P-phenylenediamine Production Process
React benzene with chlorine in the presence of $\ce{FeCl3}$ to yield chlorobenzene.

According to Ullmann's Encyclopedia of Industrial Chemistry:

Most monochlorobenzene is now produced from benzene and chlorine in continuously operated plants. Depending on the ratio of benzene to chlorine chosen, one can achieve either a low rate of benzene conversion and little dichlorobenzene
formation, or almost complete conversion of the benzene with a higher degree of dichlorobenzene formation. [...] The composition of a chlorination mixture containing the highest possible proportion of monochlorobenzene has been given as
4–5% unreacted benzene, 73% monochlorobenzene, and 22–23% dichlorobenzenes.


Nitrate chlorobenzene to obtain a mixture of 2-nitrochlorobenzene and 4-nitrochlorobenzene. Separate the isomers.

In Ullmann's Encyclopedia of Industrial Chemistry, we find:

Nitration of chlorobenzene with mixed acid (30/56/14) typically gives an isomer
mix in 98 % yield consisting of 34–36% 2-chloronitrobenzene, 63–65 % 4-chloronitrobenzene, and only ca. 1% 3-chloronitrobenzene.

&quot;Mixed acid&quot; refers to a mixture of nitric acid, sulfuric acid and water.

React 4-nitrochlorobenzene with ammonia (Nucleophilic aromatic substitution) to obtain 4-nitroaniline.
According to Ullmann's Encyclopedia of Industrial Chemistry, the reaction conditions as as follows:



4-Nitroaniline is produced in an agitated titanium reactor in 99.3 % yield by reacting 4-chloro nitrobenzene with a tenfold excess of aqueous ammonia at 175 °C and 4.2 MPa (42 bar) for 10 h.

Ullmann's Encyclopedia of Industrial Chemistry cites a patent by MONSANTO (DE1768518) as the relevant reference.

Reduce the nitro group with iron.