Hydrate, from aldehydes

Ammonia itself yields imines, R2C=NH, with carbonyl compounds but these derivatives are unstable and react with each other to form polymers of varying size. The classical aldehyde ammonias are found to be hydrated cyclic trimers, but from aldehydes carrying powerfully electron-withdrawing substituents it is possible to isolate the simple ammonia adduct [73, cf. (72), and hydrates, p. 208, hemi-acetals,... 

By analogy with the synthesis of a-hydroxy acids one can envisage a one-pot synthesis of a-hydroxy amides from aldehydes via hydrocyanation and in situ NHase-catalyzed hydrolysis to the amide. Since enantioselective NHases are very rare, the enantioselectivity should be derived from HnL-catalyzed hydrocyanation. The second step has been described for the Rhodococcus erythropolis NHase-catalyzed hydration of (R)-mandelonitrile to give the (R)-amide with retention of enantiopurity [43]. 

The atmospheric chemical kinetics of linear perfluorinated aldehyde hydrates, Cx-F2x+iCH(OH)2, have been measured for x = 1,3, and 4, focusing on formation (from aldehyde, by hydration), dehydration, and chlorine atom- and hydroxyl radical-initiated oxidation.211 The latter reaction is implicated as a significant source of perfluorinated carboxylic acids in the environment. 

Hydration of cyanohydrins.1 The cyanohydrins derived from aldehydes undergo hydration when treated with borax in water at 80° to give a-hydroxy amides in 65-85% yield. In some cases addition of a trace of KCN can increase the yield. 

Mechanism The mechanism of the Cr( VI) oxidation of aldehydes has been studied in detail in Scheme 7.14. A hydrate of aldehyde A is formed first, which reacts with chromium species to form a chromate ester B. A base abstracts a proton from the chromate ester B and Cr species leaves (E2 elimination) to give carboxylic acid. 

In acidic media, polarized multiple bonds often undergo acid catalyzed addition, and a common mode of addition is the Ad 2. Deprotonation of the nucleophile by solvent gives the neutral compound. Common examples of this easily reversible Adg2 reaction are the formation of hydrates (NuH is H2O) and, if NuH is ROH, hemiacetals (from aldehydes) and hemiketals (from ketones). Usually this reaction favors reactants. 

Water adds to an aldehyde or a ketone to form a hydrate. A hydrate is a molecule with two OH groups on the same carbon. Hydrates are also called ge/n-diols (gem comes from geminus, Latin for twin ). Hydrates of aldehydes or ketones are generally too unstable to be isolated because the tetrahedral carbon is attached to two oxygen atoms. 

The acid, which was prepared from aldehyde made from acetylene (42), was passed through heated steel tubes which were coated with a mixture of hydrated lime and magnesia. The tubes were filled with steel balls coated with the same substances which served as catalysts. The gases from the tubes were condensed after being passed through a hot solution of sodium carbonate. The 20 per cent solution of acetone thus obtained was rectified by distillation. The acetone was very pure and the yield was about 95 per cent of the theoretical. 

Contrary to some reports, electrophilic addition reactions may occur in other multiple-bond systems. In many of the reactions of aldehydes and ketones the first stage involves the addition of some entity across the carbon-oxygen bond, e.g., the formation of oximes, semicarbazones, hydrazones, hydrates (1,1-diols) and their ethers, and the aldol condensation. Most of these reactions entail a subsequent loss (elimination) of a small molecule e.g. water, ammonia, ethanol) and, while one must be careful to determine whether the rate-determining stage involves attack on the carbonyl compound or elimination from the adduct , there are some systems in which it is evident that electrophilic attack is involved in the slow stage of the reaction sequence. Examples of such reactions are the acid-catalysed formation of oximes of aliphatic - and aromatic carbonyl compounds, of furfural semi-carbazone , and of 1,1-diols from aldehydes or ketones . 

The aldehyde initially formed from the 1° alcohol (produced by a mechanism similar to the one we have just given) reacts with water to form an aldehyde hydrate. The aldehyde hydrate can then react with HCr04 (and H+) to form a chromate ester, and this can then be oxidized to the carboxylic acid. In the absence of water (i.e., using PCC in CH2CI2), the aldehyde hydrate does not form therefore, further oxidation does not take place. 

Phosphorus pentoxide Aldehydes from aldehyde hydrates... 

Nitriles are produced from aldehydes in high yield by reaction of the latter with hydrazine hydrate under cyanide-ion catalysis. A method of limited utility involves the dehydrochlorination of a-nitrosophosphonium... 

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