Acidic proton hydration

The combination of the acidic proton hydration 3-32 and the basic proton hydration 3-34 leads to the ionic dissociation of water molecule as shown in Eqn. 3-36 ... 

We can extend the general principles of electrophilic addition to acid catalyzed hydration In the first step of the mechanism shown m Figure 6 9 proton transfer to 2 methylpropene forms tert butyl cation This is followed m step 2 by reaction of the car bocation with a molecule of water acting as a nucleophile The aUcyloxomum ion formed m this step is simply the conjugate acid of tert butyl alcohol Deprotonation of the alkyl oxonium ion m step 3 yields the alcohol and regenerates the acid catalyst... 

Mechanism of Acid-Catalyzed Hydration Three steps are involved m acid catalyzed hydration (Figure 17 7 on page 718) The first and last are rapid proton transfers between... 

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. 

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. 

The acid-catalvzed hydration reaction begins with protonation of the carbonyl oxygen atom, which places a positive charge on oxygen and makes the carbonyl group more electrophilic. Subsequent nucleophilic addition of water to the protonated aldehyde ot ketone then yields a protonated gem diol, which loses H+ to give the neutral product (Figure 19.5). 

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... 

Rate-determining protonation to give a vinyl cation rather than 1,4 addition of water has been proposed as the most consistent mechanism (25) for the acid-catalyzed hydration of arylpropiolic acids in aqueous sulfuric acid. Hydration of arylpropiolic acid closely resembles the acid-catalyzed isomeriza-... 

In contrast, the acid-catalyzed hydration of arylbenzoylacetylenes differs markedly from the hydration of a-unsaturated ketones. Hydration of unsaturated ketones has been shown to proceed via a 1,4-addition mechanism where protonation occurs on oxygen to give an oxonium salt, followed by attack of water at the 0-carbon to give a hydroxy enol. The rate-limiting step has been shown to be the protonation of the hydroxy enol (27) ... 

The sequential reactions 4.1 and 4.2 represent the self-dissociation of water as the exchange of a proton between water molecules, where hydration of the proton according to reaction 4.2 is the driving force for its separation (reaction 4.1) although the proton hydration is not limited to one H20 (hydration number 1), nor is the occurrence of unhydrated OH ion realistic, the overall reaction 4.3 is generally written as the simplest form to show the principle of proton acidity. 

The most common cation is H , adding to unsaturated linkages, i.e. protonation, in for example the acid-catalysed hydration of alkenes (p. 187) ... 

Fig. 2 Free energy reaction coordinate profiles for the stepwise acid-catalyzed hydration of an alkene through a carbocation intermediate (Scheme 5). (a) Reaction profile for the case where alkene protonation is rate determining (ks kp). This profile shows a change in rate-determining step as a result of Bronsted catalysis of protonation of the alkene. (b) Reaction profile for the case where addition of solvent to the carbocation is rate determining (ks fcp). This profile shows a change in rate-determining step as a result of trapping of the carbocation by an added nucleophilic reagent.

Different rate-determining steps are observed for the acid-catalyzed hydration of vinyl ethers (alkene protonation, ks kp) and hydration of enamines (addition of solvent to an iminium ion intermediate, ks increasing stabilization of a-CH substituted carbocations by 71-electron donation from an adjacent electronegative atom results in a larger decrease in ks for nucleophile addition of solvent than in kp for deprotonation of the carbocation by solvent. 

The C02-bicarbonate buffer is a little different from buffers using the usual kind of acids and bases, but it is extremely important in maintaining the acid-base balance of the blood. The acid form of the bicarbonate buffer is actually a gas dissolved in water. Dissolved C02 is turned into an acid by hydration to give H2C03. Hydrated C02 is then much like a carboxylic acid. It gives up a proton to a base and makes bicarbonate, HCO 3. 

Further studies that demonstrate that hydration of bay-region diol epoxides under acidic conditions can occur by general acid catalysis in addition to proton catalysis have expanded our understanding of their reactivity. General acid catalyzed hydration involves H-bonding of the epoxide O-atom by the acid catalyst, followed by nucleophilic attack of the distal C-atom by H20/H0 [108][109],... 

Fig. 3-16. Acidic and basic proton levels in aqueous solution h (Hso /H20) = unitary energy of hydration of a standard gaseous proton to occupy the xmitary vacant acidic proton level 1h (H2cvoh-) = unitary energy of hydration of a standard gaseous proton to occupy the unitary vacant basic proton level Dh o = ionic dissociation energy of HjO.

The energy level of hydrated proton depends on the proton concentration. For an acidic proton in Eqn. 3-32 and a basic proton in Eqn. 3-34, the proton levels Hh- are, respectively, given in Eqns. 3-37 and 3-38 ... 

In general, the acidic and basic proton hydration processes may occur simultaneously giving the same proton level for both the acidic and the basic protons. In pure liquid water where WHgo- = Woh- io electroneutrality, the proton level is obtained from Eqns. 3-39 and 3-40 as shown in Eqn. 3-41 ... 

It follows, then, that the proton level in pure water is located midway between the unitary level of acidic proton and the unitary level of basic proton, leading to the hydrated proton concentration at pH 7. 

Fig. 9-22. Unitary proton levels of hydrated and adsorbed hydronium ions (acidic proton) and of hydrated and adsorbed water molecules (basic proton) the left side is the occupied proton level (the real potential of acidic protons), and the right side is the vacant proton level. Hi/HjO) = unitary occupied proton level of adsorbed hydronium ions (acidic proton level) H20.d = unitary vacant proton level of adsorbed hydronium ions (acidic proton level) and unitary occupied proton level of adsorbed water molecules (basic proton level) OH = unitary vacant proton level of adsorbed water molecules (basic proton level) (pHi, ) = hydrated proton level at iso-electric point pR...

Structural diffusion is favored by conditions that enhance the stiffness of the hydrogen-bonded network between water molecules low temperatures and low acid concentration. The decrease in water content leads to an effective increase in the concentration of acid protons, which in turn suppresses the contribution of structural diffusion, as found in aqueous acidic solutions. This agrees with the finding of an enhanced contribution of vehicular transport in PEMs at low hydration. Such an observation is also supported by recent studies of molecular mechanisms of proton transport in PEMs at minimal hydration. ... 

Hydrated Acidic Polymers. Hydrated acidic polymers are, by far, the most commonly used separator materials for low-temperature fuel cells. Their typical nanoseparation (also see Section 1) leads to the formation of interpenetrating hydrophobic and hydrophilic domains the hydrophobic domain gives the membrane its morphological stability, whereas the hydrated hydrophilic domain facilitates the conduction of protons. Over the past few years, the understanding of the microstructure of these materials has been continuously growing, and this has been crucial for the improved understanding of the mechanism of proton conduction and the observed dependence of the conductivity on solvent (water and methanol) content and temperature. 

Ab initio MO calculations have been carried out for two carbocation-generating reactions the 6 nI reaction of protonated 1-phenylethanol (H2O leaving group) and the acid-catalysed hydration of styrene. Optimizations were done at the MP2/6-31G level. The 6 nI transition state lies half way between the reactant and the product with respect to the bond lengths, charge distribution, and secondary deuterium isotope effects. 

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