Question

In protic solvents, the order of nucleophilicity of halides is I^⊖ > Br^⊖ > Cl^⊖ > F^⊖ but in polar aprotic solvents the order is I^⊖ < Br^⊖ < Cl^⊖ < F^⊖. Explain the reversal of the reactivity order in different types of solvent on the basis of solvation and the role of counter cations. .

Answer 1

in aprotic solvent solvation will not consider so size remains same

I⊖ > Br⊖ > Cl⊖ > F⊖   

so order nucleophillic character as follows

I⊖ < Br⊖ < Cl⊖ < F⊖

in polar protic solvents solvent forms hydrogen bond with charge

in group top atoms forms strong hydrogen bonding while moving down the group hydrogen bonding capacity will decrease.

so F^-  forms strong hydrogen bonding with protic solvent resulting nucleophillic character will decrease.

I^-  form less hydrogen bonding so act as strong nucleophile.

so in protic solvents order as follows.

I⊖ > Br⊖ > Cl⊖ > F⊖