Protonic Acid Media

Protonic Acid Media.—As with last year s Report the coverage in this section is incomplete, and more information will be found under those headings dealing with the individual acids. 

Mixtures of Hg and Dg have been found to undergo exchange reactions with the formation of HD in a number of superacids at room temperature.  

MTP International Review of Science, Inorganic Chemistry Series One, Vol. 3, p. 

Hydrogen-bonding.—The hydrogen ion complex [H(napht)2]+ (napht — 1,8-diazanaphthalene) has been isolated as its perchlorate salt. The crystal 

More recent results from the theory of hydrogen bonds and proton-transfer processes have been reviewed by Schuster. Ab initio LCAO SCF calculations have been performed on the 1 1 adduct of HCN and HF, in which HCN functions as the proton-acceptor molecule. The predicted equilibrium geometry is linear, FH- NCH, and is well described by the general hybridization model for the hydrogen bond. Del Bene has published the results of ab initio SCF calculations on the series of dimers ROH- -NHg, where R = H, Me, NHg, OH, or F. These dimer structures also fit the general hybridization model. 

Verastegui, G. Durand, and B. Tremillon, J. Electroanalyt. Chem. Interfacial Electrochem., 1974, 54, 269. 

Hydrogen-bonding.—Pedersen has analysed the hydrogen-bonding geometries of the 190 crystals, studied by neutron diffraction, in which H20 acts as a donor.195 He concludes that the equilibrium configuration of the bond is linear and that the bending of the bond is isotropic. 

or mixtures of NH3 and H20 have been measured by high-pressure mass spectrometry.198 It was deduced that NH3 forms stronger hydrogen bonds to NHt, for low values of x, than does H20. Gas-phase studies of proton solvation by donors, L, have been extended by Kebarle and Grimsrud199 to include methanol and diethyl ether. The temperature dependence of the (n, n — 1) equilibria for H+(L)n H+(L)n +L was obtained so that AG°, AH°, and AS° could be evaluated for each stage. The 

Borah and Wood201 have investigated the hydrogen-bonded complex cation Et3NHpy+, and its deuteriated analogue, by i.r. spectroscopy. The 

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When the isomers 11 and 12 were oxidized in acetonitrile in the presence of base, contrary to expectations, carbon-oxygen dimers were formed because of nitrogen participation as indicated in Scheme 1.4. However, when the nitrogen is protonated (acid medium), or acylated, the products are all carbon-carbon dimers. 

The possible protonated forms in acidic medium are shown in Scheme... 

Despite its V excessive character (340), thiazole, just as pyridine, is resistant to electrophilic substitution. In both cases the ring nitrogen deactivates the heterocyclic nucleus toward electrophilic attack. Moreover, most electrophilic substitutions, which are performed in acidic medium, involve the protonated form of thiazole or some quaternary thiazolium derivatives, whose reactivity toward electrophiles is still lower than that of the free base. 

In Table III-33 results for the methylation of thiazoles in acetic acid are given (lead tetraacetate is used as radical source), but in this case some discrepancies appear, the acidic medium being too weak, and the heterocyclic base not fully protonated. Thiazole has also been methylated by the DMSO-H2O2 method (201), and the results are in agreement with those described previously. 

The NMR determination in strongly acidic medium (trifluoroacetic acid) of the chemical shifts of the protons in the 4- or 5-position can be used to establish a reactivity scale. If the proton appears at low field, this indicates that this substitution site wiE be poorly or not at all nitrated (111). 

Tertiary amine exchangers possess —R3NH2 groups which have exchanging properties only in an acidic medium when a proton is bound to the nitrogen atom. 

A diazonium salt is a weak electrophile, and thus reacts only with highly electron-rich species such as amino and hydroxy compounds. Even hydroxy compounds must be ionized for reaction to occur. Consequendy, hydroxy compounds such as phenols and naphthols are coupled in an alkaline medium (pH > of phenol or naphthol typically pH 7—11), whereas aromatic amines such as N,N diaLkylamines are coupled in a slightly acid medium, typically pH 1—5. This provides optimum stabiUty for the dia2onium salt (stable in acid) without deactivating the nucleophile (protonation of the amine). 

Because of Us high polarity and low nucleophilicity, a trifluoroacetic acid medium is usually used for the investigation of such carbocationic processes as solvolysis, protonation of alkenes, skeletal rearrangements, and hydride shifts [22-24] It also has been used for several synthetically useful reachons, such as electrophilic aromatic substitution [25], reductions [26, 27], and oxidations [28] Trifluoroacetic acid is a good medium for the nitration of aromatic compounds Nitration of benzene or toluene with sodium nitrate in trifluoroacetic acid is almost quantitative after 4 h at room temperature [25] Under these conditions, toluene gives the usual mixture of mononitrotoluenes in an o m p ratio of 61 6 2 6 35 8 A trifluoroacetic acid medium can be used for the reduction of acids, ketones, and alcohols with sodium borohydnde [26] or triethylsilane [27] Diary Iketones are smoothly reduced by sodium borohydnde in trifluoroacetic acid to diarylmethanes (equation 13)... 

