Glycol scission

The fact that periodate fission functions best in aqueous media and LTA in organic solvents makes glycol scission oxidations possible with all types of substrates. These reagents were used extensively for structural elucidation of carbohydrates before the advent of modem spectral instrumentation. Now, they are generally used in synthetic work. 

The glycol scission products of 1,2-O-isopropylidene-a-D-glucofura-nose have been identified by n.m.r. studies. The aldehyde (23) is... 

In addition to uic-diols, other 1,2-dioxygenated groups—2-hydroxyalde-hydes, 1,2-dicarbonyl compounds, a-hydroxy and a-keto acids —and a-amino alcohols " are oxidatively cleaved both by periodate and by lead tetraacetate however, lead tetraacetate oxidizes a-hydroxy acids much more readily than does periodate,and both reagents attack 2-hydroxyaldehydes and 1,2-dicarbonyl compounds relatively slowly. " Mechanistically, these reactions appear to resemble glycol scission. ... 

W. Mackie and A. S. Perlin, Nuclear magnetic resonance spectral observations on the glycol-scission of deuterated D-glucose, Can. J. Chem., 43 (1965) 2645-2651. 

FIGURE 22 Semilog plot of the in vitro rate of hydrolytic chain scission of PCL, poly glycolic acid-co-lactic acid, and a 1 1 blend of the two polymers, demonstrating the use of blends to modify degradation rates. (From Refs. 64 and 65.)... 

The rate of hydrolysis of the partially ethoxylated polymer was retarded, although not to the extent calculated from theory (Fig. 25), suggesting some contribution to the rate of chain scission by an uncatalyzed process. End-capping poly (glycolic acid-co-lactic acid) has a similar effect on the rate of hydrolysis of this polyester (100). 

Blending of PCL and poly(glycolic acid-co-lactic acid) has been also used to control the rate of chain scission of the composite. 

Boron trifluoride in aqueous dioxane was being evaporated with nitric acid treatment (3 portions) for analysis. Final addition of perchloric acid with continued heating led to an explosion while unattended. Ring scission of dioxane to diethylene glycol and formation of diethylene glycol nitrate and/or perchlorate may have been involved. 

Glycols and their ethers undergo violent decomposition in contact with 70% perchloric acid. This seems likely to involve formation of the glycol perchlorate esters (after scission of ethers) which are explosive, those of ethylene glycol... 

At negative potentials EG can compete successfully with glycolate for adsorption sites, unless the EG concentration is too low, and this accounts for the lack of glycolate at the lowest EG concentration. The simultaneous appearance of glycolate and carbonate was taken by the authors as strong evidence for the existence of a common intermediate, identified as A in the scheme. It seems that A may desorb oxidatively or the second carbon may become bound to the surface if a suitable neighbouring adsorption site is available. Once this last process has taken place, carbon carbon bond scission occurs with complete oxidation to CO3". 

C. At higher temperatures main-chain scission occurs, but the pathways still needs to be clarified. Cyclic carbonate may also be formed (e.g., after hydrolysis with residual water), but not necessarily. CO2 and propylene glycol may result as products. Thermally more stable PPC may be processed at temperatures over 250°C as long as exposure time is short (minutes). 

Other additives such as waxes and polyethylene glycols melt at relatively low tem-peratnres and vaporize over a more narrow temperatnre range. Oxidation of the binder canses the temperature to increase more rapidly. For the thermolysis of polyethylene glycol (PEG) in air, decomposition occnrs by the mechanism of chain scission and oxidative degradation. 

Oxidation of the free hexosamines and their A-acetyl derivatives with sodium metaperiodate is not limited to scission of glycol grouping and l-ol, 2-amine groupings further oxidation occurs, the extent of which depends on the concentrations, temperature, and pH of the reactants.200 203 Struc-... 

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