Carbon of aldehydes and ketones

Amines also add to the carbonyl carbon of aldehydes and ketones, but the reactions take a different course from acylation and, with ammonia or a primary... 

Replacing the O atom of the aldehyde 9 by an N—R affords the simplest nitrogen carbonyl analog—the aldimine 10. In trying to overcome a few of the problems in alkylation reactions at the a-carbons of aldehydes and ketones (for example 9 13 ... 

The foregoing sections dealt with the protection of the carbonyl carbon of aldehydes and ketones. We now turn to the protection of the ester carbonyl. The... 

Alkylation and acylation of the a-carbon of aldehydes and ketones by means of an enamine intermediate (Sections 19.9 and 19.10). 

The signal for an aldehyde hydrogen typically appears between 8 9.5 and 8 10.1 in a H-NMR spectrum. Because almost nothing else absorbs in this region, it is very useful for identification. Hydrogens on an a-carbon (carbon directly adjacent to carbonyl) of an aldehyde or a ketone appear around 8 2.2 to 2.6. The carbonyl carbons of aldehydes and ketones have characteristic positions in the C-NMR between 8 180 and 8 215 (and can be distinguished from carboxylic acid derivatives, which absorb at a higher field). 

The a-carbon of aldehydes and ketones has special reactivity derived from the keto-enol equilibrium (referred to as tautomerization) that occurs in add or base. 

The use of enamines as synthetic intermediates for the alkylation and acylation at the a-carbon of aldehydes and ketones was pioneered by Gilbert Stork of Columbia University. This use of enamines is called the Stork enamine reaction. 

The bromine attached to the a-carbon of aldehydes and ketones can be replaced only by weakly basic nucleophiles such as a carboxylate ion. Strongly basic nucleophiles form enolate ions, which then undergo other reactions. 

In the next chapter, we shall find out how to generate nucleophilic reactivity at the a-carbons of aldehydes and ketones and how to use that reactivity in the formation of bonds with a variety of electrophiles, including the carbon atoms of other molecules. 

Bisulphite compounds of aldehydes and ketones. These substances are decomposed by dilute acids into the corresponding aldehydes or ketones with the liberation of sulphur dioxide. The aldehyde or ketone may be isolated by steam distillation or by extraction with ether. Owing to the highly reactive character of aldehydes, the bisulphite addition compounds are best decomposed with saturated sodium bicarbonate solution so um carbonate solution is generally employed for the bisulphite compounds of ketones. 

The most obvious way to reduce an aldehyde or a ketone to an alcohol is by hydro genation of the carbon-oxygen double bond Like the hydrogenation of alkenes the reac tion IS exothermic but exceedingly slow m the absence of a catalyst Finely divided metals such as platinum palladium nickel and ruthenium are effective catalysts for the hydrogenation of aldehydes and ketones Aldehydes yield primary alcohols... 

Section 17 1 The substitutive lUPAC names of aldehydes and ketones are developed by identifying the longest continuous chain that contains the carbonyl group and replacing the final e of the corresponding alkane by al for aldehydes and one for ketones The chain is numbered m the direction that gives the lowest locant to the carbon of the carbonyl group... 

In the preceding chapter you learned that nucleophilic addition to the carbonyl group IS one of the fundamental reaction types of organic chemistry In addition to its own reactivity a carbonyl group can affect the chemical properties of aldehydes and ketones m other ways Aldehydes and ketones having at least one hydrogen on a carbon next to the carbonyl are m equilibrium with their enol isomers... 

A number of novel reactions involving the a carbon atom of aldehydes and ketones involve enol and enolate anion intermediates... 

Because etiolate anions are sources of nucleophilic carbon one potential use m organic syn thesis IS their reaction with alkyl halides to give a alkyl denvahves of aldehydes and ketones... 

This suggests sp hybridization at carbon and a ct + tt carbon-oxygen double bond analogous to that of aldehydes and ketones... 

Like the carbonyl group of aldehydes and ketones the carbon of a C=0 unit m a carboxylic acid is sp hybridized Compared with the carbonyl group of an aldehyde or ketone the C=0 unit of a carboxylic acid receives an extra degree of stabilization from its attached OH group... 

The carbon-nitrogen triple bond of nitriles is much less reactive toward nucleophilic addition than is the carbon-oxygen double bond of aldehydes and ketones Strongly basic nucleophiles such as Gngnard reagents however do react with nitriles in a reaction that IS of synthetic value... 

Synthesis from Aldehydes and Ketones. Treatment of aldehydes and ketones with potassium cyanide and ammonium carbonate gives hydantoias ia a oae-pot procedure (Bucherer-Bergs reactioa) that proceeds through a complex mechanism (69). Some derivatives, like oximes, semicarbazones, thiosemicarbazones, and others, are also suitable startiag materials. The Bucherer-Bergs and Read hydantoia syntheses give epimeric products when appHed to cycloalkanones, which is of importance ia the stereoselective syathesis of amino acids (69,70). 

The most versatile method for preparing enamines involves the condensation of aldehydes and ketones with secondary amines [Eq. (1)]. Mannich and Davidsen (/) discovered that the reaction of secondary amines with aldehydes in the presence of potassium carbonate and at temperatures near 0° gave enamines, while calcium oxide and elevated temperatures were required to cause a reaction between ketones and secondary amines, although usually in poor yield. The introduction by Herr and Heyl 2-4) of the removal of the water produced in the condensation by azeotropic distillation with benzene made possible the facile preparation of enamines from ketones and disubstituted aldehydes. 

Reactions at the carbon-nitrogen double bond of iminium salts are analogous to nucleophilic reactions at the carbonyl group of aldehydes and ketones. This is why free enamines do not react with nucleophilic reagents, whereas their salts can undergo such reactions. 

---