Problem 91 - Chemistry An Atoms First Approach

Question

An important observation supporting the concept of resonance in the localized electron model was that there are only three different structures of dichlorobenzene \(\left(\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{Cl}_{2}\right) .\) How does this fact support the concept of resonance (see Exercise 89)?

Step-by-Step Solution

Answer

The fact that there are only three different structures of dichlorobenzene (ortho-, meta-, and para-dichlorobenzene) supports the concept of resonance in the localized electron model because it implies that electrons are localized and the possible bonding configurations are limited. The limited number of structures indicates that electrons are confined to specific regions or orbitals, resulting in localized structures. This supports the localized electron model and the concept of resonance, as the different structures of dichlorobenzene can be viewed as resonance structures that contribute to the overall description of the molecule.

Step 1: Review the concept of resonance and localized electron model

Resonance is a concept in chemistry where a molecule does not have a single, well-defined structure, but rather exists as a hybrid of several structures, which are called resonance structures. These structures have individual contributing effects, so the overall structure of the molecule is a mixture of all the possible resonance structures. The localized electron model is an approach to understanding the structure and bonding in molecules. In this model, electrons are assumed to be localized around specific atoms or between a pair of atoms, forming single, double, or triple bonds.

Step 2: Identify the three different structures of dichlorobenzene

Dichlorobenzene (C6H4Cl2) has three different structures due to the different possible positions of chlorine atoms on the benzene ring: 1. Ortho-dichlorobenzene: The two chlorine atoms are bonded to adjacent carbon atoms on the benzene ring. 2. Meta-dichlorobenzene: The two chlorine atoms are bonded to carbon atoms separated by one carbon atom. 3. Para-dichlorobenzene: The two chlorine atoms are bonded to carbon atoms separated by two carbon atoms (opposite sides of the benzene ring).

Step 3: Relate the structures of dichlorobenzene to resonance

The concept of resonance is supported by the fact that there are only three different structures of dichlorobenzene, as each structure represents a different way that the two chlorine atoms can be positioned on the benzene ring. This limited number of structures implies that electrons are localized and the possible bonding configurations are limited. In the localized electron model, the electrons involved in bonding are confined to specific regions or orbitals, resulting in localized structures. The fact that only three structures are found for dichlorobenzene indicates that the electrons are localized in such a way that only these specific structures can form. Furthermore, if the electrons were not localized, many more structures would be possible, as the electrons could be distributed evenly around the benzene ring. However, the limited number of structures observed for dichlorobenzene supports the localized electron model, and this, in turn, supports the concept of resonance, as the different structures of dichlorobenzene can be seen as resonance structures that contribute to the overall description of the molecule.

Key Concepts

Localized Electron Model

The localized electron model is a method used to explain molecular structures by considering electrons as localized around specific atoms or bonds. This model suggests:

• Electrons are concentrated in specific regions.
• They form distinct single, double, or triple bonds.
• The model often uses Lewis structures to visualize bonding.

Localized electrons help predict and explain the behavior of molecules by assigning fixed positions for electrons. This can contrast with models where electrons are considered more delocalized and free to move over larger structures. This fixed nature of electron placement plays a crucial role in understanding molecular shapes and reactivity. It simplifies the complex structures by breaking them into understandable components.

Dichlorobenzene Structures

Dichlorobenzene (\(\mathrm{C}_{6}\mathrm{H}_{4}\mathrm{Cl}_{2}\)) exists in three main isomeric forms, each differing in the positioning of the chlorine atoms on the benzene ring.

• Ortho-dichlorobenzene: Chlorine atoms are adjacent.
• Meta-dichlorobenzene: Chlorines are separated by one carbon atom.
• Para-dichlorobenzene: Chlorines are on opposite sides.

These configurations arise because of the different ways chlorine can attach to the carbon atoms of the benzene ring, creating diverse spatial arrangements. The limitation of having only three isomers supports the localized electron model. The position of each chlorine is dependent upon the electrons localized between specific carbon atoms, creating specific and predictable structures. These structures illustrate how electron localization restricts the configurations that can occur.

Bonding in Molecules

Bonding in molecules involves the way atoms share or transfer electrons. This includes:

• Covalent Bonds: Atoms share electrons equally.
• Ionic Bonds: Electrons are transferred, creating ions that attract each other.
• Metallic Bonds: Electrons flow freely, creating a 'sea' of electrons around metal ions.

In dichlorobenzene, covalent bonding plays a central role. The electrons shared between carbon and chlorine atoms form strong covalent bonds.

Role in Resonance

These bonds are part of the reason why resonance is possible. While the electrons are largely localized, the concept of resonance allows for the depiction of electrons as being distributed across multiple configurations, contributing to a more stable overall structure. Resonance enables chemists to depict the molecule as a compilation of these possible forms, giving a richer understanding of the molecule's stability and reactivity.