Asked in Organic Chemistry
How do you make m-nitrotoluene from toluene?
February 26, 2012 1:40PM
The instructions given by the previous answer were dangerous as it would make significant quantities of explosive compounds.
Mono-nitration of toluene predominantly yields the ortho and para isomers. When using a Sulphuric acid/Nitric acid nitrating mix you can expect the meta isomer to comprise about 4% of the mono-nitrotoluenes.
To prevent formation of explosive poly-nitrotoluenes you must add the acid mix to the toluene dropwise and keep the temperature below 30degC.
Place 117ml Toluene into a flask immersed in ice-bath with vigorous stirring of the toluene.
Add a cooled mixture of 67ml 70% HNO3 and 77ml 96% H2SO4 carefully dropwise and keep temperature below 30degC and preferably around the 20degC mark. After an hour or so the addition should be complete. Let it stand with stirring for about 30mins. Separate the the upper organic layer from the acids and wash it a couple of times with water and then with 10% NaOH solution followed by a couple more water washes.
Heat the yellow oil under a vacuum to drive off any unreacted toluene and chill in a freezer to around -10degC, p and m-nitrotoluene should crystallize out (only if you have fully removed the unreacted/excess toluene) drain the solids from the liquid and keep cold (centrifuge is better). The drained oil is about 70-80% o-nitrotoluene.
Fractionally distill the oil (using a good column) under vacuum to separate a good amount of the ortho isomer, the residue left in the flask is chilled once again to -10degC to collect more solids. Repeat this process with the oil (a eutectic mixture) until you are satisfied with the amount of solids collected.
WARNING: Do NOT distill this oil if temperature exceeded 30degC during nitration as a violent explosion could result.
Steam distill the solids and most of the para isomer will steam distill across while the meta isomer will largely remain behind as it's not so readily steam-volatilized.
This will only give a very low yield of the meta isomer as the methyl group on toluene is ortho and para directing with a para/ortho ratio of about 0.6.