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Electrophilic substitution
Electrophilic substitution in the highly activated phenol ring occurs under very mild conditions, and mononitration must be carried out with dilute aqueous nitric acid. The usual nitric acid-sulfuric acid mixture gives complex mixtures of polynitro compounds and oxidation products.
Separation of o- and p-nitrophenol can be accomplished by taking advantage of the strong intramolecular hydrogen bonding, or chelation, in the ortho isomer. In the para isomer, the hydrogen bonding is inter-molecular, leading to association of the molecules in the liquid and much lower vapor pressure. On steam distillation of the mixture the ortho isomer is obtained in pure form in the distillate. The para isomer can then be isolated from the nonvolatile residue.
[Procedure]
In a 250-ml Erlenmeyer flask, place 10 ml of concentrated nitric acid and 35 ml of water. Weigh out 8.0 g of phenol ("loose crystals") in a 50-ml beaker. (CAUTION: Avoid contact with skin.) Add 2 ml of water and allow the mixture to liquefy. With a disposable pipet, add 1- to 2-ml portions of the phenol to the nitric acid and cool as necessary by swirling in a pan of cold water to keep the temperature of the reaction mixture at 45 to 50°. After all of the phenol has been added (rinse the beaker with 1 ml of water), shake the flask intermittently for 5 to 10 minutes while the contents cool to room temperature. Meanwhile, assemble the apparatus for steam distillation.
Transfer the reaction mixture to a separatory funnel and drain the oily organic layer into a three-neck 500-ml flask. Add 150 ml of water and then carry out steam distillation until no further o-nitrophenol appears in the distillate. Collect the o-nitrophenol, air dry, and determine yield and melting point.
For isolation of the para isomer, adjust the total volume of the distillation residue to about 200 ml by adding more water or removing water by distillation. Decant the hot mixture through a coarse fluted filter or loose cotton pad, add about 1 to 2 g of charcoal to the hot filtrate, heat again to boiling, and refilter to remove charcoal. Cool a 500-ml Erlenmeyer flask in ice and pour into it a small portion of the hot solution to promote rapid crystallization and prevent separation of the product as an oil. Then add the remainder of the solution in small portions so that each is quickly chilled. Collect crystals, dry, and report yield and melting point.
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