Carbonyl, hydration

Ketones and aldehydes, because they are unsaturated, readily add water to give a 1,1-diol or hydrate  

The position of this equilibrium depends greatly on the substituents, X. Formaldehyde (X = H) exists mainly as the hydrate in aqueous solutions, while acetone (X = CH3) exists mainly in the carbonyl form (see table at right). 

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Alternatively one can make use of No Barrier Theory (NBT), which allows calculation of the free energy of activation for such reactions with no need for an empirical intrinsic barrier. This approach treats a real chemical reaction as a result of several simple processes for each of which the energy would be a quadratic function of a suitable reaction coordinate. This allows interpolation of the reaction hypersurface a search for the lowest saddle point gives the free energy of activation. This method has been applied to enolate formation, ketene hydration, carbonyl hydration, decarboxylation, and the addition of water to carbocations. ... 

Both these methods require equilibrium constants for the microscopic rate determining step, and a detailed mechanism for the reaction. The approaches can be illustrated by base and acid-catalyzed carbonyl hydration. For the base-catalyzed process, the most general mechanism is written as general base catalysis by hydroxide in the case of a relatively unreactive carbonyl compound, the proton transfer is probably complete at the transition state so that the reaction is in effect a simple addition of hydroxide. By MMT this is treated as a two-dimensional reaction proton transfer and C-0 bond formation, and requires two intrinsic barriers, for proton transfer and for C-0 bond formation. By NBT this is a three-dimensional reaction proton transfer, C-0 bond formation, and geometry change at carbon, and all three are taken as having no barrier. 

Carbonyl group, nucleophilic addition cychzation, 230, 233, 245-52, 253 Carbonyl hydrate, hydroperoxide hydrogenolysis, 156 Carbonyl oxides... 

Carbonyl hydration has been extensively studied primarily because it serves as a model for a number of important reactions, nucleophilic additions to carbonyl compounds foremost around them. While for most common carbonyl compounds the equilibrium lies far to the left (in favor of the carbonyl compound), it is possible to find compounds where the reverse is true. Because there are ample experimental data, it should be possible to identify structural and/or other characteristics which drive the equilibrium one way or the other. Alternatively, quantum chemical models can be employed. 

It is straightforward to calculate energies of hydration reactions as a function of the carbonyl compound and, once calibrated on the basis of available experimental data, use this as a criterion for selecting systems which might exist primarily as carbonyl compounds, primarily as carbonyl hydrates or anywhere in between. The disadvantage to such an approach (other than it requiring calculations on both the carbonyl compounds and their respective hydrates) is that it provides very little insight into the factors which influence the equilibrium. Another approach is to focus only on the carbonyl compounds (or only on the hydrates) and look for characteristics which correlate with the experimental equilibrium constants. This is the approach illustrated here. 

Like the case study dealing with carbonyl hydration, this exercise places calculation in the role of gathering data , and following this, seeks to use this data to design systems with specific properties. As before, the calculations offer strong advantages over experimental work. 

This example, like those in Chapter 17, dealing with carbonyl hydration and preferential stabilization of singlet or triplet carbenes, places the calculations in the role of data gathering . Unlike these previous examples, the data here take the form of images while they can be used to extract numerical data , can also serve to furnish an overview. 

Title Polymer Compositions of Carbonyl-Hydrated Ketone-Aldehyde Resins and Polyisocyanates in Reactive Solvents... 

The Prins Reaction is the acid-catalyzed of addition aldehydes to alkenes, and gives different products depending on the reaction conditions. It can be thought of conceptually as the addition of the elements of the gem-diol carbonyl hydrate of the aldehyde across the double bond. 

Scheme 6 Examples of the equilibrium between keto and hydrate forms in aqueous solution, including the respective 13C NMR shifts of the carbonyl/hydrate atoms. Glucose data from [44]...

Catalytic transformations of terpenes are well documented [213-215], comprising a wide variety of reactions hydrogenation, dehydrogenation, oxidation, hy-droformylation, carbonylation, hydration, isomerization and rearrangement, and cyclization. 

In the general acid-catalyzed dehydration of acetaldehyde hydrate, Eigen (1965) has proposed a one-encounter mechanism (transition state 17), in which both the acidity and the basicity (conjugate base) of the catalysts are important (moderated by solvent). Bell (1966) has further discussed the occurrence of cyclic paths in carbonyl hydration. Reimann and Jencks (1966) have concluded from rate and equilibrium data on the addition of hydroxylamine to an aldehyde, that proton... 

Kluger, R., Pike, D.C., and Chin, J.. Kinetics and mechanism of the reaction of dimethyl acetylphos-phonate with water. Expulsion of a phosphonate ester from a carbonyl hydrate, Can. J. Chem., 56, 1792, 1978. 

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