562
JOURNAL Of CHEMICAL EDUCATION
THE EMPIRICAL FORMULA FOR POLYSACCHARIDES KENNETH E. WRIGHT Smith College, Northampton, Massachusetts
As A
TEACEER of plant physiology the writer [has occasionally been somewhat embarassed by the reaction of his students when informing them that their method of designating the empirical formula of a polysaccharide as used in their chemistry courses is open to serious question. It is then a case of a teacher being wrongeither the botanist or the chemist-which is a had situation from the viewpoints of both teachers and pupils. The customary empirical formula for polysaccharides in texts and in classroom use is (C~HmOS), or (CnH3, Ole). depending upon whether monosaccharides or disaccharides constitute the units which are linked. The hydrolysis of a polysaccharide quite commonly is given as follows:
(CJhOrL
+xH~O
-r
zCJLnOs
Neither of these generalized representations is correct in the light of our present knowledge. The chemistry of such polysaccharides as starch, glycogen, cellulose, and inulin may-be obscure in several particulars, hut'it is fairly well established that they are condensation products of monosaccharides, formed from them by the loss of water and resulting in a chain compound. If a chain, then the number of linkages between the monosaccharides or diaaccharides forming the chain must be one less than the units in that chain. If in (C&O6). the value of x is taken as 2, then CleHeoOlo becomes the formula of a disaccharide instead of C12H9POll. The first-mentioned formula is in error by HOB. The same reasoning would apply to any chain compound whether the value of x he 2 or 200. A correct empirical designationwould he represented as fC.H,.03-HOH. . - -- -. In'the generalized equation for the hydrolysis of a polysaccharide as given previously the equation balances. However, there cannot be as many molecules of water involved in the hydrolysis as there are units in the chain. There must he one less. The generalized equation should he as follows:
-
(C6ImOdSHOH
+ (z - 1) H20
-r
xCsH,zOa
The points made in this paper may be interpreted by many as quibbling, and, therefore, not worthy of consideration. But if simplification is necessary, then from the viewpoints of classroom morale and exactness in our sciences it seems worth while that we should be more careful in our generalizations.
