Nucleophilicity and Basicity

[dttom]

Consider:

HI + Cl- <--organic solvent--> (eqm) HCl + I-

Probably the eqm bias to the right, following the principle of strong base gives a weak base upon neutralization, it seems to be right to claim I- is a weaker Lewis base than Cl-.

But my professor said I- is indeed stronger, why is that?

It may be related to the polarizability of I-, which is bigger and electric charge more delocalized, thus being more polarizable than Cl-. This makes it more reactive towards a haloalkane than Cl- does. While on the other hand, as electric charge on I- is more dispersed than in Cl-, I- is more stable than Cl-. So a more stable I- reacts more rapidly than a less stable Cl- towards a given haloalkane? It seems to be a bit paradoxical.

 

Maybe that, polarizability favours basicity as does basicity inferred from the neutralization reaction given above. So how do we take a balance, or, better, determine in between which one is a stronger Lewis base? And if in this case the factor of polarizability takes over basicity inferred, how can we say I- is a stronger Lewis base than Cl- with the reaction, HI + Cl- <--organic solvent--> (eqm) HCl + I-, in hand?

[Horza2002]

The strength of the halogen acids (HF, HCl, HBr and HI) is determined by a number of factors. You need to consider the enthalpy change (i.e. the strength of the bonds involved). In this case, H-F has the strongest bond because there is more efficient overlap between the hydrogen 1s orbital and the fluorine 2p orbtials compared to a Cl 3p, bromine 4p and an idoine 5p. That argument therefore claims that HI is a stronger acid (since it has the weakest bond).

 

But you also need to consider the stability of the resulting anions...which will then depend on the solvent that you are using...generally the I- will be more stable than a Cl-.

[dttom]

The strength of the halogen acids (HF, HCl, HBr and HI) is determined by a number of factors. You need to consider the enthalpy change (i.e. the strength of the bonds involved). In this case, H-F has the strongest bond because there is more efficient overlap between the hydrogen 1s orbital and the fluorine 2p orbtials compared to a Cl 3p, bromine 4p and an idoine 5p. That argument therefore claims that HI is a stronger acid (since it has the weakest bond).

 

But you also need to consider the stability of the resulting anions...which will then depend on the solvent that you are using...generally the I- will be more stable than a Cl-.

 

Sorry for the late answering.

 

If I- is expected to be more stable than Cl-, then I- should not be more nucleophilic than Cl-, as a more stable species should mount less attack (nucleophilic attack here). However, I- in fact is a stronger nucleophile than Cl-. One may argue that I- has a more diffused electron cloud that grants it the susceptibility, yet a counter-argument could be that it is because the electron cloud is more diffused so I- is more stable.

 

Personally, I got some inspiration while studying the HSAB theory, I think it is because the carbon centre of an alkyl halide is a soft acid, and iodide is a softer base than chloride, so equlibrium constant for the reaction is greater for using iodide than using chloride.