Aromatic ring substitution: how are MNT and DNT further nitrated if nitro groups deactivate benzene rings? This is probably a common question because it involves TNT precursors, but if an $\ce{NO2}$ group deactivates benzene rings toward electrophilic aromatic substitution reactions, then how can toluene be tri-nitrated? I understand that the methyl group will donate electron density and activate the ring toward the first substitution, but after two nitrations I'm sure the nitro groups slow the third substitution. So what's the trick? How does toluene get three nitro groups?
On a industrial scale by using highly concentrated nitric acid known as fuming nitric acid (usually > 90%, but in the last step from DNT to TNT may also be anhydrous) along with oleum, which is a mixture of $\ce{SO_3}$ and sulfuric acid. Preparation in the lab may be achieved by more or less ordinary nitration at elevated temperatures. In a micro scale experiment [1] they used two different types of nitrating acid:


Nitrating acid I contains concentrated nitric acid 19% v/v, concentrated sulfuric acid 58% v/v and water 23% v/v.
Nitrating acid II is a mixture of concentrated nitric acid 25% v/v and oleum (30% $\ce{SO_3}$) 75% v/v.


Nitrating acid I was used for $\ce{Toluene -&gt; MNT}$ and $\ce{MNT -&gt; DNT}$ (65 °C). Nitrating acid II was used for $\ce{DNT -&gt; TNT}$ (110 °C).

1 The Stepwise Nitration of Toluene: A Multistep Microscale Synthesis Based on an Industrial Process
Richard A. Russell, Robert W. Swltzer, and Robert W. Langmwe
University College, University of N.S.W., Northcotl Drive, Campbell ACT. 2600, Australia