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DNPH (Brady’s reagent)

Dear Reader,

It has come to my attention that at a series of schools Brady’s reagent (2,4-dinitrophenyl hydrazine) has been stored incorrectly, this is an orange solid which should be stored wet with water to prevent explosions.

2,4-DNPH is a very useful reagent which is used to convert aldehydes and ketones into yellow crystalline derivatives. The nitro groups on the benzene ring help make these compounds nice and crystalline. As a student I recall the many hours we spent making and purifying DNPH derivatives when I was an undergraduate at Imperial College. I recall once having to make a derivative and then recrystallize it to a constant melting point.

The great problem appears that some years ago the use of DNPH in schools was scrapped, then old bottles of this compound were left to gather dust and dry out. These bottles were later found and then as a result the police and even the military have been called out to dispose of them.

Some of you might wounder what 2,4-DNPH looks like, here is a picture of the compound. N. Okabe, T. Nakamura and H. Fukuda in Acta Crystallogr.,Sect.C:Cryst.Struct.Commun. volume 49, page 1678 reported the crystal structure of this compound in 1993. Here we can see a picture of the molecule.


The molecular structure of Brady’s reagent

Now lets look at it, the nitrogen with only one hydrogen has a trigonal planar geometry while the other nitrogen bearing hydrogens (white balls) has a distorted trigonal based pyramid geometry. This is an important difference, the trigonal flat nitrogen is likely to be sp2 hybridised. The lone pair on it is clearly part of the extended pi system formed by the benzene ring and the nitro groups (groups made of orange [oxygen atoms] and blue balls [nitrogen atoms]). The black balls are carbons while the white tiny ones are hydrogens.

The other hydrazine nitrogen looks to me like it is sp3 hybridised.


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