Organic 2 Review Problem 245

Organic 2 Review Problem 245 - PREPARATIONS 229...

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Unformatted text preview: PREPARATIONS 229 Benzaldehyde Phenylhydrazone. C 6 H 6 CHzN-NHC 6 H 6 . (Sem i-m icro Scale.) Phenylhydrazine condenses readily with aldehydes and ketones to give phenylhydrazones, which, being usually crystalline compounds of sharp C 6 H 5 CHO -f H 2 NNHC 6 H 5-> C 6 H 5 CHiNNHC 6 H 5-f H 2 O Benzaldehyde Phenylhydrazine Benzaldehyde Phenylhydrazone melting-point, can therefore be used to identify the aldehydes and ketones from which they have been formed. For this purpose phenylhydrazones are frequently more suitable than oximes (p. 93) since their greater molecular weight causes a lower solubility in most solvents, and they can therefore often be more easily isolated and recrystallised. The phenylhydrazones of the lower aliphatic aldehydes and ketones, however, often have low melting-points, and are thus not suitable for identification purposes: to overcome this difficulty, substituted phenylhydrazines such as />-nitrophenylhydrazine, (NO 2 C 6 H 4 NHNH 2 ), 2,4-dinitrophenylhydrazine (pp. 263, 346), and p-bromo-), 2,4-dinitrophenylhydrazine (pp....
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