Relative Reactivity Of Hydroxyl Groups Research Articles

Correction to "Relative Reactivities of Hydroxyl Groups in Carbohydrate Derivatives. Specific Nuclear Magnetic Resonance Spectral Assignments of Acetyl Groups in Methyl α-d-Glucopyranoside Tetraacetate and Related Derivatives".

Correction to "Relative Reactivities of Hydroxyl Groups in Carbohydrate Derivatives. Specific Nuclear Magnetic Resonance Spectral Assignments of Acetyl Groups in Methyl α-d-Glucopyranoside Tetraacetate and Related Derivatives".

Structure and solution properties of cyanoethyl celluloses synthesized in LiOH/urea aqueous solution

Cyanoethyl celluloses (CECs) with different degree of substitution (DS) were synthesized by homogeneous reaction of cellulose (cotton linter pulp and absorbent cotton) with acrylonitrile (AN) in LiOH/urea aqueous solutions. The reaction showed quick reactivity and high transfer efficiency of etherification agent. The DS values of CECs were controlled by varying the molar ratio of AN to anhydroglucose unit (AGU) and the cellulose concentration. The DS values of the CEC-1–CEC-10 increased from 0.27 to 1.78 with increasing molar ratio of AN to AGU from 0.5:1 to 9:1. While the CEC-11–CEC-21 with DS values of 0.26–1.81 could be obtained by adjusting the molar ratio from 1:1 to 27:1. The relative reactivity of hydroxyl groups is in the order of C-6 > C-2 > C-3. The DS values of the water-soluble derivatives are in the range of 0.47–1.01. As the DS values increase to 1.37, CEC samples can not be dissolved in water or dilute alkali solution, but have good solubility in organic solvents, such as DMSO, DMF and pyridine. The dilute solution properties and molecular parameters of the CEC samples were studied by static light scattering and dynamic light scattering. The results indicated that the water-soluble samples could form a small number of aggregates spontaneously in 0.9 wt% NaCl aqueous solution, while the water-insoluble samples showed extended stiff chains in 0.5% LiCl–DMAc.

A facile method for the homogeneous synthesis of cyanoethyl cellulose in NaOH/urea aqueous solutions

A series of cyanoethyl cellulose (CEC) samples were synthesized from cellulose in NaOH/urea aqueous solutions by a homogeneous method. Structure and properties of CECs were characterized with FT-IR, 1H and 13C NMR, DSC, SEC-LLS, polarized light microscopy and solubility measurements. The total DS values of the obtained CECs were in the range of 0.26 to 1.93, and the relative reactivity of hydroxyl groups is in the order C-6 > C-2 > C-3. As the molar ratio of acrylonitrile to anhydroglucose unit (AGU) of cellulose increased to 5 : 1, the C-6 hydroxyl groups of AGU were fully substituted. The total DS value for water-soluble CEC is as low as 0.54. CEC could be dissolved in many organic solvents when its total DS value reached 1.37. Organic-soluble CEC samples displayed thermotropic liquid crystalline behavior in the temperature range of 180 to 280 °C. Moreover, NaOH/urea aqueous solution was proved to be a stable medium for cyanoethylation of cellulose, the reaction products could be easy isolated and purified, and the solvents could be recycled by a simple filtration. Therefore, this work provides a facile method for the homogeneous synthesis of CEC in an aqueous system.

Relative reactivity of hydroxyl groups in inositol derivatives: role of metal ion chelation

O-Alkylation of myo-inositol derivatives containing more than one hydroxyl group via their alkali metal alkoxides (sodium or lithium) preferentially occurs at a hydroxyl group having a vicinal cis-oxygen atom. In general the observed selectivity is relatively higher for lithium alkoxides than for the corresponding sodium alkoxide. The observed regioselectivity is also dependent on other factors such as the solvent and reaction temperature. A perusal of the results presented in this article as well as those available in the literature suggests that chelation of metal ions by inositol derivatives plays a significant role in the observed regioselectivity. Steric factors associated with the axial or equatorial disposition of the reacting hydroxyl groups do not contribute much to the outcome of these O-alkylation reactions. These results could serve as guidelines in planning synthetic strategies involving other carbohydrates and their derivatives.

