Write the equations for the preparation of iodobutane from But-1-ene.    

Arrange the compounds of each set in the order of reactivity towards ${\mathrm{S}}_{\mathrm{N}}2$ displacement. 1-bromo-3-methylbutane, 2-bromo-2-methylbutane, 2-bromo-3-methylbutane.

Arrange the compounds of each set in the order of reactivity towards ${\mathrm{S}}_{\mathrm{N}}2$ displacement: 1-bromobutane, 1-bromo-2, 2-dimethylpropane, 1-bromo-2-methylbutane, 1-bromo-3-methylbutane.