Reaction classes

Application of class reactions. The apphcation of selected reactions that indicate the presence or absence of certain functional groups, with due regard to the indications provided by tests 1, 2 and 3, will locate the class (or classes) to which the compound belongs or wUl, at least, serve to eliminate all but a few classes to which the compound can be assigned. 

If an unknown compound gives a positive test with the 2 4-dinitrophenylhydrazine reagent, it then becomes necessary to decide whether it is an aldehyde or a ketone. Although the dimedone reagent (Section 111,70,2) reacts only with aldehydes, it is hardly satisfactory for routine use in class reactions. It is much simpler to make use of three other reagents given below, the preparation and properties of which have already been described (Section 111,70). 

When the compound for identification fails to respond to test 4 (aldehyde or ketone), the next class reactions to apply are the hydroxatnic acid teat and saponification, i.e., hydrolysis in alkaline solution. These are the class reactions for esters and anhydrides the rarely-encountered lactones react similarly. 

The production of both an alcohol and the sodium salt of an acid might easily be confused with the hydrolysis products of an ester (in the above instance benzyl benzoate). Such an error would soon be discovered (e.g., by reference to the b.p. and other physical properties), but it would lead to an unnecessary expenditure of time and energy. The above example, however, emphasises the importance of conducting the class reactions of neutral oxygen-containing compounds in the proper order, viz., (1) aldehydes and ketones, (2) esters and anhydrides, (3) alcohols, and (4) ethers. 

If the unknown neutral, oxygen-containing compound does not give the class reactions for aldehydes, ketones, esters and anhydrides, it is probably... 

It is convenient to summarise the more important class reactions given in this Section. 

The reactions of benzyl chloride, benzal chloride, and ben zotricbl oride may be divided into two classes (/) reactions of the side chain containing the halogen and (2) reactions of the aromatic ring. 

The majority of analgesics can be classified as either central or peripheral on the basis of their mode of action. Structural characteristics usually follow the same divisions the former show some relation to the opioids while the latter can be recognized as NSAlD s. The triamino pyridine 17 is an analgesic which does not seem to belong stmcturally to either class. Reaction of substituted pyridine 13 (obtainable from 12 by nitration ) with benzylamine 14 leads to the product from replacement of the methoxyl group (15). The reaction probably proceeds by the addition elimination sequence characteristic of heterocyclic nucleophilic displacements. Reduction of the nitro group with Raney nickel gives triamine 16. Acylation of the product with ethyl chlorofor-mate produces flupirtine (17) [4]. 

Suggestions as to the methods for identifying the above classes of compounds will be found under Class Reactions in Section XI,7. Some further remarks upon enolic compounds (see Table IV,114A) may be made here. Enols may be divided into (o) p-keto esters and (6) 1 3-diketones. With 5 per cent, sodium hydroxide solution, a p-keto ester yields the salt of the corresponding acid, which when heated with dilute hydrochloric acid is decarboxylated to a ketone ... 

No class reaction has been proposed for the measurement of total sterols. Instead, various fractionation methods, usually derived from the biochemical litreature, have been adapted to the concentrated materials collected from seawater. Certain of the more important sterols, particularly those used in the evaluation of water quality, have been determined by the use of a compound-specific reaction, after concentration from solution. Thus Wann et al. [405,... 

During protein synthesis and afterwards, proteins can undergo substantial covalent modification. The types(s) and extent of modification will depend on the protein, and often play an important role in the biological function of the protein. In general, protein modifications can be divided into two major classes reactions on the side chains of the amino acids, and cleavages of the peptide backbone. 

---