Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

172 MBTA-PHBNYLBNBDIAMINB
If it is desired to avoid working with hydrogen sulphide, commercial
sodium sulphide may be used as reducing agent. 1
The complete reduction of nitro-compounds which contain several
nitro-groups is carried out in the same way as in the case of mononitroderivatives.
If, however, only partial reduction is desired, the use of
ammonium hydrosulphide is advantageous.
/NO2
NO2
C6H4< + 3 NH4SH = C6H,< + 2 H2O + 3 S + 3 NH3 .
X
NO \NH
For the reduction of nitro-compounds containing a group which
may be attacked by nascent hydrogen, as, for example, an aldehyde
group, an unsaturated side chain, and so on, special methods must be
applied. In such cases ferrous hydroxide or iron powder (cf. Chap. VII.
5, arsanilic acid) are often used. The reduction is carried out thus : a
weighed amount of ferrous sulphate is caused to act, in the presence of
alkali (potassium or sodium hydroxide, baryta), on the substance to be
reduced. In this way it is possible to reduce, for example, o-nitrobenzaldehyde
to aminobenzaldehyde, and o-nitrocinnamic acid to aminocinnamic
acid.
The yield of m-nitraniline when prepared as described above is only
about four-fifths of the theoretically possible amount. This indicates,
quite clearly, that the intermediate reduction products are reduced
further much more rapidly than is an intact nitro-group.
When m-dinitrobenzene is reduced in an acid medium the product
is m-j>henylenediamine :
NH,
-NH,
a di-acid base of technical importance. On diazotisation it yields the
brown dye vesuvine or Bismarck brown. Hence m-phenylenediamine
is used to test for traces of nitrites in water supplies.
o- and y-Nitranilines are prepared by nitration of aniline. Since
aniline is very sensitive towards oxidising agents, the amino-group must
be protected. This is most simply secured by acetylation. Acetanilide
is nitrated and, according to the conditions chosen, a large yield of either
the o- or the y-compound can be produced. The acetyl-group is subsequently
removed by hydrolysis. y-Nitraniline is also obtained in a
simpler way. y-Nitrochlorobenzene (which can be prepared by nitration
of chlorobenzene) is caused to react with ammonia at a high temperature
and under high pressure.
1 For details see Cobenzl, Chem.-Ztg., 1913, 37, 299; see also Ullmann, Enzyhlopadie,
2nd edition, vol. i. p. 470.