Alcohols in the Gas Phase

Some qualitative aspects of the gas phase photolysis of alcohols were recognized by Harrison and Lake (172). The gas phase photolysis of methanol has been studied in more detail by Porter and Noyes (173), by Hagege et al. (75,174,175), and more recently by Herasymowych and Knight (176a), who also studied isopropanol (176b). 

In the case of methanol there is general agreement on the products and their relative yields. Hydrogen and ethylene glycol are the most important, followed by formaldehyde, methane, and 

Haglge et al. (75,174,175) report a further increase In the rate of product formation with increasing methanol pressure (up to 120 nm Hg) Herasyraowych and Knight (176a) indicate that there is a noticeable decline. 

The gas phase photolysis of methanol can be described on the basis of the reactions 22 through 25. 

The fate of the radicals is thought to be the same as discussed above for the liquid state. In the gas phase the H atoms produced in reactions 22 and 23 are expected to abstract hydrogen from the methyl and to some extent from the OH group. Hot hydrogen atoms might play a role. In the liquid phase hydrogen atom abstraction from the OH group is of even smaller likelihood than 

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Another area of gas-phase substituent effects that has attracted interest is the acidity of simple alcohols. In the gas phase, the order is r-BuOH > EtOH > MeOH 3>... 

Exercise 15-7 What order of basicity would you predict for water, methanol, isopropyl alcohol and ferf-butyl alcohol in the gas phase Give your reasoning. 

In nonaromatic systems, ionization usually plays a major role, as compared to excitation.301 Whereas, in liquids or highly disordered solids, the electron can be solvated,302 no, or very low yields of, solvated electrons are observed in solid carbohydrates, even at low temperatures.275 This implies that the electron may return to the positive hole (reaction 295). However, it cannot be excluded that ion-molecule reactions (reaction 194) precede this reaction, and that recombination occurs with the resulting ion N+, instead of with the parent ion M+ (reaction 196). Process 194 has been studied with simple alcohols in the gas phase.303,304... 

The reported halflife of ethenol (vinyl alcohol) in the gas phase at room temperature is ca. 30 min [221], far shorter than our calculated 1028-1029 s. However, the 30 min halflife is very likely that for a protonation/deprotonation isomerization catalyzed by the walls of the vessel, rather than for the concerted hydrogen migration (Fig. 5.30) considered here. Indeed, the related ethynol has been detected in planetary atmospheres and interstellar space [222], showing that that molecule, in isolation, is long-lived. Even under the more confined conditions of the lab, ethenol can be studied in the gas phase [221, 223] and in solution [224]. All three methods predict very long halflives for the uncatalyzed reaction. 

In their second paper Barnes et al.54) studied heteroassociations of alcohols in the gas phase at low alcohol pressures (about 2 Torr) and much higher pressures of the proton acceptors. In particular the IR spectra were measured for TFE with acetonitrile, acetone, methyl-isocyanide, diethylether and tetrahydrofuran and a number of amines. The dimers were obtained in all cases. Their paper contains a number of v and Av values for such systems. 

TABLE 22. Hg-sensitized decomposition of saturated alcohols in the gas phase at room... 

Triorganotin hydroxides and oxides are usually prepared by the hydrolysis of the corresponding chlorides under alkaline conditions. A recent patent reports the formation of trialkyl-, dialkyl-, and monoalkyl-tin oxides by passing an alcohol in the gas phase over tin powder in the presence of a Lewis acid at 200-400 °C,9 for example ... 

C. Why Are Carboxylic Acids and Phenols More Acidic than the Corresponding Aliphatic Alcohols, in the Gas Phase ... 

Measurement of gas phase acidity and basicity values allows a reexamination of the relationship of structure to acidity and basicity. The order of acidity of alcohols in the gas phase (Table 7.4) is f-butyl > isopropyl > ethyl > methyl, with methyl alcohol being more acidic them water. This is opposite the order of acidity of these compounds in aqueous solution (Table 7.1). The decrease of acidity in solution with increasing alkyl substitution was traditionally... 

Heating alcohols in the gas phase to temperatures above 500°C can lead to dehydration, but dehydrogenation to carbonyl compounds also occurs. Better yields of dehydration products are obtained if catalysts are added to the reaction mixture. Heating f-butyl alcohol in the gas phase at 315-422°C... 

Allison J, Ridge DP. Reactions of atomic metal ions with alkyl halides and alcohols in the gas phase. JAm Chem Soc. 1979 101 4998-5009. 

Another area of gas-phase substituent effects that has attracted interest is the acidity of simple alcohols. In the gas phase, the order is /-BuOH > EtOH > MeOH H20. This is opposite from the order in solution as revealed by the data in water and DMSO shown in Table 4.14. These changes in relative acidity can again be traced to solvation effects. In the gas phase, any substituent effect can be analyzed directly in terms of its stabilizing or destabilizing effect on the anion. Replacement of hydrogen by alkyl substituents normally increases electron density at the site of substitution, but this effect cannot be the dominant one, because it would lead to an ordering of gas-phase acidity opposite to that observed. The dominant effect is beheved to be polarizability. The methyl... 

The acidity of alcohols in the gas phase increases with increasing carbon substitution [39, 40] this order is the opposite of the acidity order in solution [40]. The free-energy data are shown in Table 12 they confirm the qualitative gas-phase results first reported by Brauman and Blair [27]. The effects of successive substitution of methyl or t-butyl for hydrogen on the a-C atom are not additive but show a regular saturation effect which is even greater than the one discussed for base strengths in Section 3.2 for the corresponding substitution in methylamine the methyl and t-butyl substituent effects are in the ratio 1.00 1.60 2.00, compared with 1.00 1.85 2.60. 

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