Methylation-GC-MS analysis of arabinofuranose- and galactofuranose-containing structures: rapid synthesis of partially O-methylated alditol acetate standards

Arabinose and galactose were treated with MeOH containing traces of H2SO4 or HCl at 25ºC to give mixtures of their methyl alpha- and beta-furanosides, as shown by 1D and 2D nuclear magnetic resonance (NMR). Oxidation of the Me alpha,beta-Araf mixture with NaIO4 preferentially oxidised the beta-isomer, to give pure Me alpha-Araf . Each product was progressively O-methylated using the Purdie reagent (MeI/Ag2O) at 25ºC and resulting mixtures of partially methylated glycosides (PMGs) were rapidly assayed by thin layer chromatography (TLC) first to favour higher yields of mono-O-methyl derivatives and later for products with higher degrees of methylation. The products were converted to complex mixtures of partially O-methylated alditol acetate derivatives (PMAAs) by successive hydrolysis, reduction with NaBD4, and acetylation. These can be used as gas chromatography-mass spectrometry (GC-MS) standards in methylation analysis of complex carbohydrates containing arabinofuranosyl and galactofuranosyl units. Of particular interest were the retention times and electron impact MS of the difficult to prepare alditol acetates of 5,6-Me2Gal, 2,5-Me2Gal, 2,5,6-Me3Gal, 3,5,6-Me3Gal, 5-MeAra, 2,5-Me2Ara, and 3,5-Me2Ara. The relative reactivities of hydroxyl groups for mixtures of Me alpha- and Me beta-Galf were HO-2 > HO-3 > HO-6 > HO-5, that of Me alpha and Me beta-Araf HO-2 > HO-3 > HO-5, and that of Me alpha-Araf HO-2 > HO-3 > HO-5.

Determination of substituents distribution in carboxymethylpullulans by NMR spectroscopy

The distribution of carboxymethyl substituents in the α-(1→6)-linked maltotriosyl repeating units of a carboxymethylpullulan (CMP) series was investigated by high resolution NMR spectroscopy on very short oligomers ( DP n =1.2–1.5) obtained by acid hydrolysis. A series of 2D NMR experiments on parent pullulan, hydrolysed pullulan and CMP was used to assign the proton and carbon chemical shifts of CMP acid hydrolysates. The degree of substitution (DS) and the relative distribution of CH 2COONa groups at OH-2, OH-3, OH-4 and OH-6 of glucose residues (DS i) were determined from 1H NMR measurements. From a set of CMP samples, widely different in degree of substitution, it was observed that the substitution at C-2 is predominant and decreases according to the order C-2>C-3>C-6>C-4. Taking into account the availability of each OH group in the parent pullulan, an order of relative reactivity of hydroxyl groups is defined according to the relation: R i=DS i/ n i, where n i is the number of free OH groups in a maltotriose unit (MTU) for a given site C-i, the reactivity order was found to be OH-2>OH-4>OH-6>OH-3.

Reaction characteristics of cellulose in the LiCl/1,3‐dimethyl‐2‐imidazolidinone solvent system

In order to elucidate the nature of the LiCl/1,3‐dimethy‐2‐imidazolidinone (DMI) solvent system as one of the homogeneous reaction media of cellulose, cellulose acetate (CA) and O‐methylcellulose (MC) were prepared using this solvent system, and the distribution of substituents within anhydroglucose units was examined by 13C‐NMR. It was found that (i) homogeneous cellulose solutions can be easily prepared by heating 2, 5–12 and 100 parts of weight of cellulose, LiCl, and DMI, respectively, and (ii) the relative reactivity of hydroxyl groups is in the order C‐6 > C‐2 > C‐3 for both CA and MC. A remarkable feature of this solvent system is that the reaction efficiency in etherification is very high compared with other homogeneous solvent systems.

13C‐NMR spectral studies on the distribution of substituents in some cellulose derivatives

Abstract13C‐NMR spectra of trityl cellulose (Tr‐Cell), tosyl cellulose (Ts‐Cell), cellulose S‐methyl xanthate (Cell‐M‐Xan), and cellulose formate (CF) in dimethylsulfoxide‐d6 were analyzed at 50.4 MHz. It was found that the distribution of substituents in the anhydroglucose units of these cellulose derivatives can be estimated from their ring carbon spectra. The results showed that (i) in Tr‐Cell having degree of substitution (DS) lower than 1, the hydroxyl groups at C‐6 carbon position are selectively tritylated, (ii) in the case of Ts‐Cell, the difference in the relative DS value among three different types of hydroxyl groups is not large, although the relative reactivities of hydroxyl groups toward tosylation decrease in the order C‐6 > C‐2 > C‐3, (iii) in Cell‐M‐Xan, the hydroxyl groups at C‐3 carbon position are mainly substituted, and (iv) the ease of formylation is C‐6 > C‐2 > C‐3. The 100.8 MHz 13C‐NMR spectra of O‐methyl cellulose (MC) revealed that the reactivity order in commercial MC prepared from alkali cellulose is C‐6 ⋍ C‐2 > C‐3. Concerning MC, its water solubility was also discussed in terms of the distribution of substituents along the cellulose chain.

