Some Common Aldehydes and Ketones

PROBLEM 9.1 Using the examples as a guide, write a structure for 

Formaldehyde (HCH=0), the simplest aldehyde, is manufactured on a very large scale by the oxidation of methanol. 

Annual world production is more than 46 billion pounds. Formaldehyde is a gas (bp -21°C), but it cannot be stored in a free state because it polymerizes readily.  

Normally it is supplied as a 37% aqueous solution called formalin. In this form, it is used as a disinfectant and preservative, but formaldehyde is mostly used in the manufacture of plastics, building insulation, particleboard, and plywood. 

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Table 16.1 lists the physical properties of a number of common aldehydes and ketones. Some aldehydes obtained from natural sources have very pleasant fragrances. The following are some in addition to those we mentioned at the beginning of this chapter. 

List some examples of common aldehydes and ketones and where you might find fhem. 

The common names of some aromatic aldehydes and ketones include the following. 

Phenylhydrazine on exposure to light slowly darkens and eventually becomes deep red in colour salts of the base share this property but to a lesser degree, the sulphate and acetate (of the common salts) being most stable to light. Phenylhydrazine is largely used in organic chemistry to characterise aldehydes and ketones as their phenyl-hydrazones (pp. 342, 345), and carbohydrates as their osazones (pp. 136-140). It is readily reduced thus in the process of osazone formation some of the phenylhydrazine is reduced to aniline and ammonia. On the... 

However, in some cases azines can be converted to hydrazones by treatment with excess hydrazine and NaOH. Arylhydrazines, especially phenyl, p-nitrophenyl, and 2,4-dinitrophenyl, are used much more often and give the corresponding hydrazones with most aldehydes and ketones.Since these are usually solids, they make excellent derivatives and are commonly employed for this purpose. Cyclic hydrazones are also known, ° as are conjugated hydrazones. a-Hydroxy aldehydes and ketones and ot-dicarbonyl compounds give osazones, in which two adjacent carbons have carbon-nitrogen double bonds ... 

The common and lUPAC names of some aldehydes and ketones are given in Table 12.1. 

Table 12.1 Common and lUPAC Names of Some Aldehydes and Ketones...

The addition of RLi and other nucleophiles to carbonyl functions in general proceeds via one of the two possible reaction pathways, polar addition (PL) and electron transfer (ET)-radical coupling (RC) sequence (equation 5). Current reaction design for the synthetic purpose of additions of common nucleophiles to aldehydes and ketones is mostly based on the polar mechanism, but apparently the ET process is involved in some reactions of, for example, Grignard reagents Mechanistically there are three possible variations the PL pathway, the ET rate-determining ET-RC route and the RC rate-determining ET-RC route. 

The commonly used reducing agents AlH and BH will reduce aldehydes and ketones to primary and secondary alkoxides.109 Similar reactivity is found for some other main group hydrides, e.g. gallium (equation 33).110... 

Schiff bases commonly employ polyamines as reagents, the donor nature of the ligands depending both on the type of aldehyde or ketone used and the nature of primary amine or diamine. Scheme 1 illustrates some ways of obtaining Schiff bases of different denticities via 1 1 reactions of aldehydes with ketones to yield mixed N, O, S, and Se donor sets. Condensation of aldehydes and ketones with a-amino acids is also known. Where two moles of a carbonyl compounds and one mole of diamine are reacted, more elaborate Schiff bases with higher denticity result, also exemplified in Scheme 1. 

Other Uses. Acetals of 2,4-pentanediol have also been prepared in order to determine the enantiomeric purity of aldehydes and ketones by analysis of diastereomers by GC or NMR. 2,i-ButanedioP is more commonly used for this purpose but has been shown to be less effective in some cases. 

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