AROMATIC ALDEHYDES, KETONES, AND QUINONES

The aromatic aldehydes are formed by reactions analogous to those made use of in the preparation of aliphatic aldehydes. The method which is best adapted to the preparation of the aromatic compounds is not the same as that used most conveniently in the aliphatic series. Acetic aldehyde and its homologues are usually prepared by oxidizing primary alcohols. Benzoic aldehyde, a typical aromatic aldehyde, is manufactured by heating benzal chloride, prepared from toluene, with water in the presence of lime — 

Acetic aldehyde can be formed in a similar way from ethylidene chloride, CH3.CHCI2, but its preparation by this reaction lacks practical value. There is marked difference between the ease with which benzal chloride and ethylidene chloride react with water the presence of the phenyl radical in the former brings about an activity not shown by the aliphatic halide. The choice of the compounds used in the preparation of the two classes of aldehydes is determined, to some extent, by the substances which can be obtained conveniently and at a low price. The alcohols are, in general, the starting point for the preparation of many aliphatic compounds, on account of their reactivity and the fact that they are prepared commercially on a large scale. In the aromatic series, on the other hand, the hydrocarbons are of the greatest importance as a source of other compounds. 

The aromatic aldehydes resemble in chemical properties the analogous fatty compounds but their behavior with certain reagents is unique, and they enter into a number of reactions of condensation which are of prime importance. A description of a typical aldehyde will suffice to illustrate the properties of the members of the class. 

Benzaldehyde, benzoic aldehyde, CeHs.CHO, occurs in amygdalin, a glucoside which is present in bitter almonds and in the kernels of various fruits it yields the aldehyde, hydrocyanic acid, and glucose on hydrolysis (353). The aldehyde is called oil of bitter almonds. It is used in flavoring extracts and perfumery, in the manufacture of certain dyes, and in the preparation of other compounds. 

Benzaldehyde is a liquid with an agreeable odor, which boils at 179°, and has the specific gravity 1.0504 at 15°. It can be formed by oxidizing benzyl alcohol, or distilling calcium benzoate with calcium formate. It is manufactured by heating benzal chloride with milk of lime, or by oxidizing benzyl chloride with a solution of lead nitrate. It has been prepared from toluene directly by electrolytic oxidation or by oxidation with air in the presence of a catalyst. 

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Reaction with Organic Compounds. Aluminum is not attacked by saturated or unsaturated, aUphatic or aromatic hydrocarbons. Halogenated derivatives of hydrocarbons do not generally react with aluminum except in the presence of water, which leads to the forma tion of halogen acids. The chemical stabiUty of aluminum in the presence of alcohols is very good and stabiUty is excellent in the presence of aldehydes, ketones, and quinones. 

Thus, by a combination of oxidation by lignin peroxidases, Mn(II)-dependent peroxidases and other active oxygen species and reductions of some aromatic aldehydes, acids and ketones to the corresponding benzylic alcohols, all aromatic rings in the lignin polymer can be either converted to ring opened products or to quinones/hydroquinones. These products are then further metabolized to CO2 by a currently unknown mechanism. 

As already indicated, carbonyl compounds such as ketones, aldehydes, enones, and quinones possess the property to act as effective electron acceptors in the excited state for generating radical anions in the presence of electron-donating partners such as alkenes, aromatics, ruthenium complexes, amines, and alcohols. We will not consider the reactivity of enones and quinones, but we will focus our attention on the behavior of the radical anions formed from ketones and aldehydes. Four different processes can occur from these radical anions including coupling of two radical anions and/or coupling of the radical anion with the radical cation formed from the donor, abstraction of hydrogen from the reaction media to produce alcohols, cyclization, in the case of ce-unsaturated radical anions, and fragmentation when a C -X bond (X=0, C) is present (Scheme 18). 

Aminochromans also arise from the reaction of phenolic Mannich bases with enamines (70JHC1311). The route is attractive for a number of reasons the starting materials are readily available its scope is considerable since the enamines may be aldehyde or ketone based and the Mannich bases may be aromatic or heteroaromatic and the products themselves are precursors of hydroxychromans and 4//-chromenes. Mechanistically, the synthesis proceeds through a quinone methide followed by addition to the enamine and cyclization, which may be a concerted process (Scheme 71). 

N03)j, a newcomer to the arena of oxidants, is useful for the acetoxylation of aromatic side chains in benzylic positions [415, 416] and for the oxidation of methylene or methyl groups that are adjacent to aromatic rings to carbonyl groups [238, 415, 417]. The reagent also oxidizes alcohols to aldehydes [418, 419, 420, 421] and phenols to quinones [422, 423], cleaves vicinal diols to ketones and a-hydroxy ketones to acids [424, 425], and converts diaryl sulfides into sulfoxides [426]. A specialty of ammonium cerium nitrate is the oxidative recovery of carbonyl compounds from their oximes and semicarbazones [422, 427] and of carboxylic acids from their hydrazides [428] under mild conditions. 

Oxidations with chromic oxide encompass hydroxylation of methylene [544] and methine [544, 545, 546] groups conversion of methyl groups into formyl groups [539, 547, 548, 549] or carboxylic groups [550, 55i] and of methylene groups into carbonyls [275, 552, 553, 554, 555] oxidation of aromatic hydrocarbons [556, 557, 555] and phenols [559] to quinones, of primary halides to aldehydes [540], and of secondary halides to ketones [560, 561] epoxidation of alkenes [562, 563,564, and oxidation of alkenes to ketones [565, 566] and to carboxylic acids [567, 565, 569]. 

Reactions of aldehydes and their derivatives have been the subject of several reports. Benzene-1,2-dicarboxaldehyde condenses" with 2,3-dihydro-naphthazarone to give the p-quinone (75), and a similar reaction occurs between diketones such as PhCOCH2CH2COPh and naphthalene-1,4-diol. 4-Nitrobenzaldehyde undergoes disproportionation in the presence of cyanide ions and methanol, resulting in methyl 4-nitrobenzoate as the main product. The acid-catalysed rearrangement of arylhydrazones (76) derived from aromatic aldehydes and diaryl ketones leads to amino-biphenyls (77). 

Periodic acid oxidation will also act on a-hydroxy aldehydes and ketones, a-diketones, a-keto aldehydes, and glyoxal. If the two neighboring hydroxyl groups are on an aromatic ring, the carbon-carbon bond is not cleave but the reactant is still oxidized. Thus, catechol is oxidized to the corresponding quinone. 

Oxidations by oxygen and catalysts are used for the conversion of alkanes into alcohols, ketones, or acids [54]-, for the epoxidation of alkenes [43, for the formation of alkenyl hydroperoxides [22] for the conversion of terminal alkenes into methyl ketones [60, 65] for the coupling of terminal acetylenes [2, 59, 66] for the oxidation of aromatic compounds to quinones [3] or carboxylic acids [65] for the dehydrogenation of alcohols to aldehydes [4, 55, 56] or ketones [56, 57, 62, 70] for the conversion of alcohols [56, 69], aldehydes [5, 6, 63], and ketones [52, 67] into carboxylic acids and for the oxidation of primary amines to nitriles [64], of thiols to disulfides [9] or sulfonic acids [53], of sulfoxides to sulfones [70], and of alkyl dichloroboranes to alkyl hydroperoxides [57]. 

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