Question

nucleophilicity order of cl-,f-,br-,i-

Answer 1

F- < Cl- < Br- < I-

Answer 2

The nucleophilicity order of halide ions (F-, Cl-, Br-, I-) depends on the solvent.

1. In Protic Solvents (e.g., water, alcohols):

   • Protic solvents can form hydrogen bonds with anions.
   • Smaller anions (like F-) have a higher charge density and are more strongly solvated by hydrogen bonding. This strong solvation hinders their ability to donate electrons and act as nucleophiles.
   • Larger anions (like I-) have a lower charge density and are less strongly solvated. Their electron cloud is also more polarizable, making them better able to form a new bond.
   • Therefore, in protic solvents, nucleophilicity increases with increasing size (down the group).
   • Order: F- < Cl- < Br- < I-

2. In Aprotic Solvents (e.g., DMSO, DMF, acetone):

   • Aprotic solvents do not form strong hydrogen bonds with anions.
   • In these solvents, nucleophilicity generally follows basicity. Basicity decreases down the group (F- > Cl- > Br- > I-), as F- is the strongest base among halides.
   • Therefore, in aprotic solvents, nucleophilicity decreases down the group.
   • Order: I- < Br- < Cl- < F-

Unless otherwise specified, discussions about the general nucleophilicity trend of halide ions usually refer to their behavior in protic solvents, where the size and solvation effects are dominant. This is also consistent with the typical context in organic reactions (e.g., SN2 reactions).

Therefore, assuming a protic solvent, the nucleophilicity order is:

F- < Cl- < Br- < I-