylene (C₂H₄), the double bond between the carbon atoms consists of one sigma bond and one localized pi bond. Here, the pi electrons are shared only between the two carbon atoms.

Localized pi bonds typically contribute to the rigidity of molecular structures, making them more predictable in behavior. However, because they do not spread their electron density over multiple atoms, they do not enhance molecular stability in the same way that delocalized bonds can.

Resonance Structures

Resonance structures are one of the telltale signs of delocalized pi bonding in molecules. These are hypothetical structures that illustrate various ways electrons can be distributed across a molecule. Importantly, the actual electron distribution—what we call the resonance hybrid—is a blend of these structures.

To determine resonance structures, we draw multiple valid Lewis structures that display different placements of pi bonds, yet are equivalent in terms of molecular legitimacy. This indicates that electrons can "resonate" or move between these configurations. It shows the delocalization of pi electrons across more than two atoms.

For example, in benzene (C₆H₆), the six carbon atoms form a ring, and the pi electrons are not confined to specific double bonds. Instead, benzene can be represented by two main resonance structures, demonstrating its delocalized pi bonding. Such arrangement increases benzene's overall stability.

Conjugated System

A conjugated system features alternating single and double bonds, allowing for enhanced electron delocalization. In this system, pi electrons are free to move between multiple adjacent p-orbitals that are aligned parallelly.

Conjugation extends the pi electron cloud across a larger section of the molecule, which decreases its potential energy and contributes to greater molecular stability. An example of a conjugated system is the 1,3-butadiene molecule (CH₂=CH-CH=CH₂), where alternating bonds allow pi electrons to be shared across the whole system.

Understanding if a molecule possesses a conjugated system is crucial. This information helps to predict the potential for resonance and the presence of delocalized pi bonds. It also significantly affects the color and chemical reactivity of organic compounds.

Molecular Stability

Molecular stability is often augmented by the presence of delocalized pi bonds due to the distribution of electron density over multiple atoms. Delocalization contributes to a lower overall energy level of the molecule, making it less reactive and more stable.

When electrons are shared over a larger area, the energy required to disrupt the molecule is increased. Therefore, such molecules tend to be less prone to chemical reactions, leading to enhanced stability. This is evident in systems like the carbonate ion (CO₃²⁻), where delocalization helps spread out negative charge evenly across the three oxygen atoms, increasing stability.

Delocalization via resonance and conjugation also leads to the phenomenon known as aromaticity, a particularly stable form of electron delocalization seen in compounds like benzene. Understanding the ways in which electron delocalization contributes to molecular stability is vital in predicting the behavior and reactivity of compounds.