25 January 2002

Paranitroacetanilide and Humongous Pharaoh's Serpents

by Norm Stanley

For a spectacular chemical demonstration the decomposition of paranitroacetanilide to yield a voluminous ash is hard to beat.  Although this compound can be purchased,  preparing it yourself is an interesting exercise in organic synthesis.

The synthesis is accomplished in two stages, starting with aniline (aminobenzene, C₆H₅NH₂): First we acetylate aniline with glacial acetic acid  to yield acetanilide:

(1)       C₆H₅NH₂ + CH₃COOH —> CH₃COONHC₆H₅

This is then nitrated with nitric acid:

 

Figure 1. Click image to enlarge

Although there are three⁽¹⁾ possible positions (ortho- meta- and para-) for the second substituent (Figure 1), in this case the nitro group, to attach to the benzene ring, formation of the para-isomer is favored..  In general the point of entry of a secondary substituent on the benzene ring is influenced by the nature of the primary group, here  acetylamino-..  It's a common stratagem in organic synthesis to select a group that yields the desired isomer.  With this established, the substituents can then be modified en route to the final product.

So much for Organic 101.  Let's head for the laboratory and acetylate some aniline:

Figure 2: Click image to enlarge.

Place 10 g (9.7 mL) of aniline and 15 g (14.2 mL) of glacial acetic acid⁽²⁾ in a 125 mL boiling flask.  Attach a reflux condenser⁽²⁾ to the flask and heat to a boil with a Bunsen burner or hot plate while maintaining a flow of cold water through the jacket of the condenser (Figure 2).  Continue to reflux the mixture for several hours.  The product will be a rose colored homogeneous liquid.  If left for several days the liquid may crystallize into a solid mass.

Acetanilide is quite soluble in hot water, but only sparingly soluble (ca. 0.5 g/100 mL) in cold water.  Add about 100 mL of distilled or deionized water to the flask and heat until the crude acetanilide dissolves.  Dark-colored droplets of immiscible liquid may be seen in the solution, apparently accounting for the color of the crude material.  Allow the mixture to cool slightly; this  unidentified by-product will settle out.  Decant off the solution of acetanilide.  To improve the yield, dissolve any acetanilide remaining in the flask in hot water and again decant.  Cool the combined solutions in an ice bath to precipitate the acetanilide as colorless leaflets.  Dissolve in hot water and recrystallize as before.  Filter off the crystals and allow them to air-dry in a shallow dish until any residual acetic acid has evaporated.  The resulting product should be free of any odor of the acid.   Nine grams or more of purified product should be recovered.  The theoretical yield  of acetanilide (m.w. 135)  from 10 g of aniline (m.w. 93) is

        x/10 135/93

        x 14.5 g

A yield of 60% was considered fairly respectable for one very inexperienced organiker.

As a test for acetanilide mix a small amount of acetanilide with an equal amount of sodium nitrite and sprinkle the mixture onto the surface of concentrated sulfuric acid.  Note the red color formed.  This is an instance of "Liebermann's reaction", used as a test for secondary amines:

(3)      

A nitrosoamine is formed; this reacts with phenol (C₆H₅OH) to produce a colored indicator which turns red in acid, blue-green in base.  The phenyl group already present in acetanilide reacts similarly to phenol.

 To nitrate the acetanilide, place 15 mL of conc. sulfuric acid in a 250 mL Erlenmeyer flask and cool by placing the flask in an ice water bath.  To keep the flask from overturning, weight it down with a lead "doughnut".  These are available from laboratory suppliers, bu t you can make your own by melting scrap lead and pouring into a suitable form. Another alternative is to buy a stick of bar solder and bend it into a circle.  Next gradually add 7 grams of acetanilide while stirring with a thermometer until all, or nearly all, has dissolved.  7.5 mL of conc. nitric acid are then added dropwise with continuous stirring.  At the outset much heat is evolved and care must be taken to keep the temperature below 20 C by adding the acid slowly and keeping ice in the bath.  Allow the flask to stand in ice water for about 15 minutes after addition of the acid.  Then pour the contents of the flask, a slightly syrupy reddish liquid, into about 500 mL of ice water while stirring vigorously;  p-nitroacetanilide will precipitate as a granular yellowish-white solid.

Filter off the precipitate on a double layer of finely-woven cotton cloth in a large glass or plastic funnel, and wash on the filter with ice water.  Take up the cloth and squeeze it gently to remove excess liquid.  Take care not to lose yield by forcing the precipitate through the cloth.  Finally, place the cloth with the precipitate in a shallow dish and allow to air dry for two or three weeks.  When the product is sufficiently dry, break up the cake and pulverize.  Bottle the p-nitroacetanilide without further purification.

Paranitroacetanilide can be used to produce a "Pharaoh's Serpent" that far outstrips the tiny ones sold as fireworks; these use the combustion of mercuric thiocyanate to produce a voluminous ash.  For a showy demonstration place a gram or two of p-nitroacetanilide in a small evaporating dish and add enough conc. sulfuric acid to make a thick paste.  Avoid too thin a mixture as this will simply fizzle instead of deflagrate.  .Place the dish on a gauze mat on a tripod and heat with a small Bunsen flame or alcohol lamp. At first the mixture will darken and bubble slightly.  Then, quite suddenly and without warning, it will deflagrate with a hissing sound and a large and dense cloud of white smoke.  Only when the smoke starts to dissipate will the "serpent" come into view, a column of black ash the diameter of the dish and extending up two feet or more.  It stays in place attached to the dish, but is so light than the dish doesn't overturn.  Despite its light weight the ash is remarkably rugged and can be broken loose and handled gently without collapsing.

This demonstration should only be done in a well-ventilated area, as the white fumes of sulfuric acid are extremely irritating.

Notes:

(1)       Given the conventional, Kekulé, representation of the structure of benzene as a ring of six carbon atoms alternately linked by single and double bonds, one might ask why there should not be four instead of three distinct isomeric forms for the disubstituted ring.   The answer is a  phenomenon called resonance wherein the bonds between adjacent carbon atoms may be thought of as existing in a intermediate state between single and double.

(2)       The Allihn condenser shown in Figure 2 is a pretty piece of glassware, but there's no specific requirement that one be used .  An ordinary straight-tube (Liebig) condenser or any of the water-jacketed columns available at bargain prices from the SAS Store will do nicely.

(3)      It is important to keep glacial acetic acid  from contact with skin as it attacks the epidermis, loosening it to form extensive blisters.  If contact does occur, immediately wash the affected area with soap and water.  As in any lab. work, wear protective goggles and  clothing.   The same caution applies to other concentrated acids (e.g., H₂SO ₄, HNO₃ ).

Similar precautions apply also to aniline, which is toxic and can be absorbed through the skin.