Benzylamine Gabriel synthesis

Benzyl phthalimide. Grind together 53 g. of finely-powdered, anhydrous potassium carbonate and 147 g. of phthalimide (Section IV,169) in a glass mortar, transfer the mixture to a 750 ml. round-bottomed flask, and treat it with 252 g. (230 ml.) of redistilled benzyl chloride. Heat in an oil bath at 190° under a reflux condenser for 3 hours. Whilst the mixture is still hot, remove the excess of benzyl chloride by steam distillation. The benzyl phthalimide commences to crystallise near the end of the steam distillation. At this point, cool the mixture rapidly with vigorous stirring so that the solid is obtained in a fine state of division. Filter the solid with suction on a Buchner funnel, wash well with water and drain as completely as possible then wash once with 200 ml. of 60 per cent, ethanol and drain again. The yield of crude product, m.p. 100-110°, is 180 g. Recrystallise from glacial acetic acid to obtain pure benzyl phthalimide, m.p. 116° the recovery is about 80 per cent. 

Benzylamine. Warm an alcoholic suspension of 118-5 g. of finely-powdered benzyl phthalimide with 25 g. of 100 per cent, hydrazine hydrate (CAUTION corrosive liquid) a white, gelatinous precipitate is produced rapidly. Decompose the latter (when its formation appears complete) by heating with excess of hydrochloric acid on a steam bath. Collect the phthalyl hydrazide which separates by suction filtration, and wash it with a little water. Concentrate the filtrate by distillation to remove alcohol, cool, filter from the small amount of precipitated phthalyl hydrazide, render alkaline with excess of sodium hydroxide solution, and extract the liberated benzylamine with ether. Dry the ethereal solution with potassium hydroxide pellets, remove the solvent (compare Fig. II, 13, 4) on a water bath and finally distil the residue. Collect the benzylamine at 185-187° the yield is 50 g. 

N- Nitrosomethylaniline (methylphenylnitrosamine). Place 53-5 g. of pure commercial monomethylaniline, 72 -5 ml. of concentrated hydrochloric acid and 200 g. of crushed ice in a 500 ml. beaker equipped with a mechanical stirrer. Support a separatory funnel with a long bent stem (as in Fig. Ill, 35,1) containing a solution of 36 g. of sodium nitrite in 125 ml. of water over the beaker. Stir the solution and run in the sodium nitrite solution during 10 minutes do not allow the temperature to rise above 10° and add more ice if necessary. Continue the stirring for a further hour. Separate the oily layer, wash it once with 50 ml. of water, and dry it with anhydrous magnesium or calcium sulphate. Distil under reduced pressure from a 100 ml. Claisen flask. Collect the V-nitrosomethylaniline (a pale yellow liquid) at 120°/l3 mm. The yield is about 65 g. 

Reduction of AT-nitrosomethylaniline. Into a 1 litre round-bottomed flask, fitted with a reflux condenser, place 39 g. of V nitroso-methylaniline and 75 g. of granulated tin. Add 150 ml. of concentrated hydrochloric acid in portions of 25 ml. (compare Section IV,34) do not add the second portion until the vigorous action produced by the previous portion has subsided, etc. Heat the reaction mixture on a water bath for 45 minutes, and allow to cool. Add cautiously a solution of 135 g. of sodium hydroxide in 175 ml. of water, and steam distil (see Fig. II, 40, 1) collect about 500 ml. of distillate. Saturate the solution with salt, separate the organic layer, extract the aqueous layer with 50 ml. of ether and combine the extract with the organic layer. Dry with anhydrous potassium carbonate, remove the ether on a water bath (compare Fig. II, 13, 4), and distil the residual liquid using an air bath (Fig. II, 5, 3). Collect the pure methylaniline at 193-194° as a colourless liquid. The yield is 23 g. 

JV-Nitrosoethylaniline (ethylphenylnitrosoamine). Use 60-5 g. of pure commercial monoethylaniline, 72-5 ml. of concentrated hydrochloric acid and 200 g. of crushed ice also 36 g. of sodium nitrite in 125 ml. of water. The yield of N-nitrosoethylaniline (a yellow liquid), b.p. 131°/20 mm., is about 65 g. 

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Chapter IV. a-Chloromethylnaphthalene (IV,23) benzylamine (Gabriel synthesis) (IV,39) i r.N -dialkylanilines (from amines and trialkyl orthophosphates) (IV,42) a-naphthaldehyde (Sommelet reaction) (IV,120) a-phenyl-cinnamic acid (Perkin reaction using triethylamine) (IV,124) p-nitrostyrene (IV,129) p-bromonaphthalene and p naphthoic acid (from 2 naphthylamine-1 -sulphonic acid) (IV,62 and IV,164) diphenic acid (from phenanthrene) (IV,165). 

Benzylamine may be obtained by the Gabriel synthesis, which depends upon the use of potassium phthalimide. The latter upon heating with benzyl... 

The Gabriel synthesis is based on work carried out by Siegmund Gabriel at the Uni versity of Berlin in the 1880s A detailed discussion of each step in the Gabriel synthesis of benzylamine can be found in the October 1975 Journal of Chemical Education (pp 670-671)... 

The Gabriel synthesis is illustrated by the preparation of benzylamine and 2-phenylethylamine (Expt 5.198). 

PRACTICE PROBLEM 20.4 Outline a preparation of benzylamine using the Gabriel synthesis. 

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