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Which is a better nucleophile, among halides ions ( fluoride, Chloride, bromide and iodide)? and Why?

Nucleophilicity of an nucleophilic depends upon nature of solvent, for example:
(1) I⁻ is a better nucleophile than F⁻ in polar protic solvents.

(2) F⁻ is a better nucleophile than Br⁻ in polar aprotic solvents.

We know that a protic solvent has an H atom bound to more electronegative elements like F, O or N. It can use its H atom to formed H-bonding with a nucleophile. Which accumulate around nucleophilic and creates a shell around the nucleophile. So that it becomes more difficult to attack the positive carbon bearing the leaving group.

F⁻ is a small ion with a high charge density. It is tightly solvated.

I⁻ is a large ion with a low charge density. It is loosely solvated. There are only a few solvent molecules to push out of the way.

Hence over all order of nucleophilicity in polar protic solvents is given as :

        [ I⁻ > Br⁻ > Cl⁻ > F⁻ ]

We know that a polar aprotic solvent does not have a hydrogen atom that can formed hydrogen bond.

But in all of them, the negative ends of the dipoles directedaway from the molecule. So that It is easy for them to solvate cations.

The positive ends of the dipoles are closer to the middle of the molecule. It is difficult for them to get close to the anions.

As the result the nucleophile has few molecules in its solvent shell. The nucleophile can more easily attack on positive carbon (electrophilic center).

Hence F⁻ becomes a much better nucleophile than chloride, 

Hence over all order of nucleophilicity in polar aprotic solvents is just reverse of that given in polar protic solvent :

        [ F⁻ > Cl⁻ > Br⁻ > I⁻ ]


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