Question

Compare the C–O bond length in the following compounds (a) \(CH_3{-}CO{-}CH_3\) (b) Benzaldehyde type structure (c) \(CH_3{-}COO^{-}\) Compare C–O bond length.

• A. \(b > a > c\)
• B. \(b > c > a\)
• C. \(a > b > c\)
• D. \(c > a > b\)

Answer 1

The Correct Option is B

Concept: Bond length depends on bond order. • Higher bond order → shorter bond length
• Lower bond order → longer bond length
Resonance can reduce bond order and increase bond length. [width=0.5]18c ans.png 
Step 1: Compound (a) \[ CH_3{-}CO{-}CH_3 \] This contains a normal carbonyl bond \(C=O\). Bond order \(=2\). Hence bond length is relatively shorter. 
Step 2: Compound (c) \[ CH_3{-}COO^- \] Carboxylate ion has resonance between two oxygen atoms. \[ R{-}C=O \leftrightarrow R{-}C{-}O^- \] Thus both C–O bonds become equivalent with bond order \(1.5\). Hence bond length is intermediate. 
Step 3: Compound (b) In this structure, resonance with the benzene ring gives a quasi-aromatic resonance form. \[ \text{Resonance structures (RS1 and RS2)} \] This delocalization decreases the effective bond order of the C–O bond. Therefore C–O bond length becomes maximum. 
Step 4: Final order \[ b > c > a \] Thus, \[ \boxed{\text{Option (2) is correct}} \]