Objective: To prepare m-dinitrobenzene from nitrobenzene

Apparatus/Glassware Required: Round-Bottom Flask, reflux condenser, beaker, volumetric flask, measuring cylinder, suction pump and Buchner funnel

Chemicals Required: Nitrobenzene, concentrated sulphuric acid, fuming nitric acid and rectified spirit

Principle:

It is prepared by nitration of nitrobenzene with concentrated nitric acid in the presence of concentrated sulphuric acid. Here, the function of the sulphuric acid is to convert the nitric acid into the highly reactive, electrophile, nitronium ion (NO₂⁺), which is the effective nitrating agent. Nitration of aromatic hydrocarbons is usually carried out with the nitrating reagent at comparatively low temperatures. Unnecessarily high temperatures should be avoided since poly nitration is more likely and oxidative break down of the aromatic system may occur.

The nitration of nitrobenzene containing an electron-withdrawing -NO₂ group does not occur readily under the above conditions, in which use forcing conditions which requires the use of fuming nitric acid and concentrated sulphuric acid need to be employed. The deactivating affect of the nitro group is largely the result of its mesomeric interaction (-M effect) with the π- electron system of the benzene ring which is supplemented by the inductive (-I) effect. The overall electron withdrawal from the ring system results in the rate of attack of the nitronium ion being substantially retarded compared to benzene. Moreover, the representation of the canonical forms of nitrobenzene, the Ortho and Para-positions are subject to the greatest reduction in electron density, so energetically unfavorable the mesomeric stabilization of this intermediate is less than that of the corresponding intermediate resulting from attack in the m-position. Because of this reason, the incoming nitro group oriented towards the m-position in the nitrobenzene.

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Procedure:

1. Place 7.0 mL of concentrated sulphuric acid and 5.0 mL of fuming nitric acid, in a 85 mL rectified spirit.
2. Add a few fragments of unglazed porcelain. Attach a reflux condenser and place the apparatus in a fume cup board.
3. Add slowly, in portions of about 1.0 mL, 4.2 mL (5.0 g) of nitrobenzene. After each addition, shake the flask to ensure thorough mixing.
4. Heat the mixture, with frequent shaking, on a boiling water bath for 30 minutes.
5. Allow the mixture to cool somewhat and pour it cautiously with vigorous stirring into about 170 mL of cold water; the m-dinitrobenzene soon solidifies.
6. Filter with suction, wash thoroughly with cold water and allow draining as completely as possible.
7. Transfer the crude m-dinitrobenzene to a 250 mL flask fitted with a reflux condenser, add 80-100 mL of rectified spirit and heat on a water bath until all the crystalline solid dissolves.
8. If the resulting solution is not quite clear, filter it through a fluted filter paper on a large funnel which has previously been warmed or through a warm Buchner funnel.

Uses: Mainly used as Synthetic intermediate

Note: Yellowish crystals; insoluble in water; M.P. 89.6° C

Precautions: Nitrobenzene is readily absorbed into the skin, and the concentrated mineral acids are corrosive. Gloves should be worn when transferring these reagents.