Problem 57 - Chemistry: The Central Science

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?

Step-by-Step Solution

Answer

(a) A localized π bond is confined between two specific atoms, while a delocalized π bond has its electron density distributed across more than two atoms in the molecule or ion. (b) To determine if a molecule or ion will exhibit delocalized π bonding, check for the presence of resonance structures, conjugation, and aromatic rings. (c) The π bond in NO2⁻ is delocalized, as it has two resonance structures where the π bond is distributed across two oxygen atoms.

Step 1: Recall the definition of π bonds

π bonds are formed by the sideways overlap of two p orbitals. This type of bond is found in molecules containing double or triple bonds, like alkenes and alkynes. They are responsible for the rigidity of molecules and their restricted rotation around the bond axis.

Step 2: Differentiate between localized and delocalized π bonds

A localized π bond is when the electron density of the bond is confined between two specific atoms. In contrast, a delocalized π bond features electron density that is spread across more than two atoms in the molecule or ion. Delocalized π bonding systems typically offer added stability to the molecule or ion due to the distribution of the electron density.

Step 3: Determine the presence of delocalized π bonding

There are a few factors that can help in determining whether a molecule or ion exhibits delocalized π bonding: 1. The presence of resonance structures: If a molecule or ion can be represented by more than one valid Lewis structure, which shows a different arrangement of electrons, it is indicative of delocalized π bonding. 2. Conjugation: Delocalized π bonding often occurs in molecules or ions with conjugation - alternating single and double bonds. 3. The presence of an aromatic ring: Aromatic rings, such as benzene, are characterized by a cyclic delocalized π bonding system.

Step 4: Analyze the NO2⁻ ion

In order to determine if the π bond in the NO2⁻ ion is localized or delocalized, we first need to draw the Lewis structure of NO2⁻: O \ N = O / O⁻ There are two resonance structures of NO2⁻, where the two oxygen atoms are double-bonded to the nitrogen atom: O \ N = O (Structure 1) / O⁻ O \ N⁻ - O (Structure 2) / O These two resonance structures indicate that the π bond in NO2⁻ is delocalized, as there is no single location for the π bond and it is distributed across two oxygen atoms.

Step 5: Conclusion

(a) A localized π bond is confined between two specific atoms, while a delocalized π bond has its electron density distributed across more than two atoms in the molecule or ion. (b) To determine if a molecule or ion will exhibit delocalized π bonding, check for the presence of resonance structures, conjugation, and aromatic rings. (c) The π bond in NO2⁻ is delocalized, as it has two resonance structures where the π bond is distributed across two oxygen atoms.