Question

In carbonate ion, all the three C-O bonds have identical bond length. Explain.

Answer 1

Answer on Question #80919, Chemistry/ Organic Chemistry

In carbonate ion, all the three C-O bonds have identical bond length. Explain.

Answer

Lewis diagram for carbonate ion is:

The carbonate ion, according to its Lewis diagram, has two types of carbon-oxygen bonds, one double bond and two single bonds, suggesting that one carbon-oxygen bond in the carbonate ion is shorter and stronger than each of the other two. Experimentally, however, the three carbon-oxygen bonds in the carbonate ion have the same bond length (136 pm). The Lewis diagram fails to explain the structure and bonding of the carbonate ion satisfactorily.

Two additional Lewis diagrams can be drawn for the carbonate ion:

However, none of them are consistent with the observed properties of the carbonate ion and, therefore, does not correctly depict the carbonate ion.

According to resonance theory the structure of the carbonate ion is not 1 nor 2 nor 3 but the average of all them. Whenever there are two or more valid Lewis structures for the same compound, the true structure of the molecule is best represented as a "resonance hybrid" of the individual structures.

Hybrid of resonance forms:

The three resonance forms of the carbonate ion, 1, 2, and 3, are identical, so they have the same stability and, therefore, contribute equally to the hybrid. Since the exact extent to which each resonance form of the carbonate ion contributes to the hybrid is known, the bond order of each carbon-oxygen bond in the hybrid can be easily determined:

Resonance structure 1 B.O. = 2 × $\frac{1}{3} + 1 \times \frac{1}{3} + 1 \times \frac{1}{3} = 1\frac{1}{3}$

Resonance structure 2 B.O. = 1 × $\frac{1}{3} + 2 \times \frac{1}{3} + 1 \times \frac{1}{3} = 1\frac{1}{3}$

Resonance structure 3 B.O. = 1 × $\frac{1}{3} + 1 \times \frac{1}{3} + 2 \times \frac{1}{3} = 1\frac{1}{3}$

According to resonance theory, each bond in the carbonate ion is one and one-third of a bond, which is consistent with the observation that the three bonds in the carbonate ion have the same bond length.

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