Of an aldehyde

Claisen reaction Condensation of an aldehyde with another aldehyde or a ketone in the presence of sodium hydroxide with the elimination of water. Thus benzaldehyde and methanal give cinnamic aldehyde, PhCH CH-CHO. 

Hydroxylamine condenses with the carbonyl group of an aldehyde or ketone to form an oxime ... 

This Condensation should not be confused with the Claisen Reaction, which is the condensation of an aldehyde with (i) another aldehyde, or (ii) a ketone, under the influence of sodium hydroxide, and with the elimination of water. For details, see Diben zal-acetone. p, 231. 

This reaction consists of the condensation of two molecular equivalents of a 1,3 diketone (or a J3-keto-ester) with one equivalent of an aldehyde and one of ammonia. Thus the interaction of ethyl acetoacetate and acetaldehyde and ammonia affords the 1,4-dihy dro-pyridine derivative (1), which when boiled with dilute nitric acid readily undergoes dehydrogenation and aromatisation" to gb e the diethyl ester of collidine (or 2,4,6-trimethyl-pyridine-3,5 dicarboxylic acid (II)). For the initial condensation the solid aldehyde-ammonia can conveniently be used in place of the separate reagents. 

This preparation is another example of the condensation of an aldehyde with malonic acid and pyridine to 3neld ultimately an ap-unsaturated acid (Doebner reaction). It is included here because, unlike the acids prepared from many of the lower aUphatic aldehydes, the product consists largely (about 95 per cent.) of the ap-isomeride and only about 5 per cent, of the PY-isomeride is present ... 

Wolff - Kishner reduction of aldehydes and ketones. Upon heating the hydrazoiie or semicarbazone of an aldehyde or ketone with potassium hydroxide or with sodium ethoxide solution (sealed tube), the corresponding hydrocarbon is obtained ... 

The conversion of an aldehyde or ketone into the corresponding amine by heating with ammonium formate or with formamide and formic acid at about... 

Knoevenagel reaction. The condensation of an aldehyde with an active methylene compound (usually malonic acid or its derivatives) in the presence of a base is generally called the Knoevenagel reaction. Knoevenagel found that condensations between aldehydes and malonic acid are effectively catalysed by ammonia and by primary and secondary amines in alcoholic solution of the organic amines piperidine was regarded as the best catalyst. 

An important general method of preparing indoles, known as the Fischer Indole synthesis, consists in heating the phenylhydrazone of an aldehyde, ketone or keto-acld in the presence of a catalyst such as zinc chloride, hydrochloric acid or glacial acetic acid. Thus acrtophenone phenylhydrazone (I) gives 2-phenyllndole (I V). The synthesis involves an intramolecular condensation with the elimination of ammonia. The following is a plausible mechanism of the reaction ... 

This reaction involves the condensation of an aldehyde or ketone with an a-halo ester in the presence of a basic condensing agent (sodium ethoxlde, sodamide, finely divided sodium or potassium iert.-butoxide) to give a glycldio (or ap-epoxy) ester. Thus acetophenone and ethyl chloroacetate yield phenyl-methylglycidic ester (I) ... 

After the umpolung of an aldehyde group by conversion to a l,3 dithian-2-ide anion (p. 17) it can be combined with a carbonyl group (D. Seebach, 1969, 1979 B.-T. GrO-bel, 1977 B). Analogous reagents are tosylmethyl isocyanide (TosMIC), which can be applied in the nucleophilic formylation of ketones (O.H. Oldenziel, 1974), and dichloromethyl lithium (G. KObrich, 1969 P. Blumbergs, 1972 H. Taguchi, 1973),... 

Terminal alkyne anions are popular reagents for the acyl anion synthons (RCHjCO"). If this nucleophile is added to aldehydes or ketones, the triple bond remains. This can be con verted to an alkynemercury(II) complex with mercuric salts and is hydrated with water or acids to form ketones (M.M.T. Khan, 1974). The more substituted carbon atom of the al-kynes is converted preferentially into a carbonyl group. Highly substituted a-hydroxyketones are available by this method (J.A. Katzenellenbogen, 1973). Acetylene itself can react with two molecules of an aldehyde or a ketone (V. jager, 1977). Hydration then leads to 1,4-dihydroxy-2-butanones. The 1,4-diols tend to condense to tetrahydrofuran derivatives in the presence of acids. 

Both the Clemmensen and the Wolff-Kishner reductions are designed to carry out a specific functional group transformation the reduction of an aldehyde or ketone carbonyl to a methylene group Neither one will reduce the carbonyl group of a carboxylic acid nor... 