The synthesis of pyrazoles and isoxazoles from l-heterobut-l-en-3-ynes is performed, as a rule, in an aqueous acid medium, i.e., under conditions favoring the hydrolysis and hydration of the initial enyne. This circumstance impedes rationalization of the experimental results. Therefore, it is reasonable to consider in brief the protonation and behavior of l-heterobut-l-en-3-ynes under the conditions of acid-catalyzed hydrolysis. 

The 2-position is largely unreactive toward electrophiles, but nucleophilic substitution occurs there with some facility, especially in acidic medium. The protonated species is about 20 times more reactive than the neutral molecule (70BSF2705). Exhaustive chlorination in the presence of antimony trichloride gave pentachlorobenzothiazole (64GEP1168911). Direct chlorination of the parent heterocycle with aluminium or ferric... 

The amount of the ester sulfonates, besides the mono- and disalt of the a-sulfo fatty acid, can be calculated by two titrations, one in the acid and one in the basic range. In the basic range both sulfonates and carbocylate functionalities are negatively charged and titrated with the cationic surfactant hyamine. In acid medium the RCOOH group is protonated and no longer available for the titration. Since hyamine-methylene blue (acid conditions) titrates only sulfonate and hyamine-phenol red (basic conditions) determines both sulfonates and carbo-cylates, substraction of the titration value with phenol red from the double value of the titration with methylene blue yields only the a-sulfo fatty acid ester. This is the only species of the three which has merely the sulfonate function [106]. 

Addition of protons (acid) to the external medium of intact mitochondria leads to the generation ofATP. 

In this section, we summarize the kinetic behavior of the oxygen reduction reaction (ORR), mainly on platinum electrodes since this metal is the most active electrocatalyst for this reaction in an acidic medium. The discussion will, however, be restricted to the characteristics of this reaction in DMFCs because of the possible presence in the cathode compartment of methanol, which can cross over the proton exchange membrane. 

We consider the case where the weak acid is initially introduced in the external aqueous medium containing the LUVs with a basic interior. Assuming that the neutral (protonated) acid is the only membrane permeable form, the rate of uptake of the weak... 

Calcium carbonate requires an acidic medium for solubility ° H2-receptor antagonists and proton pump inhibitors... 

If the dielectric constant of an amphiprotic solvent is small, protolytic reactions are complicated by the formation of ion pairs. Acetic acid is often given as an example (denoted here as AcOH, with a relative dielectric constant of 6.2). In this solvent, a dissolved strong acid, perchloric acid, is completely dissociated but the ions produced partly form ion pairs, so that the concentration of solvated protons AcOH2+ and perchlorate anions is smaller than would correspond to a strong acid (their concentrations correspond to an acid with a pK A of about 4.85). A weak acid in acetic acid medium, for example HC1, is even less dissociated than would correspond to its dissociation constant in the absence of ion-pair formation. The equilibrium... 

A similar involvement of palladium hydride, palladium alkyl, and palladium acyl complexes as intermediates in the catalytic cycle of the Pd-catalyzed hydroxycarbonylation of alkenes was reported for the aqueous-phase analogs. The cationic hydride PdH(TPPTS)3]+ was formed via the reduction of the Pd11 complex with CO and H20 to [Pd(TPPTS)3] and subsequent protonation in the acidic medium. The reaction of the hydride complex with ethene produced two new compounds, [Pd(Et)(TPPTS)3]+ and Pd(Et)(solvent)(TPPTS)2]+. The sample containing the mixture of palladium alkyl complexes reacted readily with CO to afford trans-[Pd(C(Q)Et)(TPPTS)2]+.665... 

Kinetic theory indicates that equation (32) should apply to this mechanism. Since the extent of protonation as well as the rate constant will vary with the acidity, the sum of protonated and unprotonated substrate concentrations, (Cs + Csh+), must be used. The observed reaction rate will be pseudo-first-order, rate constant k, since the acid medium is in vast excess compared to the substrate. The medium-independent rate constant is k(), and the activity coefficient of the transition state, /, has to be included to allow equation of concentrations and activities.145 We can use the antilogarithmic definition of h0 in equation (33) and the definition of Ksh+ in equation (34) ... 

The nanorods obtained are well cylindrically shaped and well crystalline (Fig. 4), although disorder is probably present at the surface as shown by HRTEM image (see Fig. 4), probably due to the strong bonding of the amine which cover the entire surface. This is demonstrated also from the fact that the nanorods display very different solubilities at different pH values in aqueous solutions due to the amphiphilic behavior of the amine group. In an acidic medium (pH < 3), the amine group is protonated, and the nanorods cannot be precipitated because they form a stable suspension.164 When the pH is increased to basic conditions (pH > 9), the nanorods precipitated within a few minutes. This could be another interesting way to develop... 

The two protonation steps of the M02(CN)4 3 complexes (M=Tc(V), Re(V) equilibria (4) and (6) in Scheme 1) occur in acidic medium while the two protonation steps for the [M02(CN)4]4 (M = Mo(IV), W(IV)) occur in basic aqueous solutions. The pKa values which demonstrate the acid/base behavior of the complexes are given in Table II. Other aspects of these complexes are also presented therein but are discussed in later sections. 

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