Heterogeneous ethoxylation of cellulose: influence of alkali and available-water concentrations on substituent distributions

O-(2-Hydroxyethyl)cellulose ( 1) as formed by the alkali-catalyzed addition of ethylene oxide to cellulose flock in a slurry process is not uniformly substituted. Most of the ethylene oxide adds to HO-6 in a chain-fashion, so that ∼20% of the d-glucose residues remain totally unsubstituted at 2.0 molar substitution. Consequently, an aqueous solution thickened by 1 is highly susceptible to enzymic degradation. Stepwise decrease in concentration of alkali during etherification gives improved stability to enzymic degradation associated with a more-uniform distribution of hydroxyethyl groups between the three hydroxyl groups of the glucose residues in cellulose. The relative reactivities of hydroxyl groups and patterns of substitution were established by matching the distribution patterns from a stochastic computer-model with the distribution of substituents as determined by chemical analyses [namely hydrolysis with sulfuric acid to determine the percentage of unsubstituted glucose residues (u- 2) and with periodate oxidation for determining the percentage of unsubstituted 2,3-vicinal diol groups per residue]. The reactivities of the three hydroxyl groups at various alkali concentrations in a heterogeneous, slurry-addition process approximate those observed under homogeneous conditions, wherein the reactivities have been determined by tedious chromatographic analyses. In the variable-alkali procedure for addition of ethylene oxide, the amount of water available to the cellulose matrix in the low-alkali ( m) sequence is important both for the stability to enzymic degradation and for obtaining gel-free, thickened, aqueous solutions. Optimal stabilities and gel-free solutions are observed at intermediate water: cellulose ratios of 1.10–1.23. At a ratio of 1.23, the stability to enzymic degradation is less sensitive to percentage variations of u- 2 than in 1 prepared at higher or lower water: cellulose ratios. Although the initial degree of degradation between 1 of high molar substitution prepared at 6.8 m alkali concentration and a similar product prepared under variable alkali conditions may be related to percentage differences of u- 2, the rate and final degree of degradation do not relate to percentage differences of u- 2. An adequate interpretation, utilizing known cellulase turnover-rates, is found in stochastic-model projections of the distribution of consecutive 2 residues not substituted at HO-2. The results indicate that ( 1) more-uniform substitution through equalization of hydroxyl reactivities is achieved by lowering the alkali concentration, and ( 2) more-uniform substitution of 2 of the many cellulose-chains being substituted is achieved by employing an optimal amount of “available” water during etherification.

Crosslinking statistics, 3. Relation between relative reactivity and accessibility of cellulose hydroxyl groups

AbstractFrom the results of a structural analysis of cellulose crosslinked by a bifunctional reagent conclusions can be drawn merely on the distribution of crosslinks among individual hydroxyl groups. No information is available to what extent this distribution is caused either by different relative reactivities of the hydroxyl groups or incomplete accessibility of glucose units. If each glucose unit had the same chance of reaction, at low levels of conversion only monosubstituted units should be present in the reaction product. If, however, some higher substituted units are present, this means that at a certain level of conversion there must have been a smaller number of units capable of reaction to which relatively more reagent had access. The relation between relative reactivity and accessibility of cellulose hydroxyl groups obeys certain statistical laws of random selection. A theory explaining these laws is developed in the present work and supported by good agreement with experimental results exemplified by microcrystalline cellulose crosslinked by epichlorohydrin (1‐chloro‐2,3‐epoxypropane). The relative reactivities of cellulose hydroxyls were found to increase in the order OH‐3, OH‐6, OH‐2 in the crosslinking reaction with epichlorohydrin.

Relative reactivities of hydroxyl groups in carbohydrate derivatives. Specific NMR spectral assignments of acetyl groups in methyl tetra-O-acetyl-.alpha.-D-glucopyranoside and related derivatives

Relative reactivities of hydroxyl groups in carbohydrate derivatives. Specific NMR spectral assignments of acetyl groups in methyl tetra-O-acetyl-.alpha.-D-glucopyranoside and related derivatives

Relative Reactivities of Hydroxyl Groups of Carbohydrates

Publisher Summary This chapter discusses the relative reactivities of hydroxyl groups in carbohydrates, which is of considerable value in the field of syntheses. The selective reactivity of hydroxyl groups of carbohydrates in many instances seems to be a consequence of spatial arrangement of the groups. This is especially indicated in those reactions, such as acyl migration, where the postulation of a cyclic intermediate seems to explain the course of the reaction. Neighboring groups in general may have a very pronounced effect on the reactivity of a given hydroxyl group. The enhanced reactivity of the 2-hydroxyl as compared to other secondary hydroxyl groups has been noted primarily in reactions requiring alkaline media. This effect is probably a result of the greater acidity of hydroxyl groups adjacent to a carbonyl or a potential carbonyl group. The greater reactivity of the primary and the 2-hydroxyl group over those of the other secondary alcohol groups is strongly indicated.