Hydrogen ascorbic acid

Sorbitol is manufactured by the reduction of glucose in aqueous solution using hydrogen with a nickel catalyst. It is used in the manufacture of ascorbic acid (vitamin C), various surface active agents, foodstuffs, pharmaceuticals, cosmetics, dentifrices, adhesives, polyurethane foams, etc. 

Chemical Properties. The most significant chemical property of L-ascorbic acid is its reversible oxidation to dehydro-L-ascorbic acid. Dehydro-L-ascorbic acid has been prepared by uv irradiation and by oxidation with air and charcoal, halogens, ferric chloride, hydrogen peroxide, 2,6-dichlorophenolindophenol, neutral potassium permanganate, selenium oxide, and many other compounds. Dehydro-L-ascorbic acid has been reduced to L-ascorbic acid by hydrogen iodide, hydrogen sulfide, 1,4-dithiothreitol (l,4-dimercapto-2,3-butanediol), and the like (33). 

Chemical Synthesis. The first synthesis of ascorbic acid was reported ia 1933 by Reichsteia and co-workers (14,39—42) (Fig. 4). Similar, iadependent reports pubHshed by Haworth and co-workers followed shordy after this work (13,43—45). L-Xylose (16) was converted by way of its osazone (17) iato L-xylosone (18), which reacted with hydrogen cyanide forming L-xylonitfile (19). L-Xylonitfile cyclized under mild conditions to the cycloimine of L-ascorbic acid. Hydrolysis of the cycloimine yielded L-ascorbic acid. The yield for the conversion of L-xylosone to L-ascorbic acid was ca 40%. 

Most current industrial vitamin C production is based on the efficient second synthesis developed by Reichstein and Grbssner in 1934 (15). Various attempts to develop a superior, more economical L-ascorbic acid process have been reported since 1934. These approaches, which have met with htde success, ate summarized in Crawford s comprehensive review (46). Currently, all chemical syntheses of vitamin C involve modifications of the Reichstein and Grbssner approach (Fig. 5). In the first step, D-glucose (4) is catalytically (Ni-catalyst) hydrogenated to D-sorbitol (20). Oxidation to L-sotbose (21) occurs microhiologicaRy with The isolated L-sotbose is reacted with acetone and sulfuric acid to yield 2,3 4,6 diacetone-L-sorbose,... 

The cleavage of the intermediate osmate ester has presented problems in the past, and a variety of procedures have been developed, including the use of mannitol and strong aqueous base, refluxing aqueous alcoholic sodium bisulfite,formaldehyde and ascorbic acid. A much milder method involves simply bubbling hydrogen sulfide into a solution of the osmate ester osmium dioxide is precipitated rapidly, and the desired organic... 

Examine atomic charges as well as the electrostatic potential map of ascorbic acid. Which hydrogen(s) is likely to be most acidic ... 

Obtain the energies of the various conjugate bases of ascorbic acid. Which one is the most stable Is it the base which results from deprotonation of the hydrogen you previously assigned as the most acidic ... 

In a study of the photoreduction of zinc(II) or magnesium chlorophyll derivatives 8 with ascorbic acid in the presence of l,4-diazabicyclo[2.2.2]octane (DABC-O), a 2,3-m-hydrogenated isobacteriochlorin 9 is formed which subsequently rearranges to a 3-ethylidenc derivative... 

FIGURE 4-18 Permselective coatings flow injection response of a poly(l,2-diaminoben-zene)-coated electrode to the following a, hydrogen peroxide (1 mM) b, ascorbic acid (1 mM) c, uric acid (1 mM) d, L-cysteine (1 mM) and e, control human serum. (Reproduced with permission from reference 63.)... 

The acidic reagents vary widely In their ability to lower NDPA levels In trifluralin The concentration of the acid Is critical to produce the desired effect In some Instances, the acid promoted additional nitrosamine formation, e g 10% hydrochloric acid, 40% phosphoric acid, ascorbic acid, etc Hydrochloric acid and hydrogen chloride gas were the most efficient at destroying NDPA Impurity ... 

The presence of o-qulnone surface waves seems, at the present time, to be coincidental to activation particularly In the case of ascorbic acid oxidation. On the other hand. Its presence may serve as a criterion of cleanliness and activation. Thus, the surface waves at 0.250 and 0.190 are Indicators or signatures for active GCE electrodes and should be used as diagnostic for a clean GCE surface as Is the hydrogen fine structure for platinum (31). It Is unfortunate that the o-qulnone peaks do not appear to be proportional to the surface area as Is the platinum fine structure. 

In chemically-cured materials, one example of an initiator/activator system is hydrogen peroxide as initiator, ascorbic acid as activator and cupric sulphate as co-activator. In light-cured materials, camphorquinone is used as a visible-light photochemical initiator, sodium p-toluene-sulphinate as activator and ethyl 4-dimethylaminobenzoate as photoaccelerator. 

The human lens is rich in ascorbate, which is required for normal collagen synthesis and acts as a water-soluble antioxidant, reacting rapidly with superoxide, hydroxyl and peroxyl radicals. However, ascorbic acid can undergo auto-oxidation and, at certain concentrations, can form hydroxyl radicals with hydrogen peroxide in the presence of light and riboflavin as described above (Delaye and Tardieu, 1983 Ueno et al., 1987). 

This is by far the most important reaction of tetrazolium salts and accounts for the bulk of their many applications. A large variety of reagents can reduce tetrazolium salts, e.g., 53 to formazans, e.g., 51. Ascorbic acid, hydrazine, and hydroxylamine have been recommended for the preparation of formazans from tetrazolium salts.245 Stronger reducing agents such as ammonium sulfide, sodium amalgam, sodium dithionite, and catalytic hydrogenation can further reduce the formazans to the amidrazones, e.g.,... 

---