THE PREPARATION OF NINHYDRIN AS A LABORATORY EXPERIMENT
0
XORGE ALEJANDRO DOMINGUEZ Instituto Tecnologico y d e Estudios Superiores d e Monterrey, Monterrey, Mexico
NINHVDRIN (triketohydrindene hydrate) is a very useful colorimetric reagent for the identification and analysis of amino acids (1, f , 5) and its use has been extended to the paper chromatography of amino acids and protein hydrolyzates (4). Several simplified techniques for the use of this reagent in undergraduate biochemical and organic experiments have been proposed (6). Unfortunately ninhydrin ir; very expensive, but it can be synthesized by any undergraduate student in three laboratory sessions, some very interesting reactions being involved. Starting with cheap phthalic anhydride, which is condensed with acetic anhydride using potassium acetate as catalyst (6) (pyridine is inactive), the phthalylacetic acid (I) obtained is converted to 1,3hydrandione-carboxylic acid with a methanol solution of sodium methcxide. This carboxylic acid is decomposed with hydrochloric acid and the l,3-hydrindanedione (11) obtained is oxidized with selenium dioxide (7) to ninhydrin, which is then purified (8).
tl
8 I
phthalic anhydride
0
0
yC'CH2 0 Il
3,
II
/C\
\c/
\ \C/
II
II
0 I1
OH
C