The type of alcohol produced depends on the carbonyl compound Substituents present on the carbonyl group of an aldehyde or ketone stay there—they become sub stituents on the carbon that bears the hydroxyl group m the product Thus as shown m Table 14 3 (following page) formaldehyde reacts with Grignard reagents to yield pri mary alcohols aldehydes yield secondary alcohols and ketones yield tertiary alcohols... 

Sodium borohydride is especially easy to use needing only to be added to an aque ous or alcoholic solution of an aldehyde or a ketone... 

Sodium borohydride and lithium aluminum hydride react with carbonyl compounds in much the same way that Grignard reagents do except that they function as hydride donors rather than as carbanion sources Figure 15 2 outlines the general mechanism for the sodium borohydride reduction of an aldehyde or ketone (R2C=0) Two points are especially important about this process... 

FIGURE 15 2 Mechanism of sodium borohydnde reduction of an aldehyde or ketone... 

FIGURE 18 1 Mechanism of acid catalyzed enolization of an aldehyde or ketone in aqueous solution... 

Substitution of deuterium for hydrogen at the a carbon atom of an aldehyde or a ketone is a convenient way to introduce an isotopic label into a molecule and is readily carried out by treating the carbonyl compound with deuterium oxide (D2O) and base... 

Reactions in which two molecules of an aldehyde combine to form an a p unsat urated aldehyde and a molecule of water are called aldol condensations... 

Formaldehyde for example cannot form an enolate but can react with the enolate of an aldehyde or ketone that can... 

Lone pair donation from the hydroxyl oxygen makes the carbonyl group less elec trophilic than that of an aldehyde or ketone The graphic that opened this chapter is an electrostatic potential map of formic acid that shows the most electron rich site to be the oxygen of the carbonyl group and the most electron poor one to be as expected the OH hydrogen... 

Esterification of carboxylic acids involves nucleophilic addition to the carbonyl group as a key step In this respect the carbonyl group of a carboxylic acid resembles that of an aldehyde or a ketone Do carboxylic acids resemble aldehydes and ketones m other ways Do they for example form enols and can they be halogenated at their a carbon atom via an enol m the way that aldehydes and ketones can ... 

The enol content of a carboxylic acid is far less than that of an aldehyde or ketone and introduction of a halogen substituent at the a carbon atom requires a different set... 

NMR Like other carbonyl groups the carbon of the —CO2H group of a car boxylic acid is strongly deshielded (8 160-185) but not as much as that of an aldehyde or ketone (8 190-215)... 

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 first stage of the mechanism is exactly the same as for nucleophilic addition to the carbonyl group of an aldehyde or ketone Many of the same nucleophiles that add to aldehydes and ketones—water (Section 17 6) alcohols (Section 17 8) amines (Sections 17 10-17 11)—add to the carbonyl groups of carboxylic acid derivatives... 

A characteristic property of an aldehyde function is its sensitivity to oxidation A solu tion of copper(II) sulfate as its citrate complex (Benedict s reagent) is capable of oxi dizing aliphatic aldehydes to the corresponding carboxylic acid... 

The presence of an aldehyde function m their open chain forms makes aldoses reactive toward nucleophilic addition of hydrogen cyanide Addition yields a mixture of diastereo meric cyanohydrins... 

The primary alcohols CH3CH2CH2CH2OH and (CH3)2CHCH20H can each be prepared by hydrogenation of an aldehyde The secondary alcohol CH3CHCH2CH3 can be prepared by hydro... 

Absolute configuration (Section 7 5) The three dimensional arrangement of atoms or groups at a chirality center Acetal (Section 17 8) Product of the reaction of an aldehyde or a ketone with two moles of an alcohol according to the equation... 

Alditol (Section 25 18) The polyol obtained on reduction of the carbonyl group of a carbohydrate Aldol addition (Section 18 9) Nucleophilic addition of an aldehyde or ketone enolate to the carbonyl group of an aide hyde or a ketone The most typical case involves two mole cules of an aldehyde and is usually catalyzed by bases... 

Baeyer-Villiger oxidation (Section 17 16) Oxidation of an aldehyde or more commonly a ketone with a peroxy acid The product of Baeyer-Vilhger oxidation of a ketone is an ester... 

Cyanohydrins are formed by nucleophilic addition of HCN to the carbonyl group of an aldehyde or a ketone Cycloadd ition (Section 10 12) Addition such as the Diels-Alder reaction in which a ring is formed via a cyclic transition state... 

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