Answer 1:
The decomposition products of p-nitroaniline
(also known as 4-nitroaniline) and sulfuric acid
are not known exactly, but the reaction has been
studied by NASA for its ability to form a
pyrostat, which is a fire quenching substance.
The decomposition reaction forms a solid brown-
black foam. A substance that forms foam when
heated is known as an aphrogen. Therefore p-
nitroaniline is an aphrogenic pyrostat. The
structure of p-nitroaniline is shown here
pnitroaniline
According to the article from
NASA, “nitroaniline-sulfuric acid decomposition
consists of three stages:”
1) “The first, or pre-expansion, stage is
the interval up to about 230°C. Water is the
major gaseous constituent, with some
nitroaniline subliming at higher mole ratios;
sulfur dioxide is absent (iodine test).
Dehydration(i) and sulfonation(ii) are the main
chemical processes; some deamination (iii) also
takes place.
2) “The second, or aphrogenic
(intumescent), stage begins at about 230°C for 4-
nitroaniline … and is over within a 50° interval
when the heating rate is 6°/min (iv). Sulfur
dioxide and water are major gaseous products
(v).
3) “The third stage, in air, represents
oxidative decomposition of the residue (the
primary product of the exothermal reaction) (vi)
and dissociation of variable amounts of ammonium
salts present in the residue (vii).”
(i) A dehydration reaction involves the loss
of water from a molecule.
(ii) Sulfonation is a reaction in which an
aromatic ring reacts with sulfuric acid to
replace a hydrogen with a sulfonic acid group
(SO3H).
(iii) Deamination is the loss of an amine
group (NH3) from a compound.
(iv) This is the part where the foam expands
rapidly.
(v) Carbon dioxide is also likely produced at
this stage.
(vi) Essentially this means that the foam
oxidizes in air after the expansion.
(vii) The deamination leads to ammonium salts
remaining in the foam.
The relative atomic proportion of elements in
p-
nitroaniline and in the foam is:
Element |
p-nitroaniline |
foam |
C |
6 |
6.0 |
H |
6 |
~3.0 |
N |
2 |
~1.8 |
S |
0 |
~0.15 |
O |
2 |
~1.3 |
The foam is most likely a cross-linked
network
formed from whatever remains of the nitroaniline
and therefore the reaction is sometimes referred
to as an explosive polymerization.
You can watch a video of the reaction on
youtube
here:
watch reaction
If you are interested, here are some other
interesting excerpts from the article:
“The mole ratio of nitroaniline to sulfuric
acid, however, is not critical; voluminous foams
have been obtained from mixtures over a mole
ratio range of 0.3 to 2.0 if a proper heating
schedule is maintained. Sulfur dioxide and water
are the principal gaseous products evolved
during these expansions.”
“Nitroaniline contains two nitrogen atoms for
every six carbons. With the exception of 3-
nitroaniline at mole ratios of 1 and 2, the
atomic proportion of residual nitrogen is
significantly less than 2. This result, along
with the presence of ammonium ions, demonstrates
extensive deamination; the extent of deamination
is greatest when the amount of sulfuric acid is
high.”
“The foams were good thermal insulators inert
to
chemicals, e.g., concentrated mineral acids,
alkalies, organic and inorganic solvents, and
oxidants.”
“If the foam is exposed to a flame, it glows
without burning and oxidizes or erodes very
slowly; the radiant glow extinguishes
immediately when the foam is taken away from the
flame.”
“The exact structural nature of the insoluble
residue constituting the foam is unknown. All
the sulfur in the residue is present as sulfonic
acid groups attached to the benzene nuclei.
Secondary amino or azinium groups resulting from
deammoniative coupling must be present in order
to account for ammonium ions and Nitrogen
content less than 2 per benzene nucleus. A high
nitrogen-to-oxygen ratio indicates that few if
any nitro groups remain. Thus, both nitro arid
sulfonic acid groups are involved in oxidative
processes.”
References:
Poshkus, A. C. and Parker, J. A. (1970), Studies
on nitroaniline–sulfuric acid compositions:
Aphrogenic pyrostats. J. Appl. Polym. Sci., 14:
2049–2064. doi: 10.1002/app.1970.070140813
aromatic sulfonation
dehudration reaction
d
eamination
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