Hydration carbonyl compounds, reaction mechanisms

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. 

In a similar fashion to the formation of hydrate with water, aldehyde and ketone react with alcohol to form acetal and ketal, respectively. In the formation of an acetal, two molecules of alcohol add to the aldehyde, and one mole of water is eliminated. An alcohol, like water, is a poor nucleophile. Therefore, the acetal formation only occurs in the presence of anhydrous acid catalyst. Acetal or ketal formation is a reversible reaction, and the formation follows the same mechanism. The equilibrium lies towards the formation of acetal when an excess of alcohol is used. In hot aqueous acidic solution, acetals or ketals are hydrolysed back to the carbonyl compounds and alcohols. 

Moreover, contrary to alkyne hydration where no adsorption of the carbonyl compound was detected, the problem is complicated here by the saturation of the strong acidic sites by the formed amide, the concentration of which shows a rapid stabilization against time (Fig.3). Consequently the reaction selectivity greatly depends on the ester percentage. The behaviour of the amide itself over the studied zeolites confirms this observation the conversion of the amide into ester goes faster on the HY2 g zeolite than on the Hg and on the HMg zeolites. This later point, together with the comprehension of the different mechanisms in relation with the zeolite properties, will be discussed in a further paper. 

The probable mechanism of the reaction is first addition of piperidine to formaldehyde to form /V-hydroxymethylpiperidine. This reacts with acidic hydrogens of the methyl group of 1,1,1-trifluoroacetone, which is hydrated because the trifluoromethyl group next to carbonyl stabilizes the hydrated form of the carbonyl compound. The resulting dihydrate is further stabilized by hydrogen bonds to the piperidine nitrogen 103. ... 

Carbene chemistry, structure and mechanism in, 7, 163 Carbon atoms, energetic, reactions with organic compounds, 3, 201 Carbonium ions (alkyl), Bpectroscopio observation in strong acid solutions, 4, 305 Carbonium ions, gaseous, from the decay of tritiated molecules, 8, 79 Carbonyl compounds, reversible hydration of, 4, 1... 

Since water adds to (at least some) carbonyl compounds, it should come as no surprise that alcohols do too. The product of the reaction is known as a hemiacetal, because it is halfway to an acetal, a functional group, which you met in Chapter 2 (p. 35) and which will be discussed in detail in Chapter 14. The mechanism follows in the footsteps of hydrate formation just use ROH instead of HOH. 

The evidence for the other forms of DHA shown in Scheme 1 is based on theory of reaction mechanisms. The fact that DHA reacts with DNPH to form a hydrazone is supportive evidence for the existence of a 2- or 3-monoketo compound. A 3-monoketo compound is required by any reasonable mechanism for the formation of the hydrated hemiketal form. The 2,3-diketo compound would be very unstable due to the high positive charge associated with the carbonyl carbons. These carbons would be very susceptible to nucleophilic attack. However, a small concentration of the 2,3-diketo form should exist in equilibrium mixtures. Direct oxidation of AA should give the 2,3-diketo form. 

Tracer studies of pyrocatechase action with O rule out a hydration step as a part of the reaction process. The intermediate formation of carbonyl compounds (as,m-muconic semialdehyde, o-benzo-quinone) or hydroxyl radicals is unlikely because no exchange with HfO is found (compare Table II). Tightly-bound enzyme-HjOj acting as a peroxidase (327) requires that o-benzoquinone be an intermediate, imlikely for reasons given above. Hayaishi and coworkers consider a direct cleavage by means of a peroxide-type addition complex a likely mechanism (equation 9) (327). 

The presence of a carbonyl substituent in the -position of the hetero-cycle is essential for the cleavage of five-membered rings. The compounds of this type which have been investigated most thoroughly are 3-acylindoles.287, 384-388 At 160-170° 3-acetylindole and hydrazine hydrate give 3-(o-aminophenyl)-5-methylpyrazole, the structure of which was proved by deamination. Other 3-acylindoles, their hydrazones, and azines, react analogously.385 For the mechanism of the reaction see Alberti.384,391 The reaction requires a fourfold excess of hydrazine hydrate,387 preferably a polar solvent,388 and about... 

Notice that the mechanisms for imine, enamine, hydrate, and acetal formation are similar. The nucleophile in each reaction has a lone pair on its attacking atom. After the nucleophile has added to the carbonyl carbon, water is eliminated from a protonated tetrahedral intermediate, forming a positively charged species. In imine and hydrate formation, a neutral product is achieved by loss of a proton from a nitrogen and an oxygen, respectively. (In hydrate formation, the neutral product is the original aldehyde or ketone.) In enamine formation, a neutral product is achieved by the loss of a proton from an a-carbon. In acetal formation, a neutral compound is achieved by the addition of a second equivalent of alcohol. 

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