Preparation of p-chlorobenzoic acid from p-chloronitrobenzene I was reading my book where I spotted the preparation of p-chlorobenzoic acid from p-chloronitrobenzene.

The preparation method there was given as such:


$\ce{Sn/HCl}$
$\ce{NaNO2 + HCl}$ ($\pu{0-5 ^\circ C}$)
$\ce{CuCN}$
$\ce{H2O/H+}$


Now, the thing is that, I recall the first two steps from the process of forming diazonium salts but I have never seen the last two steps give the $\ce{-COOH}$ group. 

Also I recall that $\ce{CuCN}$ gives us the nucleophile $\ce{CN-}$ where the negative charge is on the nitrogen atom and also that $\ce{H2O/H+}$ is used for hydration but I don't see how these two could help us form p-chloronitrobenzene from Benzenediazonium chloride.
This is simply the summery of all comments involved:

$$\ce{Cl-C6H4-NO2 -&gt;[Sn/HCl] Cl-C6H4-NH3+Cl-} \tag1$$

$$\ce{Cl-C6H4-NH3+Cl- -&gt;[\text{a. } NaNO2/HCl \ (\pu{0-5 ^\circ C})][\text{b. } CuCN] Cl-C6H4-CN} \\ \tag2$$

$$\ce{Cl-C6H4-CN -&gt;[H+/H2O][\Delta] Cl-C6H4-COOH} \\ \tag3$$

Note: The reaction $(1)$ is a typical reduction reaction, which reduce nitro group to amino group. The reaction $(2)$ is called Sandmeyer Reaction. Finally, The reaction $(3)$ is a hydrolysis condition to convert cyano group to carboxylic acid group. That hydrolysis can be done under basic conditions as well (then, final product is carboxalate).



Also note that even though it is not necessary, I put this answer up just to avoid people asking same kind of questions again and again. When they have a visual answer they might not do so.