Towards $CH X_{3} I$ in dry DMSO (a solvent in which nucleophilicity follows basicity for the same attacking atom), the correct order of nucleophilic strength of the following oxygen anions is (conjugate acid $p K_{a}$ values in DMSO: ethanol 29.8, phenol 18.0, acetic acid 12.3, trifluoroacetic acid 3.5)

A $CH X_{3} CH X_{2} O X^{-} > C X_{6} H X_{5} O X^{-} > CH X_{3} COO X^{-} > CF X_{3} COO X^{-}$
B $CF X_{3} COO X^{-} > CH X_{3} COO X^{-} > C X_{6} H X_{5} O X^{-} > CH X_{3} CH X_{2} O X^{-}$
C $CH X_{3} COO X^{-} > CF X_{3} COO X^{-} > CH X_{3} CH X_{2} O X^{-} > C X_{6} H X_{5} O X^{-}$
D $C X_{6} H X_{5} O X^{-} > CH X_{3} CH X_{2} O X^{-} > CF X_{3} COO X^{-} > CH X_{3} COO X^{-}$

Answer: A

With the same attacking atom and no hydrogen bonding to the anion, the stronger base (higher

p K_{a}

of the conjugate acid) is the stronger nucleophile. The

p K_{a}

values 29.8, 18.0, 12.3, 3.5 give the order ethoxide

>

phenoxide

>

acetate

>

trifluoroacetate.