Methanol isotopic exchange

It has been shown in Section 2.1.4 that methanol adsorbate formed from dilute solutions on a porous Pt surface, consists of COad and COHad in a ratio CO COH of ca. 20-30% [14]. The results of isotopic exchange with bulk CO seem to indicate that only the fraction present as COad can be desorbed and replaced by bulk CO. Probably the same arguments as in the case of pure COad can apply. COHad seems to be more strongly bound to the Pt surface and cannot be desorbed. 

Experiments on the bromination of equilibrated ketone-acetal systems in methanol were also recently performed for substituted acetophenones (El-Alaoui, 1979 Toullec and El-Alaoui, 1979). Lyonium catalytic constants fit (57), but for most of the substituents the (fcA)m term is negligible and cannot be obtained with accuracy. However, the relative partial rates for the bromination of equilibrated ketone-acetal systems can be estimated. For a given water concentration, it was observed that the enol path is more important for 3-nitroacetophenone than for 4-methoxyacetophenone. In fact, the smaller the proportion of free ketone at equilibrium, the more the enol path is followed. From these results, it can be seen that the enol-ether path is predominant even if the acetal form is of minor importance. The proportions of the two competing routes must only depend on (i) the relative stabilities of the hydroxy-and alkyoxycarbenium ions, (ii) the relative reactivities of these two ions yielding enol and enol ether, respectively, and (iii) the ratio of alcohol and water concentrations which determines the relative concentrations of the ions at equilibrium. Since acetal formation is a dead-end in the mechanism, the amount of acetal has no bearing on the relative rates. Bromination, isotope exchange or another reaction can occur via the enol ether even in secondary and tertiary alcohols, i.e. when the acetal is not stable at all because of steric hindrance. 

The first mechanism (a) occurs if fe t < k2 and the observed rate coefficient is given by feobs = k1. The second mechanism (b) applies if fe i > fe2 and then kohs = k2 x K where K = fe1 /fe j. The two mechanisms which correspond, respectively, to a rate-determining proton transfer and a pre-equilibrium followed by a subsequent step have been discussed in detail for isotope exchange reactions in Sect. 2.2.1. The second possibility (b) is apparently favoured by Cram [120] for racemization of 2-methyl-3-phenylpropionitrile whereas Melander [119] has interpreted his results in terms of the first (a). From the variation of the rate coefficient for racemization in different solvent mixtures of methanol/ dimethylsulphoxide a Bronsted exponent j8 = 1.1 was calculated [119] using an acidity function method which will be described fully in Sect. 4.6. 

In reactions like these, Streitwieser et al. [22, 25(c)] have proposed a method for calculating values for k l /k2 from the primary isotope effects and therefore deducing the amount of internal return. The method is illustrated by results for the isotope exchange of triphenylmethane (pK ca. 30 [127]) in methanolic sodium methoxide for which mechanism (81) is written, viz. 

Rates of methoxide ion catalysed isotope exchange for esters in methanol-D [167]... 

Mechanism and Kinetics. The most detailed study of the reaction mechanism has been made by Wachs and Madix. They used isotopic tracers and flash desorption to study the species produced when methanol is adsorbed on an oxygen-doped copper (110) single-crystal surface. While the results of such a study are of considerable interest, they are not necessarily representative of a copper catalyst continuously exposed to reaction conditions. From the desorption spectra, methanol shows exchange only of the hydroxy-hydrogen surface methoxide was identified as the most populous surface intermediate. As formaldehyde and hydrogen also appeared to be produced from the same intermediate, the mechanism (21)—(24) was proposed for the selective reaction ... 

The three different second-order processes thus exhibit widely different kinetic behaviour towards the varying base concentration at constant buffer ratio. In theory this dependence should provide a means of assigning the mechanism. An advantage over the isotopic exchange approach is that it should be possible to detect carbanion intermediates that eliminate more rapidly than they protonate. Unfortunately, the kinetics are not always clear-cut. The E2 mechanism can, under certain conditions, follow specific base catalysis, especially if one base is of much greater catalytic efficiency than the other bases present (e.g. the E2 reaction of l,l,l-trichloro-2,2-di-p-chlorophenyl-ethane with sodium thiophenoxide in methanol) . Alternatively, the base may be sufficiently powerful to produce a kinetically significant concentration of lyate ions (e.g. the E2 reaction of alkyl bromides with phenoxides in ethanol) " . 

Methanol is oxidized to formate ions, acetone and acetaldehyde to acetate species. An enolic form of adsorbed acetone is seen, which accounts for rapid isotopic exchange of -OH. On O2 adsorption two bands at 1155 and 1020 cm are formed,which are however not assigned. Mixtures of CO plus O2 lead to the obvious formation of carbonates and bicarbonates. CO interaction with presorbed NH3 forms carbamate species. Oxidizing properties of the surface are shown by the reaction of trichloroacetone to trichloroacetate species. Nitriles are, however, hydrolysed to acetimidate species R-CNH . Similar reactions with surface -OH are shown by ethyl and phenyl isocyanate, which form urethanes by reaction with isolated -OH groups, and 1,3-diethylurea with -OH pairs. Reaction with surface -OH is also shown by trimethylchlorosilane, which, however, harshly attacks the surface and forms silicone species. ... 

The sur ce FT-IR stey of a nanosize aluminum nitride powder definitely brought evidence of the specific chemical composition of its first atomic layer. The unavoidable contamination, mainly by atmospheric water, implies the presence of oxygen and hydrogen in this first layer. As a result, a comparison with the alumina sur ce appeared quite reasonable. Indeed, methanol and pyridine used as probe molecules showed the same behavior on both material sur ces. The acidity of the AIN sur ce was proven by the presence of two types of Al Lewis sites. However for AIN, a specific dissociative adsorption mechanism of acidic methanol could be possibly explained by the presence of weakly basic AI3N sites. Besides, the isotopic exchange... 

Abstract - Hydrogen isotope exchange rates in methanolic sodium methoxide of hydrocarbons having delocalized carbanions give two separate linear Br0nsted-type relations when log K values are plotted against the corresponding values,... 

Isotopic exchange of C1 in cw-[Co(en)2Cl(CN)] has been investigated using methanol and ethyleneglycol as the solvent. " ... 

With Nucleophiles.—Cyclopropane Derivatives. Simple cyclopropanes are normally resistant to nucleophilic attack, though cyclopropane itself gives a complex mixture of products in a photolytic reaction with tetrafluorohydra-zine. The isotope exchange rates in methanolic sodium methoxide of 1-deuterio- and 1-tritio-cyclopropanes with I-CF3, 1-CN, and l-S02Ph... 

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