Ninhydrin indane-1 2 3-trione hydrate

Ninhydrin (also named 1 2 3-triketoindane or 1 2 3-triketohydrindene hydrate) is prepared most simply from the inexpensive phthahc anhydride (I). The latter is condensed with acetic anhydride In the presence of potassium acetate to give phthalylacetlc acid (II) reaction of the latter with sodium methoxide in methanol yields 1 3-indanedionecarboxyhc acid, which is decomposed upon warming with dilute hydrochloric or sulphuric acid to indane-1 3-dione (or 1 3-diketohydrindene) (HI). Selenium dioxide oxidation of (III) afibrds indane-1 2 3-trione hydrate (ninhydrin) (IV). 

1 3 Indanedioiie (III) may also be prepared by condensation of diethyl phthalate (V) with ethyl acetate in the presence of sodium ethoxide the resulting sodium 1 3-indanedione-2-carboxylic ester (VI) upon warming with sulphuric acid yields (HI). 

Phthalylacetic acid. Heat a mixture of 30 g. of phthalic anhydride, 40 ml. of acetic anhydride and 5 g. of potassium acetate under reflux in an oil bath at 155-165° for 15 minutes. Pour the reaction mixture into ice-cold water, collect the yellow precipitate by suction filtration, wash it three times with 25 ml. of water and once with 10 ml. of 50 per cent, ethanol. Dry the. product at 100° the yield of crude plithalylaeetie acid is 20 g. Recrystallise from hot methanol yellow needles, m.p. 245-246°, are obtained. 

Indane-1 3-dione (1 3-diketohydrindene). Method A. To a solution of sodium methoxide, prepared from 6 1 g. of sodium and 200 ml. of anhydrous methanol, add 15 g. of phthalylacetic acid and allow to stand for 1 hour at room temperature collect the yellow precipitate by suction filtration. Mix the yellow solid with 150 ml. of 10 per cent, sulphuric acid, heat on a steam bath until no more carbon dioxide is evolved (15-20 minutes), filter the hot solution and allow to cool. Collect the yellow crystals by filtration at the pump, wash with a httle water and dry at 100°. The yield of crude 1 3-indanedione, m.p. 125-126°, is 7 g. RecrystaUise from hght petroleum, b.p. 80-100°, and thus obtain the pure product, m.p. 129-130°. 

Method B. Place 125 g. (106 -5 ml.) of diethyl phthalate and 25 g. of molecular sodium (sodium sand see Section 11,50,6) in a 500 ml. round-bottomed flask fitted with a reflux condenser and dropping funnel. Heat the flask on a steam bath and add a mixture of 122 5 g. (136 ml.) of dry ethyl acetate and 2 5 ml. of absolute ethanol over a period of 90 minutes. Continue the heating for 6 hours, cool and add 50 ml. of ether. Filter the sodium salt (VI) on a sintered glass funnel and wash it with the minimum volume of ether. Dissolve the sodium salt (96 g.) in 1400 ml. of hot water in a 3-htre beaker, cool the solution to 70°, stir vigorously and add 100 ml. of sulphuric acid (3 parts of concentrated acid to 1 part of 

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Indane-1 2 3-trione hydrate (ninhydrin). In a 500 ml. threenecked flask, fitted with a reflux condenser and mechanical stirrer, place 11 g. of resublimed selenium dioxide dissolved in 240 ml. of dioxan and 6 ml. of water. Heat the stirred solution to 60-70°, remove the source of heat, add 15 g. of crude 1 3-indanedione and reflux the resulting mixture for 6 hours. A solid separates during this period. Filter the mixture, transfer the filtrate to a distilling flask and distil off about 180 ml. of dioxan then add 100 ml. of water, boil the solution to coagulate the red tarry precipitate and remove it by filtration. Concentrate the filtrate to about 50 ml. and filter. Boil the filtrate with 0- 2-0- 3 g. of decolourising carbon, filter again, concentrate to 20-25 ml. and keep at room temperature. Collect the crystals of crude ninhydrin by suction filtration, and recrystallise from hot water with the addition of a little decolourising carlran, if necessary. The yield of colourless ninhydrin is 6 g. the crystals turn red between 125° and 130° and melt at 242-243°. 

Immiscible liquids, 12 boiling point of, 13, 14 vapour pressure of, 13, 14 Incongruent melting point, 31 Indane-1 2-dione, 903, 904 Indane-1 2 3-trione hydrate, see Ninhydrin Indigo, 980... 

Ninhydrin is the hydrate of indane-l,2,3-trione. It may cause skin irritation and discolor the skin. Use latex gloves when preparing the solution and avoid getting the spray solution on the skin. 

The simpler nitrogenous constituents of wort consist principally of a-amino acids which can be estimated by the colour reaction either with indane-1,2, 3-trione hydrate (ninhydrin) or 2,4,6-trinitrobenzenesulphonic acid. With ninhydrin, a-amino acids develop a violet colour which can be measured at 570 nm while proline, an important imino acid in wort and beer, gives a yellow colour measured at 440 nm. 2,4,6-Trinitrobenzenesulphonic acid is more specific, for amino acids and does not react with proline or ammonia. It forms yellow derivatives which can be estimated colorimetrically at 340 nm. 

Ninhydrin is the hydrate of indane-l,2,3-trione. With the exception of proline and hydroxyprohne, all of the a-amino acids found in proteins react with ninhydrin to give the same intensely colored purple anion (A ,ax 570 nm). We shall not go into the mechanism here, but notice that the only portion of the anion that is derived from the a-amino acid is the nitrogen ... 

Note. (1) Ninhydrin (p. 630) is the 2-hydrate of indane-l,2,3-trione. It reacts with a-amino acids to yield highly coloured products. Contact with the skin should be avoided since it produces a rather long-lasting purple discoloration. 

Another experimental result is also consistent with the hypothesis that cyclic 1,2,3-triketones are not stable. 2-Diazoindan-l,3-dione (9.91) is oxidized by eA butyl hypochlorite in ethanol to the 2-monoacetal 9.92 of indan-l,2,3-trione. The monoacetal undergoes hydrolysis to 2,2-dihydroxyindan-l,3-dione (ninhydrin hydrate 9.93), but the trione itself could not be identified (9-41). 

Ninhydrin (Section 23.3) The hydrate of indan-l,2,3-trione, a molecule that reacts with amino acids to give the purple dye used in quantitative analysis of amino acids. 

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