Haloalkanes and Haloarenes

The correct order of reactivity of following compounds in nucleophilic substitution $$({S}_{N}2)$$ is:
Which of the following is least reactive towards $$S_N2$$ mechanism? 
The decreasing order of rate of $$S_{N}2$$ reaction for given compounds is:
Write down the chemical equation for the preparation of Diethyl ether by Williamson's synthesis?
Write the structure of the product when chlorobenzene is treated with methyl chloride in the presence of sodium metal and dry ether.