Phyllis R. Hirsch Syracuse University
Syracuse, New York
A
I
I
Effect of Liquid NH, on Wood A demonstration of the alcohol structure of cellulose
lecture demonstration of the plasticizationof wood with liquid ammoniacan be avery effective tool for teaching the alcohol structure of cellulose to any class studying basic organic chemistry. The usual method of demonstrating the ROH (where R = any organic radical) structure of cellulose is t o outlme a laboratory experiment involving esterification. The student prepares cellulose acetate by adding 5 ml of acetic anhydride, plus two drops of HzSOa, plus 20 ml of acetic acid to 5 g of cotton. This mixture is allowed to digest for a week to provide the time required for penetration of the cellulose by the reagent. Cellulose acetate is formed and then dissolved in the acetic acid. At the next laboratory class, the student finds a solution in the beaker. He then pours this solution, in a fine stream, into a 400-ml beaker of water. The acetate, insoluble in water, forms white crystals. The visible result of this method is not very effective because the transition from solid cellulose to dissolved acetate does not occur while the student is watching. The most striking change occurs when the dissolved acetate precipitates in the water; however, the solubility of the acetate (not the chemical reaotion being studied) causes the precipitation. A new method of demonstration has been suggested by experiments performed by Dr. Conrad Schuerch, Chairman, Department of Forest Chemistry, Syracuse University. Dr. Schuerch's work was based on previously known theories about the plasticization of wood. It was evident that liquid ammonia, NH,, would atr tract the H in any ROH structure to form NH,+. The simplest example of this type of reaction,
NH.
+ HOR
-
NHdC
+ OH-
is known by all students of elementary chemistry. The ammonia reacts with the -OH groups of the cellulose and breaks the interchain hydrogen bonds. The bonding between the chains is the cause of the rigidity of wood. When the bonds are broken, the chains are free to slide and the wood becomes flexible. Thus the wood can be bent into any desired shape. Because the boiling point of ammonia is -33.35"C, it will evaporate from the treated wood upon exposure to air, leaving its borrowed H's behmd to form new interchain bonds. These bonds cause renewed rigidity and the new shape of the wood is permanent. The same procedure used by Dr. Schuerch in his laboratory can be used for a lecture demonstration. Liquid ammonia is prepared in a condenser surrounded by dry ice and kept in an insulated container. A piece of wood in a convenient form, such as a birch X 3/P X 6-in., is placed in the tongue depressor, condenser a t the beginning of the lecture. During the 15-20 min the tongue depressor must be left submerged in the ammonia to insure a complete reaction, the theory can be explained. At the end of this time, the strip of wood is removed from the condenser and bent into a spiral, knot, or any other desired shape. The shape is retained when the wood regains room temperature and the NH3 has evaporated. This demonstration is superior to the laboratory experiment now being used, because the visible result is very striking, the chemistry of the reaction is very elementary, and the method of demonstration is simple.
Volume 41, Number 1 1 , November 1964
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