Alcohols in strong acid

TTie solvolysis of propargylic substrates (199) and formation of alkynylcarbonium ions (200) has been extensively investigated. Particularly good evidence for the formation of alkynylcarbonium ions comes from the nuclear magnetic resonance spectra of alkynyl alcohols in strong acid media (200, 201). The downfield shifts of 4ppm for the proton of HC=C— and 1 ppm for CH3C=C- relative to their neutral precursors is indicative of carbonium-ion formation and shows the importance of the allenyl resonance contribution. 

Amides from nitriles and alcohols in strong acids. General scheme ... 

Reactions with secondary or tertiary alcohols in strongly acidic media yield amides (Ritter reaction) ... 

The Sn1-E1 behavior of tertiary alcohols in strong acids can be used to advantage in the preparation of tert-butyl ethers. If, for example, a mixture of tert-butyl alcohol and methanol is heated in the presence of sulfuric acid, the tertiary alcohol reacts rapidly but reversibly to produce 2-methylpropene by... 

Fluorescence lifetimes have been determined for several 9-arylxanthyl cations generated Irom their corresponding alcohol in strongly acidic media, or as their tetrafluoroborate salt in acetonitrile (Table 2) [11,28,30]. Excellent agreement between the two methods was found for the parent 9-phenyl-, p-Me-, and m-Me-substituted cations, with poorer agreement in the case of the tn-OMe cation. The error associated with the very weak fluorescence of the m-OMe-substituted cation may be responsible for the difference in the lifetime estimates. The two values reported for the fluorescence lifetime of the p-F xanthyl cation show the widest variation, with a report of 18.8 ns for the cation prepared as its tetrafluoroborate salt in acetonitrile versus 47 ns in TFA-TFE [13,28,30]. 

In another approach, Beletskaya reported the aerobic oxidation of benzyl alcohol to benzaldehyde using catalytic NH NOj in trifluoroacetic acid (TFA) (Scheme 15.2a) [11]. This was based on previous reports on the stoichiometric NO -mediated oxidation of benzyl alcohol in strongly acidic solutions [12]. In 1994, Levina disclosed a system for aerobic aliphatic alcohol oxidation using catalytic NaNOj in perchloric acid (Scheme 15.2b) [13]. 

Symmetrical ethers can be prepared through dehydration of an alcohol in strong acid. 

The amphoteric nature of the hydroxy functional group characterizes the chemical reactivity of alcohols. In strong acids, they exist as alkyloxonium ions, in neutral media as alcohols, and in strong bases as alkoxides. 

Fuithennore, a substance such as HCl that dissociates completely when dissolved in water, also dissociates completely when dissolved in an alcohol. Many important reactions of alcohols involve strong acids either as reactants or as catalysts. In all these reactions the first step is fonnation of an alkyloxonium ion by proton transfer from the acid to the alcohol. 

The differenee in reaction rates of the amino alcohols to isobutyraldehyde and the secondary amine in strong acidic solutions is determined by the reactivity as well as the concentration of the intermediate zwitterions [Fig. 2, Eq. (10)]. Since several of the equilibrium constants of the foregoing reactions are unknown, an estimate of the relative concentrations of these dipolar species is difficult. As far as the reactivity is concerned, the rate of decomposition is expected to be higher, according as the basicity of the secondary amines is lower, since the necessary driving force to expel the amine will increase with increasing basicity of the secondary amine. The kinetics and mechanism of the hydrolysis of enamines demonstrate that not only resonance in the starting material is an important factor [e.g., if... 

In the presence of strong acid, formic acid decomposes to water and carbon monoxide. In the process, reactive intermediates form which are capable of direct carboxylation of carbonium ions. Since many carbonium ions are readily generated by the reaction of alcohols with strong acid, the process of elimination and carboxylation can be conveniently carried out in a single flask. The carbonium ions generated are subject to the... 

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. 

Radiolytic reduction has been investigated as a means of producing transient Rh(II) porphyrin products, and as in the above study, the observed products were strongly dependent on pH and solvent. Radiolytic reduction of Rh(TMP)Cl in alcohol formed transient Rh(TMP)- which was prevented from dimerization by the bulky TMP ligand. In alkaline 2-propanol the product is [Rh(TMP)r. in weakly acidic 2-propanol the hydride Rh(TMP)H is formed, and in strongly acidic 2-propanol the alkylated rhodium(III) porphyrins Rh(TMP)CH3 and Rh(TMP) (C(CH 3)20H) are observed. The alkyl products result from reaction of Rh(TMP)-with CH3- and C(CH3)20H formed by radiolysis of the 2-propanol solvent. 

The same is trne with an El process. If the substrate is an alcohol, a strong acid will be required in order to protonate the OH group ... 

Oxime carbamates have high polarity and solubility in water and are relatively chemically and thermally unstable. They are relatively stable in weakly acidic to neutral media (pH 4-6) but unstable in strongly acidic and basic media. Rapid hydrolysis occurs in strongly basic aqueous solutions (pH > 9) to form the parent oxime/alcohol and methylamine, which is enhanced at elevated temperature. Additionally, oxime carbamates are, generally, stable in most organic solvents and readily soluble in acetone, methanol, acetonitrile, and ethyl acetate, with the exception of aliphatic hydrocarbons. Furthermore, most oxime carbamates contain an active -alkyl (methyl) moiety that can be easily oxidized to form the corresponding sulfoxide or sulfone metabolites. 

Apart from the alkyl halide-Lewis acid combination, two other sources of carbo-cations are often used in Friedel-Crafts reactions. Alcohols can serve as carbocation precursors in strong acids such as sulfuric or phosphoric acid. Alkylation can also be effected by alcohols in combination with BF3 or A1C13.37 Alkenes can serve as alkylating agents when a protic acid, especially H2S04, H3P04, and HF, or a Lewis acid, such as BF3 and A1C13, is used as a catalyst.38... 

N-nitro amines, RNHN02, decompose to alcohols and nitrous oxide in strong acid media. Rate constants obtained for R = methyl in sulfuric acid222 224 are illustrated as excess acidity plots in Fig. 12.119 This shows multiple curvature, but analysis according to equation (59) shows that one water molecule is involved in the reaction up to about 80 wt% H2S04, and one bisulfate ion above this point, see Fig. 13. The proposed mechanism is shown in Scheme 2. 19... 

Electronic absorption spectra of alcohols in strong proton acids (H2SO4) were obtained by Rosenbaum and Symons (1059, 1960). They observed for a number of simple aliphatic alcohols absorption maxima... 

Substituted anhydro-5-hydroxy-l,2,3,4-oxatriazolium hydroxides (4) decompose on heating in strong acids <320912,56JA5124>. The 3-alkyl substituted compounds produce carbon dioxide, azo-imide (HN3), and the alcohol corresponding to the 3-substituent. The 3-aryl substituted analogues produce carbon dioxide and aryl azide. In some cases the initially formed aryl azide decomposes further to anilines under the conditions of the reaction <82JCS(P2)70l>. Kinetic studies indicate that 3-methyl-anhydro-5-hydroxy-l,2,3,4-oxatriazolium hydroxide (24) reacts via a bimolecular A-2... 

In the case of secondary and tertiary alcohols the strong acid HRu(C0) I causes a very rapid dehydration of the substrates to olefins which have been found in very large amount in the reaction mixtures together with their oligomerization products. 

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