In this lesson, we'll learn how to draw resonance structures for organic molecules. Resonance structures are different ways of representing the same molecule, showing how electrons can be distributed differently while keeping the atomic positions fixed. We'll work through two examples, drawing all resonance structures and identifying the most stable one.
Our first molecule is a five-membered carbon ring with two double bonds and a negatively charged oxygen atom attached to the ring. The negative charge on the oxygen indicates it has an extra electron. We need to draw two additional resonance structures by moving electrons within the molecule while keeping the atomic framework unchanged.
The first resonance structure is the one given in the problem. Here, the negative charge is localized on the oxygen atom, which has two lone pairs of electrons. This structure serves as our starting point for generating the other resonance forms.
For the second resonance structure, we move the negative charge from the oxygen to a carbon atom in the ring. This requires shifting the positions of the double bonds. The electrons from one of the double bonds move to the oxygen, neutralizing its charge, while electrons from the oxygen's lone pair form a new double bond with an adjacent carbon.
The third resonance structure shows the negative charge on a different carbon atom in the ring. Again, we use arrow pushing notation to show how electrons move to form this structure. The arrows indicate the movement of electron pairs, either from lone pairs or from existing double bonds, to create new double bonds or charges in different locations.
Arrow pushing is a formal way to show how electrons move between resonance structures. We use curved arrows to represent the movement of electron pairs. These arrows start from either a lone pair or a pi bond and point to where the electrons are moving. This helps us visualize the transformation from one resonance structure to another.
To determine which resonance structure contributes most to the resonance hybrid, we need to consider the stability of each structure. The structure with the negative charge on the oxygen atom is the most stable because oxygen is more electronegative than carbon and can better stabilize the negative charge. This makes it the greatest contributing resonance structure.
The resonance hybrid is a weighted average of all resonance structures. It represents the actual molecule more accurately than any single resonance structure. In the hybrid, we see partial double bonds between all atoms in the conjugated system, and the negative charge is delocalized over the oxygen and carbon atoms. This delocalization makes the molecule more stable than any individual resonance structure would suggest.
Our second molecule is a six-membered carbon ring with two double bonds and a positive charge on one of the carbon atoms. The positive charge indicates a carbon atom missing an electron. We'll draw the resonance structures for this molecule, showing how the positive charge can be delocalized through the ring.
For this six-membered ring, we can draw three resonance structures. The positive charge can be delocalized to three different carbon atoms in the ring. Each structure shows the positive charge on a different carbon, with the double bonds shifting positions accordingly. This delocalization of the positive charge contributes to the overall stability of the molecule.
Using arrow pushing notation, we can show how the positive charge shifts between the three carbon atoms. The movement of pi electrons around the ring results in the delocalization of the positive charge. These arrows help us visualize the electron movement that connects the different resonance structures.
The resonance hybrid for this molecule shows the positive charge delocalized over three carbon atoms. This means each of these carbon atoms has a partial positive charge. Additionally, all the bonds in the ring have partial double bond character due to the delocalization of pi electrons. This hybrid structure represents the actual molecule more accurately than any single resonance structure.