November 13, 1943
Dr. G. V. Caesar
Stein-Hall and Company
285 Madison Avenue
New York, New York
Dear Dr. Caesar:
I thank you for your chart of the nitration of cellulose by nitrogen pentoxide, which I return herewith.
I do not think that the linear relation between logarithm of percent nitrogen and logarithm of nitration time has any significance. I would interpret the data in the following way. If each hydroxyl group of the cellulose were to be nitrated independently of the other, the percent of nitrogen would approach the limiting value 14.14 according to the exponential function derived from the first order reaction rate theory; that is, it would reach the halfway point, 7.07 percent, in a given time, the three-quarters point in twice that time, the seven-eighths point in three times that time, and so on. Your data indicate that the reaction deviates from this by slowing down with the passage of time. This presumably means that as some hydroxyl groups are nitrated the remaining ones become more difficult to nitrate.
A possible explanation of the practical independence of the rate of nitration of the N2O5 concentration is that a relatively small amount of N2O5 is needed to form complexes with the hydroxyl groups, and that the reaction of nitration is then essentially a unimolecular rearrangement within these complexes; under these circumstances the rate of the reaction would be independent of the N2O5 concentration so long as enough N2O5 were present to keep nearly all of the hydroxyl group combined.
Sincerely yours,
Linus Pauling
LP:jr
November
13, 1943
