3.50. One of the anions is resonance stabilized, with the negative charge spread

Question

3.50. One of the anions is resonance stabilized, with the negative charge spread over two oxygen atoms. That anion is the weakest (most stable) base. Among the remaining three anions, they do not differ from each other in any of the four factors (ARIO), but they are expected to differ from each other in terms of solvent effects. That is, an anion will be less stable (stronger base) if it has steric bulk in close proximity with the negative charge. The steric bulk reduces the stability of the anion by limiting its ability to interact with solvent molecules, as described in Section 3.7 Increasing base strength 51. The proton highlighted below it

Explanation / Answer

The basic strength is decided by either acceptability of accepting an H+ ion or donating an OH- ion in the solvent

In the first structure, the negative charge on Oxygen is stabilized by other Oxygen presen in the resonance

Hence they form same resonating structures, hence it can easily accept an H+ to form carboxylic acid

In the second structure, the attack of H+ is very easy, since there is no steric hindrance because of presence of bulky group, therefore H will easily attack the OH- ion

In the third and fourth structure, the O- has a bulky group attached to it, which will provide steric hindrance to the incoming H+ leading to difficulty in the formation of bond

Hence the correct order will be 1 > 2 > 3 > 4

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