Resonance structures are a way of representing the delocalization of electrons in molecules that have multiple bonds or lone pairs. Pi systems are a type of molecular orbital that result from the overlap of parallel p orbitals.
In this article, we will explore how resonance structures and pi systems are related, how to draw them, and what are their implications for the stability, reactivity, and properties of molecules.
What are Resonance Structures?
Resonance structures are not real structures of molecules, but rather a way of showing the different possible arrangements of electrons in a molecule that has more than one valid Lewis structure. For example, consider the molecule of ozone, O$_3$. We can draw two different Lewis structures for ozone, as shown below:
!Ozone resonance structures
Both structures obey the octet rule and have the same number of valence electrons. However, neither structure accurately represents the true nature of ozone. In reality, ozone has a bond order of 1.5 between each oxygen atom, meaning that there is a partial double bond and a partial single bond. The electrons are delocalized over the three oxygen atoms, creating a resonance hybrid that is a blend of the two resonance structures.
!Ozone resonance hybrid
To indicate that a molecule has resonance structures, we use a double-headed arrow between them. We can also use brackets to enclose the resonance structures and show that they are part of the same molecule. For example:
!Ozone resonance notation
The actual structure of ozone is somewhere between the two resonance structures, but closer to the one with lower energy. The lower energy structure is called the major contributor, while the higher energy structure is called the minor contributor. The relative energies of the resonance structures depend on factors such as bond strength, charge separation, and formal charge.
What are Pi Systems?
Pi systems are a type of molecular orbital that result from the sideways overlap of parallel p orbitals. Pi orbitals have a nodal plane that passes through the nuclei of the atoms involved in the overlap. Pi orbitals can be bonding or antibonding, depending on whether the overlap is constructive or destructive.
!Pi bonding and antibonding orbitals
Pi systems are important for understanding resonance structures because they allow for electron delocalization. When a molecule has more than one pi bond, the pi orbitals can interact with each other and form a continuous network of overlapping orbitals. This network is called a conjugated system, and it allows for electrons to move freely within it.
!Conjugated system
A conjugated system can have different resonance structures depending on how the electrons are distributed in the pi orbitals. For example, consider the molecule of benzene, C$_6$H$_6$. Benzene has six carbon atoms arranged in a ring, each with one hydrogen atom attached. Each carbon atom has one sp$^2$ hybrid orbital that forms a sigma bond with another carbon atom or a hydrogen atom, and one unhybridized p orbital that forms a pi bond with an adjacent carbon atom.
!Benzene structure
We can draw two different resonance structures for benzene, as shown below:
!Benzene resonance structures
Both structures have three double bonds and three single bonds in alternating positions. However, neither structure accurately represents the true nature of benzene. In reality, benzene has a bond order of 1.5 between each carbon atom, meaning that there is a partial double bond and a partial single bond. The electrons are delocalized over the six carbon atoms, creating a resonance hybrid that is a blend of the two resonance structures.
!Benzene resonance hybrid
To indicate that benzene has resonance structures, we use a circle inside the ring to show the delocalization of electrons. We can also use brackets to enclose the resonance structures and show that they are part of the same molecule. For example:
!Benzene resonance notation
The actual structure of benzene is somewhere between the two resonance structures, but closer to the one with lower energy. The lower energy structure is called the major contributor, while the higher energy structure is called the minor contributor. The relative energies of the resonance structures depend on factors such as bond strength, aromaticity, and formal charge.
How to Draw Resonance Structures and Pi Systems?
To draw resonance structures and pi systems, we need to follow some rules and guidelines. Here are some steps to follow:
- Identify the atoms that have p orbitals and can form pi bonds. These are usually atoms with sp$^2$ or sp hybridization, such as carbon, nitrogen, oxygen, and sulfur.
- Draw the Lewis structure of the molecule and assign formal charges to each atom. Use the formula FC = V – N – B/2, where FC is the formal charge, V is the number of valence electrons, N is the number of non-bonding electrons, and B is the number of bonding electrons.
- Identify the pi bonds and the conjugated system in the molecule. A pi bond is formed by the overlap of two p orbitals. A conjugated system is a network of overlapping pi orbitals that allows for electron delocalization.
- Draw the different resonance structures by moving electrons within the conjugated system. Do not break sigma bonds or change the positions of atoms. Only move electrons in pi bonds or lone pairs that are part of the conjugated system. Use curved arrows to show the movement of electrons. Make sure that each resonance structure obeys the octet rule and has the same net charge as the original structure.
- Compare the relative energies of the resonance structures using factors such as bond strength, charge separation, and formal charge. The lower energy structure is the major contributor, while the higher energy structure is the minor contributor. The actual structure of the molecule is a resonance hybrid that is a blend of all the resonance structures, but closer to the major contributor.
- Draw the pi system of the molecule by showing the overlapping p orbitals and their phases. Use solid and dashed lines to indicate bonding and antibonding interactions. Use plus and minus signs to indicate constructive and destructive overlaps.
Here is an example of drawing resonance structures and pi systems for nitrate ion, NO$_3^-$:
!Nitrate ion example
What are the Implications of Resonance Structures and Pi Systems?
Resonance structures and pi systems have important implications for the stability, reactivity, and properties of molecules. Here are some examples:
- Resonance structures increase the stability of molecules by lowering their energy and spreading their charge over a larger area. For example, benzene is more stable than cyclohexene because it has more resonance structures that delocalize its electrons over six carbon atoms instead of two.
- Resonance structures affect the reactivity of molecules by changing their electron density and polarity. For example, nitrate ion is more reactive than nitrite ion because it has more resonance structures that make it more negative and polarizable.
- Pi systems affect the properties of molecules by creating regions of high and low electron density that can interact with electric or magnetic fields. For example, conjugated systems can absorb or emit light at specific wavelengths, giving rise to color or fluorescence.
Conclusion
Resonance structures and pi systems are two concepts that help us understand how electrons are distributed in molecules that have multiple bonds or lone pairs.