Alcohols, acidity proton affinity

Additional gas-phase reactivity data, such as gas-phase acidities of alcohols [41], proton affinities of alcohols and ethers [41], and proton affinities of carbonyl compounds [42] could equally well be described by similar equations. 

The gas-phase esterification reaction has not been observed with formic acid. The reason for this behaviour has been explained on the basis of the proton affinities of the acid and alcohols as indicated in (73) and (74). Thus,... 

Compounds with a high HOMO and LUMO (Figure 5.5c) tend to be stable to selfreaction but are chemically reactive as Lewis bases and nucleophiles. The higher the HOMO, the more reactive. Carbanions, with HOMO near a, are the most powerful bases and nucleophiles, followed by amides and alkoxides. The neutral nitrogen (amines, heteroaromatics) and oxygen bases (water, alcohols, ethers, and carbonyls) will only react with relatively strong Lewis acids. Extensive tabulations of gas-phase basicities or proton affinities (i.e., —AG° of protonation) exist [109, 110]. These will be discussed in subsequent chapters. 

In the substrates, special attention has been devoted to zeolites and fullerenes [34, 38, 39, 44], in the reactions to acid-base equilibria and the influence of hardness and softness on both sides of the equilibrium (acidity of carboxylic acids [45], alkyl [46] and halogenated alcohols [47], hydrides [48] and recently hydrofullerenes [49], basicity of amines [50, 51] and proton affinities of amino acids [52]). For reviews of these studies, see [18, 53, 54],... 

Studies of the effect of other sorbed species give considerable insight into the nature of spillover. Boudart et al. studied the influence of sorbed water on the rate of spillover (114). The ratio of W03 conversion at room temperature to the hydrogen bronzes and the sorption of hydrogen from the gas phase were dramatically increased by the presence of water. Levy and Boudart used a spectrum of alcohols and acids as coadsorbents, and found that the increase in the rate of reduction of W03 was related to the proton affinity of the coadsorbent (39,115). The proton, produced on the Pt, is proposed to be... 

Reviews by Gorte and coworkers [35, 36] deal with the adsorption complexes formed by strong and weak bases with acid sites in zeolites. They examine the adsorption enthalpies of a series of strongly basic molecules such as alkylamines, pyridines and imines. These workers also performed studies of the adsorption properties of weak bases, including water, alcohols, thiols, olefins, aldehydes, ketones and nitriles. They report a poor correlation between the differential heats of adsorption on H-MFl zeolites and the enthalpies of protonation in aqueous solutions, but a much better correlation with gas-phase proton affinities [37]. 

Mechanistic Studies. - The mechanism of the reaction of tetra-zole-activated phosphoramidites with alcohols has been studied. A series of diethyl azolyl phosphoramidites (85) was prepared from diethyl phosphorochloridite and fully characterized, and the same compounds shown to be formed from the phosphoramidite (86) and azole. The degree of formation of (85) from (86) increases with the acidity of the azole, and the proposed mechanism is a fast protonation of (86), followed by a slow, reversible formation of (85) and a fast reaction of (85) with alcohols. Another study was concerned with the influence of amine hydrochlorides on the rate of methanolysis of the phosphoramidites (87) or (88), or tris(diethylamino)phosphine.The chloride content was measured to be 10-20 mM in doubly distilled samples which explains that "uncatalysed alcoholysis is possible. Intensive purification, including treatment with butyllithium and distillation from sodium, brought the chloride content down to 0.1-1 mM. The methanolysis reaction, in methanol as the solvent, was found to be first-order in catalyst concentration. An aJb initio calculation on N- and P-protonated aminophosphine (89) gave similar proton affinities for N and P this contrasts with earlier MNDO calculations which had ff-protonated species as the most stable. The M-protonated compound had an electronic structure reminiscent of a phosphenium ion-ammonia complex. 

Acetonitrile is also an interesting molecule for probing acid sites in catalysts [39,47-49]. It is a weak base, so no protons are abstracted and actual hydroxyl groups can be observed. It also allows the investigation of both Lewis and Bronsted acidities. While it is normally considered to be a weak base, it actually has a moderately high proton affinity (798 kJ mof, compared to 857 kJ mof for ammonia and 773 kJ mol for methanol). Other nitriles and alcohols have also been used to probe the acid sites of catalysts [50,51],... 

Having established the importance of the calculated parameters in the studies of physical data we turned our attention to the problem of chemical reactivity. It could be shown that the values for the inductive and polarisability effect can be used to calculate reactivity data of some fundamental polar reactions in the gas phase (Figure 6). Linear equations could be developed to calculate proton affinities of amines/ of alcohols and ethers/ and of thiols and thioethers/ as well as gas phase acidity data of alcohols. ... 

Parthaserathi et al. [238] have used the group philicity index to estimate the pK s of several phosphoric acid esters and related compounds They included the calculations in a group with anilines and alcohols, but the numerical results for the phosphoric acids appear generally good Moser, Range, and York [301] have calculated gas-phase proton affinities and basicities for a large collection of acyclic and cyclic phosphates. 

Whether to consider these heteromolecular associates as real chemical adducts is more terminology than a chemical problem. In most cases investigators truly assume that in systems formed of two alcohols or of two carboxylic acids, specific interaction does not exist. In the case of mixed solvents formed of two carbon acids, that is hue only when components have similar proton affinity, as is the case of a system sueh as aeehe acid-propionie aeid (see further paragraph 9.2.8). 

From a purely chemical standpoint, the ICR spectrometer can provide information regarding ionic and radical heats of formation, bond strengths, proton affinities, the excited states of neutral molecules, and the chemical behavior of organic ions in the absence of solvent molecules. Indeed, one of the more interesting studies carried out with the ICR spectrometer has shown that the relative acid strengths in a series of alcohols and amines are quite different from the values previously accepted from solution chemistry. "" ... 

Further, the application of mass-spectrometric techniques gives just the same order for the proton affinities of amines in the gas phase,and a considerable amount of quantitative information is now available. It is of interest in this connection that Just the same effects of alkyl substitution are observed on the tendency of amines to lose a proton,i.e., in the absence of solvation effects alkyl groups have a similar stabilizing influence both on cationsand on anions2>N. This suggests that the effect of alkyl groups on the acid-base properties of amines, and also alcohols," is best explained by the stabilization of a localized charge by a polarizable environment, and casts considerable doubt on the usual explanation in terms of the inductive effect. 

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