IIT JEE , AIEEE Forum - Chemistry , Organic Chemistry

Milo_Breakin_free

If a reaction is carried out in a protic solvent, whose molecules have a hydrogen bonded to an oxygen or to a nitrogen, the larger atom is a better nucleophile in an S_N2 reaction. In other words, the weaker base is the better nucleophile in a protic solvent. For example, the iodide ion is better than a fluoride ion as a nucleophile. However, if the reaction is carried out in an aprotic solvent, whose molecules do not have hydrogen bonded to an oxygen or to a nitrogen, then the stronger base is the better nucleophile. In this case, the fluoride ion is better than the iodide ion as a nucleophile.

Can u please explain this statement and why this is true???

srujana

Organic Doubt,based on SN2 reaction

srujana

"If a reaction is carried out in a protic solvent, whose molecules have a hydrogen bonded to an oxygen or to a nitrogen, the larger atom is a better nucleophile in an SN2 reaction."

A protic solvent usually consists of R-O-H or N-H groups.
They form hydrogen bonding with the hydrogen atom of the nucleophiles.

An SN2 reaction requires that the nucleophile attacks the carbon atom.

Hence the nucleophile should be free from the solvant molecules which solvate it(by forming hydrogen bonding)

Now less the size of the anion, more easily it is solvated, as it can appear more close to it and form stronger hydrogen bonds.
Hence in case of small anions in polar protic solvents, it requires that more energy is provided to it to strip off the solvent molecules forming the Hydrogen bond with it.This reduces the strengt hof the nucleophile.

In case of polar aprotic solvents there is no solvation of the anions hence it follows the usual trend "a strong base is a better nucleophile"

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