As early as 1932, Hurd and Isenhour' employed the results of experiments involving glycerol, 1,4-butanediol, and 1,2,4-butanetriol to gain insight into the mechanism of furfural formation from pentoses. They detected no products from glycerol after refluxing for 2 h in 3.9 N aq. HCl, thereby demonstrating the acid-catalyzed dehydration mechanism to be very slow at temperatures near 100". On the other hand, after heating a solution of 1,4-butandiol in sulfuric acid for 2 h at 65570", the same authors obtained a 76% yield of tetrahydrofuran.
Similarly, they noted that 1,2,4butanetriol may be converted into 3-hydroxytetrahydrofuran on boiling with water. Since that time there have been extensive studies of anhydride formation from aldi-tols3,4, however all such studies have generally utilized strong acids and high temperatures. These results clearly establish and alerted us to the potential role of acidcatalyzed, intramolecular nucleophilic substitution (etherification) mechanisms in the dehydration of aldoses and ketoses. in subsequent papers we shall elaborate upon the involvement of such reactions in the formation of furfural and 5-(hydroxymethyl)-2furaldehyde (HMF) from aldoses.
Recently Ramayya et al5 reported experimental investigations of the acid-catalyzed dehydration of ethanol, 1 -propanol, and glycerol in liquid water at 34.5 MPa and temperatures ranging from 320" to 400". After 20 s, in the presence of l-30 mM sulfuric acid, essentially 100% relative molar yields of ethylene were obtained from 0.5~ (or less) ethanol. Similar results were obtained from I-propanol. Clearly, the high temperatures involved in these experiments facilitated the acid-catalyzed elimination reaction, leading to the selective dehydration of the alcohol. When the concentration of the alcohol reactant exceeded M, the appropriate ether was detected in the reaction products. Thus, *Part 2 of the series Kinetic studies of the reactions of ketoses and aldoses in water at high temperature.
For part 1, see ref. 1.