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

To compare the C–O bond length in the given compounds, we need to consider the nature of bonding and resonance effects present in each compound, which affects bond lengths.

1. In compound (a), \(CH_3{-}CO{-}CH_3\) (Acetone), the C–O bond is a carbonyl group (C=O). Typically, double bonds are shorter than single bonds due to increased bond order. Hence, the C–O bond in acetone is relatively short.
2. In compound (b), Benzaldehyde, the structure contains an aldehyde group attached to a benzene ring. The C–O bond here is also a double bond (C=O). However, due to resonance with the benzene ring, there is partial double bond character from the resonance, which slightly lengthens the bond compared to typical carbonyl bonds.
3. In compound (c), \(CH_3{-}COO^{-}\) (acetate ion), the C–O bonds are resonating between single and double bond characters, due to the delocalization of the negative charge over both oxygen atoms. This results in bond length that is intermediate between a typical single bond and a double bond but generally longer than typical carbonyl C=O bonds due to resonance.

Considering these observations, the bond lengths are affected in the following way: the bond in benzaldehyde (b) is the longest due to resonance with the benzene ring, followed by the acetate ion (c) which has resonance effects leading to partial double bond characters, and then the acetone (a) with a typical carbonyl bond.

Thus, the order of increasing C–O bond lengths is: \(b > c > a\).