342 PRACTICAL ORGANIC CHEMISTRY After 1-2 minutes, a flocculent precipitate of the phenylhydrazone, RCH:NNHC 6 H 5 , is produced. Filter and recrystallise from ethanol and take the m.p. (M.ps., p. 539-540.) The phenylhydrazones of formaldehyde and acetaldehyde are difficult to isolate and are seldom prepared. ( b ) 2^-Dinitrophenylhydrazones. To a few drops of formalin, add a few drops of 2,4-dinitrophenylhydrazine reagent A (p. 261): a yellow precipitate is produced in the cold. Acetaldehyde gives an orange-coloured precipitate. Dissolve a few drops of benzaldehyde or salicylal-dehyde in 2 ml. of methanol and then add a few drops of the reagent B: an orange-coloured precipitate is obtained. In each case filter and recrystallise from ethanol. Take the m.p. (M.ps., p. 539-540.) (c) Semicarbazones (cf. p. 258). Add 0-5 g. of semicarbazide hydro-chloride to 5 ml. of water, add 0-5 g. of anhydrous sodium acetate and warm gently until a clear solution is obtained. Then add a solution of 0-5 ml. of benzaldehyde in 2-3 ml. of ethanol, and warm on a water-
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This note was uploaded on 09/19/2011 for the course CHM 2211 taught by Professor Castalleano during the Fall '06 term at University of Florida.