Water molecules reaction mechanisms

I.2. Energetics of the Benzylation by o-QM in the Gas Phase and in Aqueous Solution The Gibbs energy profiles for the NH3, H20, and H2S addition reactions to o-QM, in the gas phase and in aqueous solutions, both in the presence (water-catalyzed mechanism) and in the absence of an ancillary water molecule (uncatalyzed mechanism) have been explored, and are displayed in Scheme 2.3.13... 

The key feature of this reaction mechanism is the fact that a certain proton mobility is induced on the surface silanols by the interaction with ammonia.23 The excellent leaving group, H20, is subsequently replaced by NH3. The rate-limiting step in this reaction is the formation and desorption of the water molecule. This mechanism could explain why Fink was able to reach a much higher degree of ammoniation, using a flow system, compared to other researchers using a static system. 

If the rate-determining step involves a reaction of SH+ with a water molecule (A2 mechanism) a Van der Waals bond of a length of ca. 3.6 A goes over to a reacting bond of a length of ca. 1.65 A. This corresponds to a volume decrease of ca. 2 x 10-2 3 cm3 per particle or 12 cm3 per mole. Another bond length may be somewhat increased at the same time, therefore we expect for a bimolecular reaction step that [31] An V % -11 cm3. 

As a second point in our examination of numerical results, we shall consider the active role of solvent molecules (in particular water) in reaction mechanisms. This problem is more complex than tautomeric equilibria considered in the previous subsection, and its analysis would require longer discussions. For this reason we shall confine ourselves to show examples of two basic patterns of active intervention of additional water molecules. The reader is warned that the reactive role of the solvent molecules is not limited to these two basic mechanisms. Both mechanisms have been considered in a recent report by Rivail et al. (1994) which we take as a starting point for our analysis. In this report Rivail et al. compare two different reactions, the hydrolysis of formamide and the ionic dissociation of HC1 in water. We shall examine the two cases separately. 

Ethers react with hydrogen halides to form an alkyl halide and an alcohol. The alcohol in turn can react to form a second molecule of alkyl halide and water. Thus in the presence of two mole-equivalents of hydrogen halide, an ether produces two moles of alkyl halide and one of water. The reaction mechanism is analogous to that of alcohols and hydrogen halides. The ether is protonated first to form an oxonium ion. In... 

One final thing to point out is that when highly reactive gases are used, the purity of the gas is not so critical because the impurity is almost insignificant in determining the ion-molecule reaction mechanism. On the other hand, with collision and low-reactivity gases that contain impurities, such as carbon dioxide, hydrocarbons, or water vapor, the impurity could be the dominant reaction pathway as opposed... 

The carboxylation of alcohols is an interesting reaction for the synthesis of carbonates that requires a better understanding in order to avoid catalyst deactivation by water. The reaction mechanism has been investigated for the Sn, Nb and DCC systems. Scheme 1.18 shows two different possible intra- and inter-molecular mechartisms. The intramolecular mechanism that operates with Sn and DCC is based on a double base-activation of CH3OH and produces an E=0 double bond (E = C) that reduces the activity of the catalyst or generates an inert polymer (E = Sn). The intermolecular mechanism, that seems to be operative with Nb systems, can follow two routes that differ with respect to the intermediacy of one or two alcohol molecules. In the latter case, the reaction follows a base plus acid activation of methanol, and the catalysts perform much better and do not lose activity over several cycles [114]. The water formed in the reaction must be eliminated in order to push the equitibrium to right and avoid the destruction of the catalysts. 

MO (STO-3G) calculations on the gas-phase hydration reaction of formaldehyde suggest a concerted process involving two water molecules as a low-energy mechanism for hydration. 

The breakthrough came with stopped-flow techniques, applied first by Ritchie and Wright (1971a, 1981b). Stopped-flow measurements allow evaluation of observed rates in more detail. It was possible to show that the forward reaction occured not only with hydroxide ions but also with water molecules, followed by fast deprotonation by hydroxide ions. The mechanism of the latter reaction will be discussed in Sections 5.2 and 5.3. 

The Aq dependence of the oxidation in strongly acidic media was taken by Rodek as implying that no water molecule is involved in the transition state and that consequently the ester mechanism as portrayed above cannot hold. However, the Zucker-Hammett hypothesis upon which this argument is based, i.e. that a reaction forming a transition state containing a water molecule will follow a [H30 ] dependence, but that otherwise an dependence will be followed, may not be valid, and in any case the ester can be depicted as breaking down as fol-lows - ... 

The reaction mechanism is based on protonation of the hydroxyl moiety, rearrangement of the phenyl group and simultaneous cleavage of water, creating a carbocation as intermediate [135]. This cation is hydroxylated by water. Thereby, an unstable hemiacetal is formed that splits into two molecules, phenol and water. 

The reaction mechanism for the heterogeneous and homogeneous acid-catalysed esterification were reported to be similar (17). However, there is a major difference concerning the snrface hydrophobicity. Reaction pockets are created inside a hydrophobic environment, where the fatty acid molecules can be absorbed and react further. Water molecules are unlikely to be absorbed on sites enclosed in hydrophobic areas. 

These profiles clearly show that in the gas phase the alkylations of both ammonia and water by o-QM are assisted by an additional water molecule H-bonded to o-QM (water-catalyzed mechanism), since S4 and S5 TSs are favored over their uncatalyzed counterparts (SI and S2) by 5.6 and4.0 kcal/mol [at the B3LYP/6-311 + G(d,p) level], respectively. In contrast, the reaction with hydrogen sulfide in the gas phase shows a slight preference for a direct alkylation without water assistance (by 0.8 kcal/mol). 

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