Crystalline derivatives

Occasionally benzoyl chloride gives a product which does not crystallise well in such cases the difficulty may frequently be overcome by using p-nitrobenzoyl chloride, N02C,H4C0C1, or 3,5-dinitrobenzoyl chloride, (NOiljCjHjCOCl (p. 240), which usually give highly crystalline derivatives. 

A crystalline derivative of benzyl alcohol cannot be obtained by using benzoyl chloride, because the benzyl benzoate, C HiCOOCHiCaHj, so obtained has m.p. 18°, and is thus usually liquid the present preparation illustrates therefore the use of a substituted benzoyl chloride (p-nitrobenzoyl chloride, m.p. 75°) in order to obtain a crystalline derivative of suitably high m.p. 

In eachof the Sections 10-27, the chief physical propertiesof the compounds concerned are Erst briefly indicated a list of the gen eral reactionsf is then given, followed by practical directions for the application or illustration of these general reactions. The types of crystalline derivatives which can be most readily and reliably prepared are then given, with practical examples. Finally, any important special reactions of individual members are described. 

Urethanes. The reaction with phenylisocyanate should be used for crystalline derivative formation (see below), and not as a general reaction for alcohols. 

Aliphatic and aromatic acids, simple and substituted, vary con siderably in their properties, and no one reaction for the preparation of crystalline derivatives is general. The following are recommended as most promising. 

Aqueous solutions give with ferric chloride the characteristic reactions of the neutral salt of the acid (p. 348). Identify the carboxylic acids by the tests already given (Section 14, p. 347), or by the preparation of one of the crystalline derivatives below. 

D) No general reaction can be cited for the preparation of crystalline derivatives of Class (iii). Triphenylamine, when nitrated in acetic acid with fuming nitric acid, gives tri-/>-nitrophenylamine, m.p. 280°. The presence of substituents in the phenyl groups may however complicate or invalidate nitration. 

These are not essential if the acid and amine obtained on hydrolysis are crystalline but crystalline derivatives of the acid and amine can be prepared by the standard methods for acids (p. 349) and amines (pp. 374, 376) respectively. 

No very general rules can be given with regard to the formation of crystalline derivatives of aromatic hydrocarlwns. Their reactivities towards a particular reagent vary considerably and complications often arise owing to the production of isomers. 

As stated above, ethers do not lend themselves to the ready formation of crystalline derivatives. The following however, are recommended. 

Tables IV-XXVIII (pp. 560—561) record the m.ps. of crystalline derivatives of a much wider range of compounds, and are intended primarily for the more advanced student.

First, considerably greater emphasis has been placed on semimicro techniques and their application to preparations, separations, analysis and physical determinations such as those of molecular weight. We have therefore greatly expanded the section on Manipulation on a semi-micro scale which was in the Third Edition, and we have described many more preparations on this scale, some independent and others as alternatives to the larger-scale preparations which immediately precede them. Some 40 separate preparations on the semi-micro scale are described in detail, in addition to specific directions for the preparation of many classes of crystalline derivatives required for identification purposes. The equipment required for these small-scale reactions has been selected on a realistic basis, and care has been taken not to include the very curious pieces of apparatus sometimes suggested as necessary for working on the semi-micro scale. 

In Part III, dealing with the Reactions and Identification of Organic Compounds, greater emphasis has now been placed on the preparation of suitable crystalline derivatives. Quite apart from the importance of these derivatives for purposes of identification, encouragement is thereby given to the student to gain experience in small-scale preparative work. 

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