Synthesis of m-Dinitrobenzene from Nitrobenzene

Aim: To synthesize m-Dinitrobenzene from Nitrobenzene by nitration reaction

Reference: F. G. Mann and B. C. Saunders, Practical Organic Chemistry, 4^th Ed, 1974, P. No. 160.

Requirement:

Apparatus: Beaker, Stirrer, Filtration assembly, RBF, Condenser, etc.

Chemicals: Fuming nitric acid, sulphuric acid, nitrobenzene

Theory/Principle:

           Here nitration is occurring on nitrobenzene. It is an electrophilic aromatic substitution in the presence of NO₂, which is a strong electron-withdrawing group and it directs the incoming substituents to the meta position. Here nitronium ions act as the electrophile which is generated from fuming nitric acid in the presence of conc. sulphuric acid. Nitration is an electrophilic substitution reaction on an aromatic ring.

Fig. 1 Synthesis of m-Dinitrobenzene from Nitrobenzene 

Fig. 2 Mechanism of Reaction

Procedure:

1. This preparation must be performed in a fume-cupboard because nitrous fumes are evolved during the nitration. A ground-glass flask and air condenser should preferably be used
2. Place 1.5 ml of fuming nitric acid in 100 ml. flask and add carefully with shaking 2.0 ml. (3.7 g) of concentrated sulphuric acid and then some fragments of unglazed porcelain.
3. Fit a reflux air-condenser securely to the flask, and then add slowly down the condenser 1.2 ml of nitrobenzene: do not add more than 0.3 ml. of the nitrobenzene at a time, and after each addition shake the flask to ensure thorough mixing of the contents.
4. Now heat the flask on a boiling water-bath for 1 hour, both the flask and the condenser being securely clamped in position if joined by a cork, which the acid fumes evolved may attack and weaken.
5. Shake the flask vigorously from time to time throughout this period of heating.
6. Finally, pour the mixture carefully with stirring into an excess of cold water (about 300 ml), whereupon the heavy oily dinitrobenzene will rapidly solidify.
7. Filter the crystalline material at the pump, wash thoroughly with water to remove all acid, and then drain as completely as possible.
8. Thus obtain crude m-dinitrobenzene, m.p. 90°.