Q.How will you prepare n-propyl iodide from n-propyl bromide?

Two named reactions convert an EXISTING chloro- or bromoalkane into a different haloalkane by swapping only the halogen, leaving the carbon skeleton untouched -- useful because iodides and fluorides are often harder to make directly. Finkelstein reaction: a chloro- or bromoalkane heated with a concentrated solution of sodium iodide in dry acetone gives the corresponding iodoalkane (bromoethane + NaI/acetone/heat gives iodoethane + NaBr). Mechanistically it is a straightforward SN2 displacement, driven strongly forward because NaBr and NaCl are insoluble in dry acetone (while NaI is soluble) and precipitate out of solution as they form, continuously removing product and pulling the equilibrium toward completion (Le Chatelier). Swarts reaction: a chloro- or bromoalkane heated with a metallic fluoride -- AgF, SbF3 or Hg2F2 -- gives the fluoroalkane (bromoethane + AgF gives fluoroethane + AgBr). The same name, 'Swarts reaction', is reused later in the chapter for the industrial manufacture of Freon-12 from carbon tetrachloride + HF/SbCl5 -- the same halogen-exchange chemistry, just applied to a tetrahalomethane instead of a simple monohaloalkane, so a question naming 'the Swarts reaction' could correctly point to either example.