Question

Draw all of the reasonable resonance structures that satisfy the octet rule for each of the

following substances:

a) CO_3^2-

b) HCO_3^- (the H is attached to one of the O atoms in CO_3^2-)

c) H₂CO₃ (each H is attached to one of the O atoms in CO₃^2-)

d) Each of the three substances in problem (a) (b) (c) contain three carbon oxygen bonds. For eachsubstance, how many of the carbon oxygen bonds are the same length?

Answer 1

Carbonate ion, the conjugate base of bicarbonate ion and carbonic acid is a polar dianion which contains 1 C=O and two C-O bonds with the latter's O atoms bearing a negative charge. In this structure, carbon with its 4 valence electrons gets two electrons via a double bond with oxygen and one each through the remaining two negatively charged oxygens. Due to intense charge localization on two anions quiet close to each other, the carbonate anio tends to be unstable, abstract protons from the medium forming bicarbonate or carbonic acid in equilibrium with itself. To prevent such a strong repulsion from two anions, the trigonal planar anion goes in resonance as given below.

The bicarbonate ion has the same kind of C-O bonds as carbonate and only here, the charge of one of the oxides is balanced with a proton. Due to this, unlike carbonate ion, bicarbonate does not have extensive resonance through all its C-O bonds. Instead, resonance puts only the C=O and C-O^- in resonance.

Similar to bicarbonate ion, carbonic acid has two, not one C-OH bonds. It is a neutral species, harbouring one C=O and two C-OH bonds. Due to the charge satisifaction in all oxygens, no anionic repulsion is present to nullify through resonance, giving this molecule no resonance structure.

From the resonance structures of all the three species given, it can be seen that in carbonate ion, all three C-O bonds are equal, possessing partial double/single bond character while in bicarbonate, two C-O bonds are equal with the one C-OH bond possessing pure single bond character and finally in carbonic acid, the C=O has pure double bond character and the two C-OH bonds having pure single bond character.