SOME IMPORTANT ALDEHYDES

Formaldehyde is a colorless gas with a pungent odor. It is the simplest aldehyde and was discovered by the Russian chemist A. Butlerov in 1859. It is readily soluble in water and it is most commonly used as a 40% solution in water known as formalin. Formaldehyde is widely used to manufacture building materials and numerous household products. 

If H2 is removed from a molecule, it is called dehydrogenation. If H2 is added, it is called hydrogenation. Hydrogenation is a reduction reaction and dehydrogenation is an oxidation reaction. 

As aldehydes are formed by the dehydrogenation of alcohols, their names are derived from the first syllables (al- and dehyde-) of these words. 

It is seen that the copper foil is covered with black copper (II) oxide (CuO) 

Hot copper oxide reacts with ethyl alcohol, it quickly oxidizes the ethanol to ethanal and it itself is reduced to bright metallic copper as before. 

---

The structures of some important aldehydes and ketones are shown below ... 

Instead of a-halo esters, related reactants can be used e.g. the a-halo derivatives of ketones, nitriles, sulfones and A,A-disubstituted amides. The Darzens condensation is also of some importance as a synthetic method because a glycidic acid can be converted into the next higher homolog of the original aldehyde, or into a branched aldehyde (e.g. 5) if the original carbonyl substrate was a ketone ... 

Betterton, E. A., Hoffmann, M. R. (1988) Henry s law constants of some environmentally important aldehydes. Environ. Sci. Technol. 22, 1415-1418. 

The nucleophilic displacement of suitable para substituents in analogues of malachite green is used as a means of synthesis of some important acid dyes. For example, Cl Acid Blue 83 (6.176 R = H) is made by the aldehyde method, using 4-chlorobenzaldehyde and N-ethyl-N-(3-sulphobenzyl)aniline. The resulting leuco base is oxidised to the colour salt,... 

Of some importance as textile dyes are aza analogues of polymethine (cyanine) dyes. Azacarbocyanines result when Fischer s aldehyde is heated with primary aromatic amines. Thus Cl Basic Yellow 11 (6.220) is obtained when Fischer s aldehyde is condensed with 2,4-dimethoxyaniline. The equivalent reaction with 2-methylindoline gives Cl Basic Yellow 21 (6.221), which has superior light fastness but has been classified by ETAD as toxic [73]. The tinctorially strong golden yellow diazacarbocyanine dye Cl Basic Yellow 28 (6.222) is prepared by coupling diazotised p-anisidine with Fischer s base (6.223), followed by quaternisation with dimethyl sulphate. Some triazacarbocyanine dyes are also used commercially. 

IPreparation of Mldehydes Some important methods for the preparation of aldehydes and Ketones and ketones are as follows ... 

The photocycloaddition of a carbonyl compound to an alkene was discovered as early as 1909 by Paterno and Chiefifi [78] who employed sunlight as the irradiation source. In the 1950s the reaction was more intensively investigated by Biichi et al. [79] using artificial light sources. The Paterno-Biichi reaction has been studied mechanistically [80] and some important aspects are summarized in Scheme 37. Upon n r -excitation (1=280-350 nm), aldehydes... 

Partially hydrogenated quinoline cores are also present in some important bioactive compounds. For example, the 4-aza-analogs of Podophyllotoxin, a plant lignan that inhibits microtubule assembly, revealed to be more potent and less toxic anticancer agents. In 2006, Ji s group reported a green multicomponent approach to a new series of these derivatives, consisting of the reaction of either tetronic acid or 1,3-indanedione with various aldehydes and substituted anilines in water under microwave irradiation conditions (Scheme 26) [107]. For this efficient and eco-friendly transformation, the authors proposed a mechanism quite similar to the one that was postulated for the synthesis of tetrahydroquinolines in the precedent section. 

Figures 4.26, 4.27, and 4.28 show typical UV absorption spectra for some simple aldehydes and ketones (Rogers, 1990 Martinez et al., 1992 see also Cronin and Zhu, 1998, for n-pentanal). Formaldehyde stands out from the higher aldehydes and ketones in that it has a highly structured spectrum and furthermore, the absorption extends out to longer wavelengths. The latter difference is particularly important because the solar intensity increases rapidly with wavelength here (Chapter 3.C.1) and hence the photolysis rate constant for HCHO and the rate of production of free radicals...

