Methanol proton transfer

R. C. Dunbar, Energy dependence of methanol proton transfer reaction rate, J. Chem. Phys. 52, 2780-2781 (1970). 

Step 4 Proton transfer from methanol to the anion gives 1 4 cyclohexadiene H H H... 

This study is particularly noteworthy in the evolution of QM-MM studies of enzyme reactions in that a number of technical features have enhanced the accuracy of the technique. First, the authors explicitly optimized the semiempirical parameters for this specific reaction based on extensive studies of model reactions. This approach had also been used with considerable success in QM-MM simultation of the proton transfer between methanol and imidazole in solution. 

A similar drop in electrical conductivity, though not so marked, is observed on adding a trace of water to a dilute solution of HC1 in methanol, which is attributed to the proton transfer... 

Autoprotolysis of the Solvent. While studying these proton transfers, there is another type that may be discussed at the same time, namely, the self-dissociation of the solvent itself. As is well known, highly purified solvents show at least a small electrical conductivity. In methanol, for example, it is generally recognized that this conductivity arises from the fact that, a certain number of protons havo been transferred according to the process... 

The Dissociation Constant of Nitric Add. Alodcrately Weak Acids. The Variation of J with Temperature. Proton Transfers between Solute Particles. A Proton Transfer in Methanol Solution. Proton Transfers with a Negative Value for. / . The Hydrolysis of Salts. Molecules with Symmetry. Substituted Ammonium Ions. Deuteron Transfers in D2(). The Dissociation of Molecular Ions. 

A Proton Transfer in Methanol Solution. Although in non-aqueous solutions several proton transfers have been studied at room temperature, very few investigations have been made over a wide range of temperature in very dilute solution. Table 16 gives some results obtained in methanol solution by NisonofT and Kuhn1 for a proton transfer of the following type ... 

Returning now to proton transfers in methanol solution, we see that the family of curves in Fig. 44, calculated with S = 185.4 is very similar to those for aqueous solution. The curve for a = 0.0 passes through its maximum at the characteristic temperature 185.4°K. We now find that we can interpret the experimental results recorded in Table 16 for we see in Fig. 44 that curves drawn for values of a between —3 and —4 pass through zero near room temperature. The row of circles give values of... 

Electrodes and Galvanic Cells. The Silver-Silver Chloride Electrode. The Hydrogen Electrode. Half-cells Containing an Amalgam, Electrode. Two Cells Placed Back to Back. Cells Containing Equimolal Solutions. The Alkali Chlorides as Solutes. HC1 in Methanol or Ethanol Containing a Trace of Water. The Alkali Chlorides in Methanol-Water Mixtures. The Heal of Solution of HC1. Proton Transfer Equilibrium from Measurements of E.M.F. 

In their measurements with the cell (195) containing methanol, Non-hebel and Hartley1 verified by direct experiment that the addition of a small drop of water to either side of the cell, sufficient to give a mole fraction of I120 equal to about 0.001, produced a change in the e.m.f. equal to a few millivolts. This was attributed mainly to the proton transfer (44). The curve for HOI in methanol-water mixtures must thus have a very steep slope, as has been sketched on the right-hand side of Fig. 01. 

Similarly, at 25°C a liter of methanol weighs 785 grams and contains 24.5 moles. We find then for the proton transfer (44)... 

Proton Transfers in Various Solvents. The Autoprotolysis of Methanol. Formic Acid as Solvent. The Sulfate Ion. Autoprotolysis of Formic Add. The Urea Molecule. Sulfuric Add and Liquid Ammonia as Solvents. 

The ionization of (E)-diazo methyl ethers is catalyzed by the general acid mechanism, as shown by Broxton and Stray (1980, 1982) using acetic acid and six other aliphatic and aromatic carboxylic acids. The observation of general acid catalysis is evidence that proton transfer occurs in the rate-determining part of the reaction (Scheme 6-5). The Bronsted a value is 0.32, which indicates that in the transition state the proton is still closer to the carboxylic acid than to the oxygen atom of the methanol to be formed. If the benzene ring of the diazo ether (Ar in Scheme 6-5) contains a carboxy group in the 2-position, intramolecular acid catalysis is observed (Broxton and McLeish, 1983). 

A somewhat different example of this conversion is observed in the reaction of CHO + ions from methanol with CD4. Here proton transfer... 

Itoh, M. Adachi, T. Transient absorption and two-step laser excitation fluorescence studies of the excited-state proton transfer and relaxation in the methanol solution of 7-hydroxyflavone. J. Am. Chem. Soc. 1984, 106, 4320 -324. 

Sakurai et al. have provided what is probably the most important mechanistic finding in the area of intermolecular additions of silenes in recent years, namely a detailed proposal for the mechanism of alcohol addition to the silicon-carbon double bond.68 A cyclic silene 116 was synthesized in the presence of various amounts of methanol and other alcohols, and varying proportions of methanol adducts 117 and 118 were obtained. It was concluded that the methanolysis involved two steps, the first being the association of the oxygen lone pairs with the sp 2-hybridized silicon atom of the silene. The second step, proton transfer, could occur in two ways. If the proton was transferred from the complexed methanol molecule (path a) its delivery would result in syn addition. However, if a second molecule of methanol participated (path b), it would deliver its proton... 

The origin of the sequence corresponding to protonated methanol peaks is a rapid intracluster proton transfer reaction following ionization of the neutral clusters. This reaction has a well-known bimolecular counterpart that proceeds at near collision rate 104... 

As a result, similar to 7AI, 3CNAI, and 3,5CNAI undergo methanol-catalyzed excited-state double proton transfer (ESDPT), revealing dual (normal and proton transfer) emission. However, proton transfer is prohibited for 5CNAI and DiCNAI... 

Specific Electron Capture. It is now customary to use solvents such as methanol (usually CD3OD) or methyl tetrahydrofuran (MTHF) as solvents if electron-capture by AB is required. These solvents form good glasses at 77 K, and for sufficiently dilute solutions of the substrate, AB, electron ejection occurs overwhelmingly from solvent molecules, so that AB+ centres are not formed. Electrons are fairly mobile, and hence AB radicals are formed provided AB has, effectively, a positive electron affinity. The hole centres, such as CD30D+, are not mobile because proton transfer to surrounding solvent molecules occurs rapidly at all temperatures. 

The reactions of the vinylcarbenes 7 and 15 with methanol clearly involve delocalized intermediates. However, the product distributions deviate from those of free (solvated) allyl cations. Competition of the various reaction paths outlined in Scheme 5 could be invoked to explain the results. On the other hand, the effect of charge delocalization in allylic systems may be partially offset by ion pairing. Proton transfer from alcohols to carbenes will give rise to carbocation-alkoxide ion pairs that is, the counterion will be closer to the carbene-derived carbon than to any other site. Unless the paired ions are rapidly separated by solvent molecules, collapse of the ion pair will mimic a concerted O-H insertion reaction. 

The ratio of isomeric ethers is strongly affected by polar substituents which induce an asymmetric distribution of charge in allylic cations. Photolysis of methyl 2-diazo-4-phenyl-3-butenoate (20) in methanol produced 24 in large excess over 25 as the positive charge of 22 resides mainly a to phenyl (Scheme 8).19 As would be expected, proton transfer to the electron-poor carbene 21 proceeds reluctantly intramolecular addition with formation of the cyclopropene... 

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