Uses of Aldehydes and Ketones

Kitson, G. E., Hudson, H. E. and Dickinson, N. A. The effect of various sugars on the stability of 2 amino 6 methyl 5 oxo A n propyl 4,5 tlihytlro > triazolo [1,5-tf] pyrimidine Part II The use of aldehydic and ketonic compounds to investigate the functional groups involved. Pharm. Acta. Helv. 54 359—362 1979. 

The use of aldehydes and ketones as well as carboxylic acids and their derivatives is discussed in Chapters 5-8, where their reactions with other species (such as alcohols and alkyl halides) is introduced. 

Preparation and Uses of Aldehydes and Ketones Aldehydes can be produced by the oxidation of a primary alcohol with an oxidizing agent such as dichromate ion in acidic solution. However, the aldehyde so formed is readily oxidized further to a carboxylic acid. 

NH2-C0-NH NH2,CH5N30. Colourless crystalline substance m.p. 96" C. Prepared by the electrolytic reduction of nitrourea in 20% sulphuric acid at 10 "C. Forms crystalline salts with acids. Reacts with aldehydes and ketones to give semicarbazones. Used for the isolation and identification of aldehydes and ketones. 

The most commonly used protected derivatives of aldehydes and ketones are 1,3-dioxolanes and 1,3-oxathiolanes. They are obtained from the carbonyl compounds and 1,2-ethanediol or 2-mercaptoethanol, respectively, in aprotic solvents and in the presence of catalysts, e.g. BF, (L.F. Fieser, 1954 G.E. Wilson, Jr., 1968), and water scavengers, e.g. orthoesters (P. Doyle. 1965). Acid-catalyzed exchange dioxolanation with dioxolanes of low boiling ketones, e.g. acetone, which are distilled during the reaction, can also be applied (H. J. Dauben, Jr., 1954). Selective monoketalization of diketones is often used with good success (C. Mercier, 1973). Even from diketones with two keto groups of very similar reactivity monoketals may be obtained by repeated acid-catalyzed equilibration (W.S. Johnson, 1962 A.G. Hortmann, 1969). Most aldehydes are easily converted into acetals. The ketalization of ketones is more difficult for sterical reasons and often requires long reaction times at elevated temperatures. a, -Unsaturated ketones react more slowly than saturated ketones. 2-Mercaptoethanol is more reactive than 1,2-ethanediol (J. Romo, 1951 C. Djerassi, 1952 G.E. Wilson, Jr., 1968). 

Many of the most interesting and useful reactions of aldehydes and ketones involve trans formation of the initial product of nucleophilic addition to some other substance under the reaction conditions An example is the reaction of aldehydes with alcohols under con ditions of acid catalysis The expected product of nucleophilic addition of the alcohol to the carbonyl group is called a hemiacetal The product actually isolated however cor responds to reaction of one mole of the aldehyde with two moles of alcohol to give gem mal diethers known as acetals... 

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... 

Addition of sodium dithionite to formaldehyde yields the sodium salt of hydroxymethanesulfinic acid [79-25-4] H0CH2S02Na, which retains the useful reducing character of the sodium dithionite although somewhat attenuated in reactivity. The most important organic chemistry of sodium dithionite involves its use in reducing dyes, eg, anthraquinone vat dyes, sulfur dyes, and indigo, to their soluble leuco forms (see Dyes, anthraquinone). Dithionite can reduce various chromophores that are not reduced by sulfite. Dithionite can be used for the reduction of aldehydes and ketones to alcohols (348). Quantitative studies have been made of the reduction potential of dithionite as a function of pH and the concentration of other salts (349,350). 

The aromatic primary and secondary stibines are readily oxidized by air, but they are considerably more stable than their aHphatic counterparts. Diphenylstibine is a powerful reducing agent, reacting with many acids to Hberate hydrogen (79). It has also been used for the selective reduction of aldehydes and ketones to the corresponding alcohols (80). At low temperatures, diphenylstibine undergoes an addition reaction with ketene (81) ... 

Owing to the number of different reactions in which they can be used, pyrazole aldehydes and ketones are the most interesting group of C-linked substituents. Transformations of the CHO group into CO2H, CH(OR)2, CH=CHR, etc., and of the COR group into... 

