Acidity Protic solvents

In the case of Lewis acids, protic solvents such as water or alcohol can strongly influence their reactivity, cause it to react via an alternative path to the one desired, or even cause decomposition. Recently, rare earth metal triflates were used to develop water tolerant Lewis acids that can be used in many organic reactions. ... 

The reactions in a non-basic aprotic solvent CH2C12 provided solely 10, the product of carbon protonation, while those carried out in an acidic protic solvent HFIP give exclusively 8K, the product of oxygen protonation. The equilibrium protonation may be favored in a protic solvent having abundant protons available. In other basic solvents, the proton donor involved in the reaction should be the conjugate acid of the solvent, and many factors may delicately control the selectivity of the reaction. 

Tris(l-pyrazolyl)methaneis a white, crystalline compound which is soluble in most organic solvents, but sparingly soluble in hydrocarbons. The ligand is stable in basic and neutral solutions. In acidic protic solvents, decomposition to formic acid and pyrazole occurs readily. The H NMR (80 MHz, CDC13) exhibits signals at 8 6.35 (d of d, 3H, J = 2.5,1.8 Hz), 7.57 (d of d, 3H, J = 2.4,... 

Strong bases Extremely corrosive React violently with acids, protic solvents... 

There are cases when the cobalt dioxygen complexes are only intermediates in the irreversible oxidation of the cobalt(II) complex en route to the cobalt(III) derivatives. The situation is largely dependent on the ligands, solvent and acidity. Protic solvents promote irreversible metal-centered oxidation, which may involve some complex pathways. It is usually the p-peroxodicobalt species which is regarded as the precursor of the cobalt(III) product. The reduction product from 0 may be either or H O. 

A series of aryl chloroformates 69, which are very unstable at room temperature, being readily decomposed by amines, acids, protic solvents, and atmospheric moisture, could be prepared with diphosgene and triphosgene in the presence of ethyl diisopropylamine (Table 4.3) [44]. 

In a Lewis-acid catalysed Diels-Alder reaction, the first step is coordination of the catalyst to a Lewis-basic site of the reactant. In a typical catalysed Diels-Alder reaction, the carbonyl oxygen of the dienophile coordinates to the Lewis acid. The most common solvents for these processes are inert apolar liquids such as dichloromethane or benzene. Protic solvents, and water in particular, are avoided because of their strong interactions wifti the catalyst and the reacting system. Interestingly, for other catalysed reactions such as hydroformylations the same solvents do not give problems. This paradox is a result of the difference in hardness of the reactants and the catalyst involved... 

Solvent Effects on the Rate of Substitution by the S 2 Mechanism Polar solvents are required m typical bimolecular substitutions because ionic substances such as the sodium and potassium salts cited earlier m Table 8 1 are not sufficiently soluble m nonpolar solvents to give a high enough concentration of the nucleophile to allow the reaction to occur at a rapid rate Other than the requirement that the solvent be polar enough to dis solve ionic compounds however the effect of solvent polarity on the rate of 8 2 reactions IS small What is most important is whether or not the polar solvent is protic or aprotic Water (HOH) alcohols (ROH) and carboxylic acids (RCO2H) are classified as polar protic solvents they all have OH groups that allow them to form hydrogen bonds... 

The zwitterion (6) can react with protic solvents to produce a variety of products. Reaction with water yields a transient hydroperoxy alcohol (10) that can dehydrate to a carboxyUc acid or spHt out H2O2 to form a carbonyl compound (aldehyde or ketone, R2CO). In alcohoHc media, the product is an isolable hydroperoxy ether (11) that can be hydrolyzed or reduced (with (CH O) or (CH2)2S) to a carbonyl compound. Reductive amination of (11) over Raney nickel produces amides and amines (64). Reaction of the zwitterion with a carboxyUc acid to form a hydroperoxy ester (12) is commercially important because it can be oxidized to other acids, RCOOH and R COOH. Reaction of zwitterion with HCN produces a-hydroxy nitriles that can be hydrolyzed to a-hydroxy carboxyUc acids. Carboxylates are obtained with H2O2/OH (65). The zwitterion can be reduced during the course of the reaction by tetracyanoethylene to produce its epoxide (66). 

Thickeners. Thickeners are added to remover formulas to increase the viscosity which allows the remover to cling to vertical surfaces. Natural and synthetic polymers are used as thickeners. They are generally dispersed and then caused to swell by the addition of a protic solvent or by adjusting the pH of the remover. When the polymer swells, it causes the viscosity of the mixture to increase. Viscosity is controlled by the amount of thickener added. Common thickeners used in organic removers include hydroxypropylmethylceUulose [9004-65-3], hydroxypropylceUulose [9004-64-2], hydroxyethyl cellulose, and poly(acryHc acid) [9003-01-4]. Thickeners used in aqueous removers include acryHc polymers and latex-type polymers. Some thickeners are not stable in very acidic or very basic environments, so careful selection is important. 

Most ozonolysis reaction products are postulated to form by the reaction of the 1,3-zwitterion with the extmded carbonyl compound in a 1,3-dipolar cycloaddition reaction to produce stable 1,2,4-trioxanes (ozonides) (17) as shown with itself (dimerization) to form cycHc diperoxides (4) or with protic solvents, such as alcohols, carboxyUc acids, etc, to form a-substituted alkyl hydroperoxides. The latter can form other peroxidic products, depending on reactants, reaction conditions, and solvent. 

The predominant, if not exclusive, formation of 5/7-fused hydroxy ketones was observed in the case of 4-alkylated dienones [(204) (205) (R = CH3) 6 1 from (201) (R = CH3)] ° and of prednisone 21-acetate [(206)-> (207)]. It appears therefore likely that intermediates which represent the conjugate acids of the postulated zwitterionic intermediates in the dienone photoisomerizations [c/. (202), (203)] participate both in the acid-catalyzed transformations of (200) and in the dienone photochemistry in protic solvents. 

Water (HOH), alcohols (ROH), and carboxylic acids (RC02FI) are classified as polar protic solvents they all have OH groups that allow them to for-rn hydrogen bonds... 

The reaction is generally performed between 0 and 100 °C with the majority of the reactions being mn at reflux. Polar protic solvents such as methanol, ethanol, isopropanol, and water are commonly used as solvents. Addition of acid or use of acetic acid as solvent generally helps push sluggish reactions. The use of P-ketoesters as the dicarbonyl partner occasionally requires added base for cyclization to occur to form the pyrazolone. When using alkyl hydrazine salts, base may be required to deprotonate the hydrazine for the reaction to take place. 

The ultraviolet absorption spectra of the anhydro-bases in acid solution or in protic solvents are those of the 3,4-dihydro-)3-carbolinium ion (Ajnax 355 mp, for 438b and 438c). In alkaline solution and in nonionizing solvents absorption at a shorter wavelength (A ax 315 m/x) is observed. In general, solutions of the anhydro-bases in acid and in protic solvents are more deeply colored than their solutions in basic or in non-ionizing media. 

Hydrogen bonding with protic solvents or reagents occurs widely in azines even when they are not appreciably basic and the protic compounds are very poor acids. The latter do not have to be present... 

Hydroxy-l-alkenyl diisopropylcarbamates 2 (X = OCb), in this respect, occupy a medium position since they are stable in strongly acidic and basic protic solvents. For deblocking vinyl carbamates, the presence of catalytic amounts of mercuric or palladium(II) salts is required. Due to this stability, several reactions of homoallylic alcohols, proceeding with high diastereo-selectivity, e g., epoxidation, are applicable in order to introduce further hetero-substituents. 

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