Using strong acids

In the presence of strong acids, most alcohols are dehydrated to give the thermodynamically more stable olefin. It is not safe to assume that such a strict adherence to the Say tzeff rule is indicative of a carbonium ion mechanism, as under the reaction conditions the less stable olefins are rapidly isomerised. Such thermodynamic control greatly restricts the synthetic utility of dehydra- 

A number of different types of kinetic evidence have been used to show that the dehydration reactions proceed via carbonium-ion intermediates in an El process. Simple rate studies show that the order of reactivity decreases along the series tertiary alcohol secondary primary, the order of decreasing carbonium-ion stability. Skeletal rearrangements typical of carbonium ions have also been observed On a number of occasions, the rate of olefin formation has been shown to be a slower process than the exchange of the hydroxyl function with the reaction medium -. This can conveniently be demonstrated by studying the reaction in water enriched with OH2 or, in the case of an optically active alcohol, by comparing the rates of racemisation and dehydration - . These observations indicate that exchange occurs before (184), and not simultaneously with, elimination. 

For tertiary alcohols, oxygen exchange occurs via the loosely solvated (encumbered) carbonium ion (A 2) whereas for the less stable primary carbonium ions, exchange most plausibly occurs via a symmetrically solvated carbonium ion (185) or a direct displacement reaction (186), viz. 

The nature of exchange mechanism with secondary alcohols will depend on the stability of the intermediate carbonium ion and possibly alcohols possessing an alpha-phenyl substituent simulate the reaction mode of tertiary alcohols, whereas the simple secondary alkyl alcohols possibly utilise a mechanism similar to that of the primary substrates. The extensive kinetic investigations of Manassen and Klein on the dehydration of secondary butanol have been interpreted in terms of carbonium-ion intermediates which are symmetrically solvated. Supporting evidence for this idea is provided by the earlier studies on the rate of exchange of 0 in optically active secondary butanol Race- 

Whether such arguments are justified is debatable, as racemisation/ eiimmation varies widely for similar structures, being 58 for 1,2-diphenylethanol but exceeding 100 for 1-phenylethanoF . For optically active 1-phenylethanol the ratio, A exchange/ raceinisati n. in aqueous sulphuric acid, was 0.82. This result indicates that exchange occurs at a partially shielded carbonium ion or alternatively by a simultaneous mixture of unimolecular and bimolecular mechanisms .  

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Acid catalysis using strong acid catalysts, especially 2eohtes which enhance selectivity because of pore si2e restrictions, has been used for a variety of alkenes and dienes (9—11). /-Butyltoluenediamine [106398-83-8] (/-BTDA) (C H gN2) is available on a semicommercial basis (12). 

Cyclizations, which failed to occur under thermal conditions, have been forced by using strong acids as solvent. Such cyclizations required careful temperature control in order to cyclize while maintaining the 3-carboxyl substituent. ... 

The Nef reacdon was originally carried out rmder acidic condidons using strong acid such as aqueous HCl.However, the use of base followed by acid is incompadble v/ithpolyfuncdoniil substrates in addidon, some componnds are prone to undergo side reacdons or fail to react, as discussed in the references. Thus, various modified methods have been devebped, and they... 

Protonation of an amide using strong acid occurs on oxygen rather than on nitrogen. Suggest a reason for this behavior, taking resonance into account. 

This material is acetylated with 35 ml. of pyridine and 35 ml. of acetic anhydride for 6 hours at room temperature. The mixture is then poured onto 250 g. of ice and extracted with five 75-ml. portions of ether. The combined ether extracts are washed successively with three 10-ml. portions each of saturated aqueous sodium bicarbonate and water and then dried over magnesium sulfate. It is important not to use strong acids such as hydrochloric or sulfuric acid to remove pyridine as was done in Part A, since they can destroy the acid-sensitive product. 

Benzothiophenes have always been of interest for medicinal chemistry and can be found in a number of marketed drugs such as Sertaconazole (Gineder-mofix), Zileuton (Leutrol) and Raloxifene (Evista). The classical synthesis of benzo thiophenes starts from thiophenols, reacting with bromoacetaldehyde dimethyl acetal, followed by cycUzation using strong acid. An alternative and more convenient route was also described starting from benzaldehydes which... 