The absolute values of the absorption cross sections of HCHO have been somewhat controversial. This appears to be due to a lack of sufficient resolution in some studies as discussed in Chapter 3.B.2, if the spectral resolution is too low relative to the bandwidth, nonlinear Beer-Lambert plots result. The strongly banded structure means that calculations of the photolysis rate constant require actinic flux data that have much finer resolution than the 2- to 5-nm intervals for which these flux data are given in Chapter 3 or, alternatively, that the measured absorption cross sections must be appropriately averaged. One significant advantage of the highly structured absorption of HCHO is that it can be used to measure low concentrations of this important aldehyde in the atmosphere by UV absorption (see Sections A.ld and A.4f in Chapter 11.). 

Table 8.3 shows the solubilities of some potentially important aldehydes in the form of the Henry s law constant (H) and the effective Henry s law constant (// ) (Betterton and Hoffmann, 1988 Olson and Hoffmann, 1989). These aldehydes not only dissolve in aqueous solutions but also hydrate to form gera-diols (Buschmann et al., 1980, 1982) ... 

The Cannizzaro reaction, that is, the base-catalysed disproportionation of a carbonyl compound to an alcohol and a carboxylic acid, has gained some importance as an economically viable alternative to the reduction with borohydrides. However, the reaction is restricted to carbonyl compounds without any a-hydrogen, which do not undergo competing aldol reactions. Thus, mainly aromatic aldehydes are used for this kind of transformation. The protocols developed for microwave applications typically involve solvent-free conditions using alumina as the solid support. Under these conditions, a significant acceleration of the reaction was achieved. 

Linoleum was analyzed by Jensen, WolkofFand Wilkins (1995,1996) using GC—O and GC—MS. The most important odorants identified were butanal, hexanal, heptanal, octanal, nonanal, decanal, acetic acid, propionic acid, butyric acid, hexanoic acid, toluene, 2-pentylfuran. The authors state that they might have overlooked some unsaturated aldehydes with very low odor thresholds. They were able to closely simulate the odor of linoleum by mixing the compounds according to... 

The assessment of the chemistry and the rehearsal of the monomer set were run in parallel, with some important findings. Many natural a-amino acid esters (Mi) gave good results, as did the aromatic aldehydes (M2) in general, while aliphatic and deactivated aromatic aldehydes were less satisfactory and thus rejected. Commercially... 

Finally, some important studies of the influence of pH in the reaction between formaldehyde and amine in biological. systems, in the aminomethylation of polyacrylamide, and in the use of aldehydes other than foiTnaldchyde- - lead to the conclusion that under acidic conditions, aldehyde attack by the amine is the rate-determining step. Under neutral or basic conditions the rate-determining step is hydroxyl elimination from the methylolamine HO—CH2—NR2 with formation of the methyl-cncimmonium cation. - Polarographic methods demonstrate that the maximum concentration of this last species occurs at a pH near 10. 

Finally, mention should be made of the Grundmann method which, though long winded and unlikely to be of use today, was once of some importance. It is summarized in Scheme 9. In favorable cases the sequence could be carried through in over 50% overall yield, and it was valued for its ability to synthesize aldehydes inaccessible via the classical Rosenmund. 

Scheme 21). Some important a-sulfinyl carbanions (42) are derived from dithioacetal S-oxides (43), and on subsequent alkylation and hydrolysis they yield the aldehydes (40) (Scheme 22).

Catalytic oxidative transformations of lower alkanes attract the attention as possible ways to transfer these substances into more suitable chemicals - olefins and oxygenates (alcohols, aldehydes, acids, etc.) - and to involve them into the industrial use as raw materials for chemical and petrochemical synthesis. However, the yields of desirable products reached up to date are not sufficiently high. The progress in the studies of intrinsic mechanism of catalytic partial oxidation of lower alkanes is not sustainable either. We believe that these two facts are correlated and that the analysis we performed in the present work can brighten up some important details of the mechanism of catalytic oxidation of lower alkanes. ... 

---