Question

(a) Give the resonance structure of the conjugate bases of the following :

(b) Compare the stabilities of their bases and the acidites of the parent compounds.

Answer 1

(a) (I)
(II)
(III)
(b) (I) Anion is stabilised due to aromaticity, althrough it has negative charge on less electronegative `C` atom.
(II) Anion is stabilished by `(p pi - p pi)` bond formation. It also has negative charge on less electronegative `C` atom but its anion is less stable than the anion of `(I)`.
(III) Anion is also stabilised by `(p pi - p pi)` bond formation, but the negative charge is delocalised to electronegative `O` atom, but some aromaticity of benzene is lost. So, anion is more stable than that of `(II)` but less stable than that of `(I)`
Therefore, stability order of the anions is :
`I gt III gt II`
The conjugate base is more stable if the negative charge is delocalised to more `EN` atom. Bit if an anion retains its aromaticity, it is even more stable.
The more stable the base (or weaker the base), the more acidic is the parent.
Acidic order is : `I gt III gt II`.