Addition of nucleophilic solvents such as alcohols and carboxylic acids can be effected by using strong acids as catalysts.10... 

Vanadium pentoxide (V Oj) is a reddish-yellow powder extracted from minerals using strong acids or alkalies. In addition to being used as a catalyst for many organic chemical reactions, it is used in photography and in UV-protected windowpanes and to color ceramics and dye cloth. 

However, this method possesses several disadvantages such as long reaction time and complicated work-up procedure. For example, in the case of Sc(OTf)3-catalyzed reaction, the treatment required 72 h to get completed at the ambient temperature. After that a pure product was isolated from the reaction mixture by the capture of the solid phase by using strongly acidic cation exchange resin, followed by washing of the solvent and final treatment of resin with 2 M methanolic ammonia. 

Classical methods use strongly acidic medium. Oximes may be converted to their ester or ether derivatives and these may be used to promote the rearrangement (less vigorous reaction conditions are usually needed). Some other milder reaction conditions and reagents may be used (Section VI.C). 

Classically, rearrangement reactions have been performed using strong acidic reaction conditions. Several Brpnsted and Lewis acids have been used over the years to promote a variety of chemical transformations that afforded several new structures that revealed potentially interesting bioactivities. Thus, considering the acid character of Bi(III) salts and their environmentally friendly nature when compared to other previously used Brens ted and Lewis acids, the study of their application in the referred rearrangements is an interesting subject. 

The influence of strong Lewis acids, such as aluminum trichloride and boron trichloride, on similar reactions has been studied but it was found that they did not have a significant role.50 However, it is reported that using strong acidic reaction media is very helpful and the fluorination proceeds very favorably. 4-Nitroanisole and 4-fluorobenzoic acid arc thus converted into 2-fluoro-4-nitroanisole (6) and 3,4-difluorobenzoic acid (7), respectively, in good yields.51... 

In order to metallize a polymer surface, electroless plating can be applied. This process typically consists of a pretreatment process in order to improve the adhesion. In the second step a surface seeding of the electroless catalyst is done. Wet chemical methods of pretreatment are using strong acids such as chromic acid, sulfuric acid and acidified potassium permanganate in order to achieve a surface modification of the polymers (96). 

There are several reactions of coordinated methyleneamido ligands cited in the literature. The methylineamido ligands can be displaced by carbon monoxide or by protolysis using strong acids (equations 102 and 103). 

Sodium hypochlorite, NaCIO, is the active ingredient in many bleaches. Calculate the ratio of the concentrations of CIO and HCIO in a bleach solution having a pH adjusted to 6.50 by using strong acid or strong base. 

Downstream processing may consist of several operations such as liming to precipitate the metabolite as the calcium salt, washing of the precipitate with water to remove soluble impurities, acidification using strong acids to convert the salt in its free acid form. The acidic liquor may be demineralized using IER, decolorized using active carbons, concentrated under vacuum, and finally crystallized. 

High-performance liquid chromatography (HPLC) is a well-established separation technique it is able to solve numerous analytical problems and there is the possibility of acting on the mobile phases with appropriate additives to improve the quality of the peak. Of course, any additive must be compatible with the MS detector nonvolatile buffer or eluent additives cannot be used strong acids such as trifhioroacetic acid (TFA) may cause significant signal suppression in positive ionization. Different stationary phases are used as an alternative to the classical C18 Phenyl, HILIC, fluorinated, etc. 

The proposition of a one-electron mechanism for the electron-transfer reduction of dioxygen and the associated conclusions present significant ramifications relative to the development of improved fuel cells and metal-air batteries. To date the practical forms of such systems have used strongly acidic or basic electrolytes. Such solution conditions normally cause atom transfer to be the dominant reduction process for molecular oxygen at metal electrodes. Hence, the search for effective catalytic materials should be in this context rather than in terms of a one-electron-transfer process. 

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