Which is the better nucleophile in no solvent, in an aprotic solvent, and a prot

Question

Which is the better nucleophile in no solvent, in an aprotic solvent, and a protic solvent? Cl- or Br-

I was under the impression that Cl- was the better nucleophile because it is smaller and can not stabilize the charge as well as Br- thus making it a better nucleophile when in no solvent/ aprotic solvent

I also thought that because the charge is more concentrated in Cl- it was more suscelptible to being protonated in a protic solvent, thus making Br- the better nucleophile in a protic solvent

Please explain why...

Explanation / Answer

Bromine is a better nucleophile than Cl(-) in polar protic solvents, but chloride is a better nucleophile than Br(-) in polar aprotic solvents.

This can be explained by three key trends here.

1) nucleophilicity is decreased by hydrogen bonding – and a nucleophile in a protic solvent will be surrounded by solvent molecules it is hydrogen-bonded to. Br- > Cl-

2) hydrogen bonding ability decreases as one goes down a column in the periodic table

3)In polar aprotic or no solvent, in the absence of hydrogen bonding, nucleophilicity increases with basicity.

Cl- > Br-

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