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). 

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... 

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. 

Hydration of several 1,2,3-triones including indane derivatives (70 Scheme 4) has been studied in dioxane-water mixtures.1053 Monohydration gives a 2,2-diol (71) forward rates and equilibrium constants have been measured over a wide range of solvent composition. Based on activation parameters, kinetic isotope effects, a Hammett treatment, and a second-order rate dependence on water, two water molecules are suggested to play distinct roles, one as nucleophile, the other as general acid-base, similar to dialdehydes.105b,c... 

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. 

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. 

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 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 ... 

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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