Finkelstein reaction.

Explanation

It is a halide exchange method and iodoalkanes are best prepared by this method from the corresponding chloro or bromoalkanes by heating with sodium iodide in acetone or methanol.
`CH_(3)CH_(2)-Br+NaIunderset(Delta)overset("Acetone or methanol")(to)CH_(3)CH_(2)I+NaBr`
Sodium bromide (or NaCl in case of chloroalkanes), being less soluble than NaI in acetone, gets deposited This reaction is very useful for preparing such alkyl iodides which cannot be prepared by direct addition of HI to alkenes. For example, n-propyl iodide cannot be prepared by direct addition of HI to propene. However, this can be prepared using Finkelstein reaction as follows :
`CH_(3)-CH=CH_(2)underset("peroxide")overset("HBr")(to)CH_(3)CH_(2)CH_(2)Brunderset(Delta)overset("NaI/Acetone")(to)CH_(3)CH_(2)CH_(2)I`