For 10 points, identify this type of reaction in which a nucleophile forces out a leaving group and takes its place in a single step.
In 2008, a roundabout mechanism for this type of reaction was observed in gas phase interactions between chloride ions and methyl iodine gas. The Finkelstein reaction, which involves two halides, proceeds through this mechanism, as does a common method of producing primary and secondary amines.  It is greatly enhanced in the presence of crown ethers due to solvation effects freeing up anions, and this mechanism's transition state is trigonal bipyramidal as a change in the stereochemistry of the system occurs known as the Walden Inversion. For 10 points, identify this type of reaction in which a nucleophile forces out a leaving group and takes its place in a single step.
In 2008, a roundabout mechanism for this type of reaction was observed in gas phase interactions between chloride ions and methyl iodine gas.
SN2 Reaction [or bimolecular nucleophilic substitution (accept in any order)]