Write contributing (important) resonance structures for each of the following compounds and predict their relative C=O vibrational frequencies based on the importance of the contributing resonance structures. List in them order from highest to lowest frequency.

a.) Cyclohexanone
b.) 2-cyclohexenone
c.) cyclohexanecarbaldehyde

These boards don't allow us to draw structures.

To determine the contributing resonance structures for each compound, we need to analyze their molecular structures and identify potential resonance forms. Resonance occurs when electron movement can be delocalized within a molecule, resulting in multiple possible arrangements of electrons. This movement is important for determining the relative vibrational frequencies of the C=O bond.

a.) Cyclohexanone:
The resonance structures for cyclohexanone can be determined by examining its molecular structure, which consists of a cyclohexyl ring attached to a carbonyl group (C=O). The possible resonance structures for cyclohexanone are:
1. The double bond (pi bond) of the carbonyl group can resonate onto any of the carbon atoms in the cyclohexyl ring, resulting in a resonance structure where a double bond exists between that carbon atom and the oxygen atom.
2. Similarly, the lone pair on the oxygen atom in the carbonyl group can resonate onto any of the carbon atoms in the cyclohexyl ring, resulting in a resonance structure where a double bond exists between that carbon atom and the oxygen atom.

b.) 2-cyclohexenone:
In 2-cyclohexenone, the molecular structure consists of a cyclohexenyl ring attached to a carbonyl group (C=O). The possible resonance structures for 2-cyclohexenone are:
1. As there is a double bond (pi bond) in the cyclohexenyl ring adjacent to the carbonyl group, the pi electrons in this double bond can resonate onto the carbonyl oxygen atom, resulting in a resonance structure where the double bond shifts from the ring to the carbonyl group.
2. Similarly, the lone pair on the oxygen atom in the carbonyl group can resonate onto the carbon atom adjacent to the double bond in the cyclohexenyl ring, resulting in a resonance structure where the double bond shifts from the carbonyl group to the ring.

c.) Cyclohexanecarbaldehyde:
The molecular structure of cyclohexanecarbaldehyde consists of a cyclohexyl ring attached to an aldehyde group (C=O and -H). The possible resonance structures for cyclohexanecarbaldehyde are:
1. As in cyclohexanone, the double bond (pi bond) in the carbonyl group can resonate onto any of the carbon atoms in the cyclohexyl ring, resulting in a resonance structure where a double bond exists between that carbon atom and the oxygen atom.
2. Additionally, the lone pair on the oxygen atom in the carbonyl group can resonate onto any of the carbon atoms in the cyclohexyl ring, resulting in a resonance structure where a double bond exists between that carbon atom and the oxygen atom.
3. With the presence of the aldehyde group, the double bond (pi bond) between the carbon atom of the carbonyl group and the adjacent carbon atom can resonate onto the oxygen atom, resulting in a resonance structure where the double bond shifts from the carbonyl group to the oxygen atom.

Now, to predict the relative vibrational frequencies of the C=O bonds in each compound, we need to consider the importance of the contributing resonance structures. The more significant or stable the resonance form, the more it will contribute to the overall bonding characteristics, and thus affect the vibrational frequency of the C=O bond.

In general, resonance forms with greater electron delocalization and lower energy will be more important and will influence the bond strength and frequency. Based on this principle, we can predict the relative vibrational frequencies from highest to lowest as follows:

b.) 2-cyclohexenone (highest frequency): This compound has a significant resonance form where the double bond shifts from the ring to the carbonyl group, resulting in increased electron delocalization and higher bond strength.
c.) Cyclohexanecarbaldehyde (intermediate frequency): It has resonance forms involving both the delocalization of the carbonyl group and the aldehyde group, resulting in some electron movement and bond strengthening.
a.) Cyclohexanone (lowest frequency): Although cyclohexanone also has resonance structures involving electron delocalization in the ring, the overall electron movement is relatively less compared to the other compounds.

Remember that these predictions are based on the assumption that resonance structures affect bond strength and frequency. Experimental results can validate or adjust these predictions.