Question

Explain the order of the following nucleophiles in weakly polar aprotic solvents, e.g., acetone.
(a) `LiI gt LiBr gt LiCI gt gt LiF`
(b) `CsF gt RbF gt KF gt NaF gt LiF`
(c) `Bu_4 N^(oplus) CI^(Ө) gt Bu_4 N^(oplus) Br^(Ө) gt Bu_4 N^(oplus) I^(Ө)`.

Answer 1

(a) Ion-pairing influences the reactivity of `N overset (Ӫ) u`. The counter ion plays an important role in ion-pairing. The negative charge on the largest anion `I^(Ө)`. Is most diffused, so `I^(Ө)` forms least ion-pairing. Hence it is most reactive.
The negative charge on the smallest `F^(Ө)` ion is least diffused. So the `F^(Ө)` ion forms ion pairs readily and hence are less reactive. So the order is :
`LiI gt LiBr gt LiCI gt gt LiF`.
(b) As the size of cation decreases, ion-pairing with `X^(Ө)` increases and nucleophilicity of `X^(Ө)` decreases. So the order is : `CsF gt RbF gt KF gt NaF gt LiF`.
(c) `Bu_4 N^(oplus)` has positively charged `N` with four `Bu` atoms and has no tendency to ion-pair or H-bonding with `X^(Ө)`. So the reactivity is directly related to basicity. Basic order is : `CI^(Ө) gt Br^(Ө) gt I^(Ө)`.
So the order is : `Bu_4 N^(oplus) CI^(Ө) gt Bu_4 N^(oplus) Br^(Ө) gt Bu_4 N^(oplus) I^(Ө)`.