Nucleophilicty of a species is its ability to attack an electrophlic carbon whilie basicity is the ability of a species to remove H⁺.

The order of nucleophilicity of different species depends on the nature of solvent used. For instance, for F^-,Cl^-,Br^- and I^- with their counter cation as Na⁺ and see their nucleophilicity in solvents, namely :

1) Non-polar(CCl₄)

2) Polar protic(H₂O)

3) Polar aprotic(CH₃SOCH₃)

4) Weakly polar aprotic ( CH₃COCH₃)

 

Polar solvents are able to dissociate the salts i.e. ion-pairs can be dissociated. On the other hand,non-polar and weakly polar solvents are unable to do so. 

 

In non-polar and weakly polar aprotic solvents, all the salts will exist as ion-pairs. The ion-pairing will be strongest according to the FAJAN's RULE, i.e the ion-pairing will be strongest with the samallest anion(F^-) and weakest with the largest anion(I^-). Thus,the reactivity of X^- decreases with with decreasing size. Thus, the nucleophilicity order of X^- in such solvents would be :-

    I^- > Br^- > Cl^- > F^-  

 

In polar solvents, hydrogen bonding or ion-dipole interaction diminishes the reactivity of the anion. Stronger the interaction, lesser is the reactivity of anion. F^- ion will form strong H-bond with polar protic solvent while weakest ion-dipole interaction will be with I^- ion. Thus, the order of nucleophilicity in polar protic solvent would be:-

       I^-  > Br^- > Cl^- > F^-

 

Polar aprotic solvents have the ability to solvate only cations, thus anions are left free. The reactivity of anions is then governed by their negative charge density(i.e their basic character). Thus, the order of nucleophilicity of X^- in polar aprotic solvents would be:-   

                    F^- > Cl^- > Br^- > I^-

 

On this basis, certain nucleophility orders are:-

1) In polar solvents. HS^- > OH^-

2) Bases are better nucleophiles than their conjugate acids.

   e.g:-  OH^- > H₂O    and,  NH₂^- > NH₃

2) In non-polar solvents, ^-CH3 > ^-NH₂ > ^-OH > ^-F

3) When nucleophilic and basic sites are same, nucleophilicity parallels basicity. e.g:-   RO^- >OH^- > RCOO^-. In this case, the basic/nucleophilic site is the negatively charged oxygen atom 

 

4) When atom bonded to nucleophilic site also has an unshared pair of electrons, nucleophilicity of species increases.

  e.g:-   HOO^- > HO^- and, H₂N-NH₂ > NH₃

5) For the atoms of same group, the one having larger size has more nucleophility.

 e.g:-  RS^- > RO^-

 

 

RS^- > ArS^- > I^- > CN^- > OH^- > N₃^- > Br^- > ArO^- > Cl^- > pyridine > AcO^- > H₂O