RSSiMe3 [R = Me, Et, (-CH2-)3], Zn, Et20, 0-25°, 70-95% yield. This method is satisfactory for a variety of aldehydes and ketones and is also suitable for the preparation of 1,3-dithianes. Methacrolein gives the product of Michael addition rather than the thioacetal. The less hindered of two ketones is readily protected using this methodology. ... 

Phenylhydrazine has been used in the synthesis of indole derivatives. The hydrazones of aldehydes and ketones contain-... 

The [ 2 + 4]-cycloaddition reaction of aldehydes and ketones with 1,3-dienes is a well-established synthetic procedure for the preparation of dihydropyrans which are attractive substrates for the synthesis of carbohydrates and other natural products [2]. Carbonyl compounds are usually of limited reactivity in cycloaddition reactions with dienes, because only electron-deficient carbonyl groups, as in glyoxy-lates, chloral, ketomalonate, 1,2,3-triketones, and related compounds, react with dienes which have electron-donating groups. The use of Lewis acids as catalysts for cycloaddition reactions of carbonyl compounds has, however, led to a new era for this class of reactions in synthetic organic chemistry. In particular, the application of chiral Lewis acid catalysts has provided new opportunities for enantioselec-tive cycloadditions of carbonyl compounds. 

The term Knoevenagel reaction however is used also for analogous reactions of aldehydes and ketones with various types of CH-acidic methylene compounds. The reaction belongs to a class of carbonyl reactions, that are related to the aldol reaction. The mechanism is formulated by analogy to the latter. The initial step is the deprotonation of the CH-acidic methylene compound 2. Organic bases like amines can be used for this purpose a catalytic amount of amine usually suffices. A common procedure, that uses pyridine as base as well as solvent, together with a catalytic amount of piperidine, is called the Doebner modification of the Knoevenagel reaction. 

The pinacol rearrangement reaction is of limited synthetic importance although it can be a useful alternative to the standard methods for synthesis of aldehydes and ketones." Especially in the synthesis of ketones with special substitution pattern—e.g. a spiro ketone like 5—the pinacol rearrangement demonstrates its synthetic potential ... 

Phosphonium salts containing a benzyl group may be converted into ylides by the use of only moderately strong bases such as sodium ethoxide. The preparation of benzyli-dene derivatives of aldehydes and ketones is therefore easily done. The procedure below is for the preparation of a substituted butadiene, which in turn is ideally suited for use in the Diels-Alder reaction (see Chapter 8, Section I). 

TSie processes depending on the use of sodium bisulphite or sulphite, and in which the aldehyde or ketone compounds dissolve in the solution of the reagent, are known as absorption processes, and are those most commonly employed for oils containing a high proportion of aldehydes and ketones, the use of sodium bisulphite being probably still the method most usually adopted for aldehydes, though the use of neutral sodium sulphite is the official process in the British Pharmacopoeia of 1914, and is also that most suitable for the estimation of ketones. 

Lithium aluminum hydride, LiAIH4/ is another reducing agent often used for reduction of aldehydes and ketones. A grayish powder that is soluble in ether and tetrabydrofuran, LiAlH4 is much more reactive than NaBH4 but also more dangerous. It reacts violently with water and decomposes explosively when heated above 120 °C. 

The most common reaction of aldehydes and ketones is the nucleophilic addition reaction, in which a nucleophile, Nu , adds to the electrophilic carbon of the carbonyl group. Since the nucleophile uses an electron pair to form a new bond to carbon, two electrons from the carbon-oxygen double bond must move toward the electronegative oxygen atom to give an alkoxide anion. The carbonyl carbon rehybridizes from sp2 to sp3 during the reaction, and the alkoxide ion product therefore has tetrahedral geometry. 

ThomsonNOW Click Organic Interactive to use a web-based palette to draw structures of aldehydes and ketones based on their IUPAC names. 

A particularly common cr-substitution reaction in the laboratory is the halogenation of aldehydes and ketones at their a- positions by reaction with Cl2, Br2, or I2 in acidic solution. Bromine in acetic acid solvent is often used. 

Triarylamines have been employed in arylene vinylene AB copolymers 38 by Horhold et al. using a Homer polycondensation route of aldehydes and ketones 36 with fois-phosphonate 37 (Scheme 1-12) 164]. Phenylamines have remarkably low redox potentials and their charge transport properties have been investigated extensively [65]. EL devices comprising triarylamines have demonstrated low driving voltages. 

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