Hydration of proton

Explain what is meant by the statement The relative heat of hydration of T is -1385.7 kJ mor. Halliwell and Nyburg s value for the heat of hydration of protons was -1087.8 kJ mol". Use this value to establish the absolute heat of hydration of T. 

In the case of apparent general acid catalysis of acetylimidazole hydrolysis, the mechanism can be defined as a specific acid-general base process by comparison with the general base catalysis of N-methyl,N -acetylimidazolium ion. The rate of disappearance of N-methyl,N -acetylimidazolium ion in water at 25° is proportional to the concentration of the basic form of buffer components such as acetate, phosphate, N-methylimidazole, etc., (equation 30) (Wolfenden and Jencks, 1961). The buffer terms show a 1 1 correlation with the general acid-catalyzed rate of acetylimidazole disappearance (Jencks and Carriuolo, 1959) in water at 25°, when the rate expression for the latter reaction is written in terms of equation (32) rather than equation (31), that is, in terms of a general base-catalyzed hydration of protonated acetylimidazole (pX= 3-6). 

The dynamics of proton hydration is too fast to be directly measured, but the thermodynamics of stepwise hydration of proton was measured as clustering of water molecules around free protons in a gas phase (Searcy and Fenn 1974 Kebarle, 1975). Clusters with varying size were observed... 

It will become obvious from the following outline that an important fact to remember when looking at polymer membrane is the hydration of protons in the aqueous medium. The protons always exist in the form of species H" " nHjO. During discharge of the cell, when current flows, the hydrated protons migrate in the membrane from the anode toward the cathode, each proton dragging along n water molecules in the same direction. 

Klassen JS, Blades AT, Kebarle P. Determinations of ion-molecule equilibria involving ions produced by electrospray. Hydration of protonated amines, diamines, and some small peptides. J Phys Chem. 1995 99 15509-17. 

The dissociation of acids to anions in aqueous solutions is due to a high energy of ion hydration. In a vacuum water dissociation to and OH requires an energy of 1630 kJ/mol, whereas in the liquid phase the dissociation of water occurs with an expense of 60 kJ/mol only. When the hydration of protons is presented as a stepwise process, the change in the enthalpy by steps is the following ... 

In an aquo-complex, loss of protons from the coordinated water molecules can occur, as with hydrated non-transition metal ions (p. 45). To prevent proton loss by aquo complexes, therefore, acid must usually be added. It is for these conditions that redox potentials in Chapter 4 are usually quoted. Thus, in acid solutions, we have... 

Iron Sulfur Compounds. Many molecular compounds (18—20) are known in which iron is tetrahedraHy coordinated by a combination of thiolate and sulfide donors. Of the 10 or more stmcturaHy characterized classes of Fe—S compounds, the four shown in Figure 1 are known to occur in proteins. The mononuclear iron site REPLACE occurs in the one-iron bacterial electron-transfer protein mbredoxin. The [2Fe—2S] (10) and [4Fe—4S] (12) cubane stmctures are found in the 2-, 4-, and 8-iron ferredoxins, which are also electron-transfer proteins. The [3Fe—4S] voided cubane stmcture (11) has been found in some ferredoxins and in the inactive form of aconitase, the enzyme which catalyzes the stereospecific hydration—rehydration of citrate to isocitrate in the Krebs cycle. In addition, enzymes are known that contain either other types of iron sulfur clusters or iron sulfur clusters that include other metals. Examples include nitrogenase, which reduces N2 to NH at a MoFe Sg homocitrate cluster carbon monoxide dehydrogenase, which assembles acetyl-coenzyme A (acetyl-CoA) at a FeNiS site and hydrogenases, which catalyze the reversible reduction of protons to hydrogen gas. 

Fig. 2. Hydration of the surface stmcture of manganese dioxide (2) and subsequent reactions of hydrous manganese oxide (3) showing proton transfer (4)...

The spectra of protonated polyaza heterocycles are frequently complicated by the occurrence of covalent hydration. This is more common with polycyclic systems, e.g. pteridine. 

The UV spectra have been used in studies of protonation and related covalent hydration, structural assignments and tautomerism (see appropriate Sections), as well as in studies of bridgehead addition to 5-deazapterins (79MI21500, 78TL2271) and related 5-deazaflavin derivatives (80JA1092). 

