ESTIMATION OF CARBOXYL, ALDEHYDE, AND KETONE GROUPS IN CHROMIUM TRIOXIDE OXYSTARCHES

Abstract

Under identical conditions, corn starch amylose was more readily oxidized than the amylopectin by chromium trioxide dissolved in acetic acid – acetic anhydride, which was a nonswelling system. These differences in rate nearly vanished for samples dried through solvent exchange when the oxidant was dissolved in a swelling medium, 0.2 M aqueous sulphuric acid. The absolute rate of oxidation, however, was greatly reduced.Carboxyl groups in the oxystarches were satisfactorily estimated either by ion-exchange with calcium acetate or sodium bromide solutions, or by direct titration to about pH 8.5 with aqueous alkali. The samples retained chromium compounds which grossly interfered with the determination of total carbonyl groups by condensation with hydroxylamine hydrochloride; condensation with excess sodium cyanide, and estimation of the ammonia from the hydrolysis of the cyanohydrins, gave better results.Aldehyde groups in an oxystarch containing 0.16 M. of carboxyl and 0.14 M. of carbonyl groups were selectively oxidized with chlorous acid or alkaline hypoiodite, or were selectively condensed with sodium bisulphite solution. All three estimations indicated that about one-third of the carbonyl groups were aldehydes probably occupying the sixth positions in the glucose residues. The cyanohydrin of the oxystarch, when saponified and then hydrolyzed and reduced with boiling hydriodic acid, yielded the lactone of 2-methyl-4-hydroxyhexanoic acid, the recovery of which showed that at least 17% of the carbonyl groups occurred as 2-ketoglucose residues.