Alkaline elimination reaction, effect

Extraction of hemiceUulose is a complex process that alters or degrades hemiceUulose in some manner (11,138). Alkaline reagents that break hydrogen bonds are the most effective solvents but they de-estetify and initiate -elimination reactions. Polar solvents such as DMSO and dimethylformamide are more specific and are used to extract partiaUy acetylated polymers from milled wood or holoceUulose (11,139). Solvent mixtures of increasing solvent power are employed in a sequential manner (138) and advantage is taken of the different behavior of various alkaUes and alkaline complexes under different experimental conditions of extraction, concentration, and temperature (4,140). Some sequences for these elaborate extraction schemes have been summarized (138,139) and an experimenter should optimize them for the material involved and the desired end product (102). 

Extraction of hemicellulose is a complex process that alters or degrades hemicellulose in sonic manner. Alkaline reagents that break hydrogen bonds arc the most effective solvents hut they de-esterily and initiate /(-elimination reactions. Polar solvents such as DMSO and dimethyllormumidc are more specific and arc used to extract partially acctylated polymers from milled wood or holoeellulnse. 

Many steroid secondary alcohols are resistant to acid-catalysed dehydration, but can be converted into olefins by elimination reactions of their sulphonate esters, usually the toluene 3 -sulphonate ( tosylate ) or methanesulphonate ( mesylate ). These elimination reactions always accompany attempted nucleophilic substitution of the sulphonate (p. 41), and can be made to predominate by appropriate choice of reaction conditions. Activated alkaline alumina has been used to prepare A - and AH-olehns from the sulphonates of 3 - [131] and i2a-alcohols [132]. In the latter case the naphthalene-i-sulphonate was most effective, and the methanesulphonate distinctly less so, suggesting that steric acceleration occurs with the more bulky naphthalenesulphonate [ 132c]. This view is strengthened by the enhanced efficiency of elimination of a i2a-sulphonate in the presence of a 17a-methyl group, which increases the compression at C(i2) [132b]. The mechanism and stereochemical features of these heterogeneous reactions have not been described in detail. 

Thermolysis of sparteine N-oxides occurs readily when an aqueous acidic solution is heated in the presence of an excess of chloride ion. In the case of the N-16-oxide (7), alkaline work-up gives the carbinolamine (8) a study of the effect of the concentration of chloride ion and of the pH of the medium on the reaction rate led to the proposed mechanism (Scheme 2). Sparteine N-1-oxide and a-isosparteine N-1-oxide undergo similar jS-elimination reactions, but the products are enamines. A preliminary investigation of the thermolysis of sparteine epi-N- 16-oxide (9) showed that the carbinolamine (8) was the sole product, formed apparently by a yn-elimination. 

In low-polarity media, specific interaction with protic species (water) dramatically affects the reactivity (nucleophilicity or basicity) of anions with high charge density (OH. F. oxanions. carbanions, etc.). Basicity of OH in the Hofinann elimination reaction of (hexyl)4 N 0H /7H20 (Eq. 9), carried out in a chlorobenzene-water two-phase system, increases 50.000 times by reducing the hydration number n of the anion from 11 to 3. The enhancement is extrapolated to be more than nine powers of 10 for the hypothetical anhydrous hydroxide. This indicates that the largely dehydrated hydroxide, extracted in a low-polarity solvent (chlorobenzene) from concentrated alkaline soluiions. is an extremely powerful base. Results account for the dramatic effect produced by an increase of base on the rate of reactions promoted by alkali hydroxides under LL-PTC conditions, such as carbanion formation and alkylation, alkene isomerization, H/D exchanges in carbon acids, and acid-base equilibria ... 

The first two reactions of the sequence are similar to reactions that occur in acidic medium. The 1,2- and 2,3-enediols, and the unsaturated elimination-products derived from them, are present both in acidic and basic solutions. In general, however, reactions in basic solution are much faster than in acidic solution, because of the greater catalytic effect of the hydroxyl ion on the transformation reactions Mechanistic differences between the media become operative in steps c and d. In acid, further dehydration, if it is possible, occurs rapidly, before equilibrium of the deoxy-enediol with the dicarbonyl compound has been established,17 and the products are furans. In alkaline solution, the rapid formation of the tautomeric dicarbonyl compound permits the benzilic acid rearrangement42 to proceed. 

Two mechanisms (i.e. direct hydrolysis and alternatively a path via an unstable acyl phosphate intermediate) are involved in the hydrolysis in phosphate buffer of N-arylsulfonyl / -lactams such as (130).107 The acyl phosphate intermediate can be trapped with hydrazine. The alkaline hydrolysis of some torsionally distorted lactams, i.e. the bridged benz[rfe]isoquinolin-l-ones (131), in 70% (v/v) DMSO-water has been compared under the same conditions with the hydrolysis of AvA-dimethyl-1 -naphthamide (132). The relative rates of reaction and activation parameters indicate the effect of torsional distortion.108 The reaction of the tricyclic azetidinones (133) with trifluoroacetic acid gives the bicyclic thioesters (135). The mechanism may involve acid-catalysed elimination of methanethiol to give an azetinone intermediate (134) which, after nucleophilic attack of the thiol, is converted into (135).109... 

Electrolytes affect dispersed polysaccharides through water inactivation, specific ion binding, and polyanion neutralization. Each effect is valence-dependent, but is less on neutral polysaccharides than on ionic polysaccharides. Di- and polyvalent cations gel or precipitate a constant amount of polysacchride at much lower concentrations than do monovalent cations. The precipitation reaction is used to advantage in isolating pectin with alkaline Al3+, because this cation and polymeric forms of Al(OH)3 readily precipitate and entrain pectinic acid from apple tissue homogenates. Other di- and polyvalent cation effects are crosslinking (Prud homme et al., 1989) and an increased rate of (3 elimination over monovalent cations (Sajjaanan-... 

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