Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: nudephil on 12/10/2020 17:37:40

Title: Explain the order of nucleophilicity F->Cl->Br->I-?
Post by: nudephil on 12/10/2020 17:37:40

Here's a question that came in from listener V.:

I know that in protic solvents, I- is a very good nucleophile and F- is a very poor one due to hydrogen bonding. But why should there be a reversal of order of nucleophilicity i.e. F->Cl->Br->I-?

Can anyone answer?

Title: Re: Explain the order of nucleophilicity F->Cl->Br->I-?
Post by: chiralSPO on 12/10/2020 19:41:56

An important aspect to note is that nucleophilicity is typically used to indicate relative rates of reactions (ie a *kinetic* comparison) vs indicating the relative stability of the products and reactants (ie a *thermodynamic* comparison, like ΔG or pK_a or K_eq)

It's good to think about solvent effects, but even in polar aprotic solvetns, iodide is a better nucleophile (and leaving group) than bromide, which is in turn a better nucleophile (and leaving group) than chloride (fluoride is terrible as a nucleophile, and as leaving group for S_N1 and S_N2 reactions, though it is ok as a leaving group for S_NAr reactions and in acyl or phosphoryl fluorides).

The key here is polarizability. During the course of an S_N2 reaction, electrons have to be able to get from the non-bonding orbitals on the nucleophile into the σ* orbital of the C–LG bond. The more electrons there are, and the larger the orbitals they are in, the easier it is for the electrons to move to the right place. (this also. applies to the leaving groups, for which the C–LG bond must polarize during the course of the reaction).