Question

Consider the following molecules/species:
The correct order of carbon - oxygen double bond length is :

• A. \(x>y>z\)
• B. \(y>z>x\)
• C. \(z>x>y\)
• D. \(x>z>y\)

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The bond length of a covalent bond is inversely proportional to its bond order. A pure double bond is shorter than a bond that has partial single bond character due to resonance or conjugation. 
 
Step 2: Key Formula or Approach: 
Analyze the resonance and conjugation in each species: 
1. (y) Propanone: Isolated \(C=O\) double bond (Bond order \(\approx 2\)). 
2. (x) Benzoquinone-like structure: \(C=O\) is in conjugation with the ring \(\pi\) system (Bond order \(< 2\)). 
3. (z) Acetate ion: \(C=O\) is in complete resonance with the other oxygen atom (Bond order \(\approx 1.5\)). 

Step 3: Detailed Explanation: 
In species (y), the carbonyl group is localized, making it a strong double bond with the highest bond order and shortest length. 
In species (x), the carbonyl oxygen is conjugated with the cyclohexadiene ring. Resonance slightly reduces the double bond character, increasing the bond length relative to (y). 
In species (z), the acetate ion (\(CH_3COO^-\)) exhibits equivalent resonance structures where the negative charge is delocalized over both oxygen atoms. The bond order for each C-O bond is exactly 1.5, which is much lower than 2. This results in the longest bond length among the three. 
Therefore, the length order is: \(z (1.5)>x (\text{conjugated})>y (2.0)\). 

Step 4: Final Answer: 
The correct order of bond length is \(z>x>y\).