The hydration reaction has been extensively studied because it is the mechanistic prototype for many reactions at carbonyl centers that involve more complex molecules. For acetaldehyde, the half-life of the exchange reaction is on the order of one minute under neutral conditions but is considerably faster in acidic or basic media. The second-order rate constant for acid-catalyzed hydration of acetaldehyde is on the order of 500 M s . Acid catalysis involves either protonation or hydrogen bonding at the carbonyl oxygen. 

The enzyme carbonic anhydrase promotes the hydration of COg. Many of the protons formed upon ionization of carbonic acid are picked up by Hb as Og dissociates. The bicarbonate ions are transported with the blood back to the lungs. When Hb becomes oxygenated again in the lungs, H is released and reacts with HCO3 to re-form HgCOj, from which COg is liberated. The COg is then exhaled as a gas. 

There has been considerable discussion about the extent of hydration of the proton and the hydroxide ion in aqueous solution. There is little doubt that this is variable (as for many other ions) and the hydration number derived depends both on the precise definition adopted for this quantity and on the experimental method used to determine it. H30" has definitely been detected by vibration spectroscopy, and by O nmr spectroscopy on a solution of HF/SbFs/Ha O in SO2 a quartet was observed at —15° which collapsed to a singlet on proton decoupling, 7( 0- H) 106 Hz. In crystalline hydrates there are a growing number of well-characterized hydrates of the series H3O+, H5O2+, H7O3+, H9O4+ and H13O6+, i.e. [H(0H2) ]+ n = 1-4, Thus... 

The synthesis of pyrazoles and isoxazoles from l-heterobut-l-en-3-ynes is performed, as a rule, in an aqueous acid medium, i.e., under conditions favoring the hydrolysis and hydration of the initial enyne. This circumstance impedes rationalization of the experimental results. Therefore, it is reasonable to consider in brief the protonation and behavior of l-heterobut-l-en-3-ynes under the conditions of acid-catalyzed hydrolysis. 

Since hydration of biomolecules is of particular importance in molecular biology, uracil - water (U-W) complexes have been studied by many groups [98 JCS(F) 1277, 98JST307, 99JPC(A)1611, 00PCCP1281]. In the cyclic U-W complex the most stable hydrogen bond is formed at the site characterized by the lowest proton... 

Mechanism of the acid-catalyzed hydration of an alkene to yield an alcohol. Protonation of the alkene gives a carbocation intermediate that reacts with water. 

One of the steps in fat metabolism is the hydration of crotonate to yield 3-hydroxybutvrate. The reaction occurs by addition of —OH to the Si face at CM, followed by protonation at C2, also from the Si face. Draw the product of the reaction, showing the stereochemistry of each step. 

Al(III) is an example of an aquatic ion that forms a series of hydrated and protonated species. These include AlOrf Al(OH)J, Al(OH)3, and other forms in addition to AP. (For simplicity, we omit the H2O molecules that complete the structures of these complexes.) Most of these species are amphoteric (able to act as an acid or a base). Thus the speciation of Al(III) and many other aquatic ions is sensitive to pH. In this case, an aggregate variable springs from the conservation of mass condition. In the case of dissolved aluminum, the total dissolved aluminum is given by... 

However, the rates of hydration of these compounds are about a factor of ten to one hundred faster than predicted by means of the Taft relation. Anchimeric assistance by the hydroxyl group in the transition state for protonation, 9, has... 

Clearly, a large body of diverse evidence indicates that the acid-catalyzed hydration of alkynyl ethers and thioethers proceeds via a rate-determining protonation through a vinyl cation. However, these vinyl cations are unique in that they have a resonance form where the positive charge resides on the... 

As the data in Table 1 indicate, there is a strong dependence of the hydration on the electron demands of the substituents, with a rho of -4.3 in the Yukawa-Tsuno (18) equation, where logk is plotted against p[a + t a — a)]. Partial hydration of CsHsC CT and recovery of the unreacted starting material did not result in any loss of specific activity, which indicates that the protonation of the triple bond is not significantly reversible and hence is rate determining. 

The experiments of Bott (17) and Noyce (19-21) show that a vinyl cation best represents the intermediate in the hydration of phenylacetylenes. In particular, the large solvent Isotope effects observed indicate a rate-limiting protonation and formation of a vinyl cation, for these values are not in agreement with solvent isotope effects observed for compounds which react by other possible mechanisms, such as one involving equilibrium formation of the vinyl cation followed by the slow attack by water. 

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