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Basics of Organic Chemistry question

2003 · Shift 0 · Q12
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Basics of Organic Chemistry question

2003 · Shift 0 · Q12

JEE MainChemistryBasics of Organic ChemistryMCQ+4 / −1
In the anion HCOO −-− the two carbon-oxygen bonds are found to be of equal length. What is the reason for it?
  1. A
    The C = O bond is weaker than the C-O bond
  2. B
    The anion HCOO−HCOO^-HCOO− has two resonating structures
  3. C
    The anion is obtained by removal of a proton from the acid molecule
  4. D
    Electronic orbitals of carbon atom are hybridised
View written solutionFree

Correct answer: B

  1. We need the reason why in the formate ion, HCOO−\mathrm{HCOO^-}HCOO−, the two carbon–oxygen bonds have equal length.

  2. Write the two important Lewis/resonance structures of the formate ion:

H−C(=O)−O−↔H−C(−O−)=O\mathrm{H-C(=O)-O^- \leftrightarrow H-C(-O^-)=O}H−C(=O)−O−↔H−C(−O−)=O

In one structure, the double bond is to one oxygen; in the other, the double bond is to the other oxygen.

  1. Because of resonance, the actual structure is a resonance hybrid. So:
  • the negative charge is delocalized over both oxygen atoms,
  • both C−O\mathrm{C-O}C−O bonds become equivalent,
  • each bond has bond order between 1 and 2 (approximately 1.51.51.5).

  1. Equal bond order leads to equal bond length. Hence the two carbon–oxygen bonds are equal because of resonance.

  2. Check options:

  • A: Incorrect. Equality of bond lengths is not because C=O\mathrm{C=O}C=O is weaker than C−O\mathrm{C-O}C−O.
  • B: Correct. HCOO−\mathrm{HCOO^-}HCOO− has two resonating structures.
  • C: Incorrect. Formation by loss of proton does not by itself explain equal bond lengths.
  • D: Incorrect. Hybridisation of carbon alone does not make the two bonds equal; resonance does.

Therefore, the correct answer is B.

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