environment. Nitrates in the soil are
taken up by plant roots and may
then pass along food chains into animals. Decomposing bacteria convert
nitrogen-containing compounds (especially ammonia) in plant and animal wastes and dead remains back
into nitrates, which are released into
the soil and can again be taken up by
plants (see nitrification). Though nitrogen is essential to all forms of life,
the huge amount present in the atmosphere is not directly available to
most organisms (compare carbon
cycle). It can, however, be assimilated by some specialized bacteria
(see nitrogen fixation) and is thus
made available to other organisms
indirectly. Lightning Ûashes also
make some nitrogen available to
plants by causing the combination of
atmospheric nitrogen and oxygen to
form oxides of nitrogen, which enter
the soil and form nitrates. Some nitrogen is returned from the soil to
the atmosphere by denitrifying bacteria (see denitrification).
nitrogen dioxide See dinitrogen
tetroxide.
nitrogen Üxation A chemical
process in which atmospheric nitrogen is assimilated into organic compounds in living organisms and
hence into the *nitrogen cycle. The
ability to Üx nitrogen is limited to
certain bacteria (e.g. Azotobacter, Anabaena). Rhizobium bacteria are able to
Üx nitrogen in association with cells
in the roots of leguminous plants,
such as peas and beans, in which
they form characteristic root nodules; cultivation of legumes is therefore one way of increasing soil
nitrogen. Various chemical processes
are used to Üx atmospheric nitrogen
in the manufacture of *fertilizers.
These include the *Birkeland–Eyde
process, the cyanamide process (see
calcium dicarbide), and the *Haber
process.
nitrogen monoxide (nitric oxide)
A colourless gas, NO; m.p. –163.6°C;
b.p. –151.8°C. It is soluble in water,
ethanol, and ether. In the liquid state
nitrogen monoxide is blue in colour
(r.d. 1.26). It is formed in many reactions involving the reduction of nitric acid, but more convenient
reactions for the preparation of reasonably pure NO are reactions of
sodium nitrite, sulphuric acid, and either sodium iodide or iron(II) sulphate. Nitrogen monoxide reacts
readily with oxygen to give nitrogen
dioxide and with the halogens to give
the nitrosyl halides XNO (X = F,Cl,Br).
It is oxidized to nitric acid by strong
oxidizing agents and reduced to dinitrogen oxide by reducing agents. The
molecule has one unpaired electron,
which accounts for its paramagnetism and for the blue colour in the liquid state. This electron is relatively
easily removed to give the nitrosyl
ion NO
+ , which is the ion present in
such compounds as NOClO 4 , NOBF 4 ,
NOFeCl 4 , (NO) 2 PtCl 6 and a ligand in
complexes, such as Co(CO) 3 NO.
In mammals and other vertebrates,
nitrogen monoxide is now known to
play several important roles. For example, it acts as a gaseous mediator
in producing such responses as dilation of blood vessels, relaxation of
smooth muscle, and inhibition of
platelet aggregation. In certain cells
of the immune system it is converted
to the peroxynitrite ion (
–
O–O–N=O),
which has activity against tumour
cells and pathogens.
nitrogen mustards A group of nitrogen compounds similar to *sulphur mustard. They were used as
chemotherapy agents in cancer treatment. Like sulphur mustard, they are
powerful blistering agents. Large
quantities were made during World
375
nitrogen mustards
n
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taken up by plant roots and may
then pass along food chains into animals. Decomposing bacteria convert
nitrogen-containing compounds (especially ammonia) in plant and animal wastes and dead remains back
into nitrates, which are released into
the soil and can again be taken up by
plants (see nitrification). Though nitrogen is essential to all forms of life,
the huge amount present in the atmosphere is not directly available to
most organisms (compare carbon
cycle). It can, however, be assimilated by some specialized bacteria
(see nitrogen fixation) and is thus
made available to other organisms
indirectly. Lightning Ûashes also
make some nitrogen available to
plants by causing the combination of
atmospheric nitrogen and oxygen to
form oxides of nitrogen, which enter
the soil and form nitrates. Some nitrogen is returned from the soil to
the atmosphere by denitrifying bacteria (see denitrification).
nitrogen dioxide See dinitrogen
tetroxide.
nitrogen Üxation A chemical
process in which atmospheric nitrogen is assimilated into organic compounds in living organisms and
hence into the *nitrogen cycle. The
ability to Üx nitrogen is limited to
certain bacteria (e.g. Azotobacter, Anabaena). Rhizobium bacteria are able to
Üx nitrogen in association with cells
in the roots of leguminous plants,
such as peas and beans, in which
they form characteristic root nodules; cultivation of legumes is therefore one way of increasing soil
nitrogen. Various chemical processes
are used to Üx atmospheric nitrogen
in the manufacture of *fertilizers.
These include the *Birkeland–Eyde
process, the cyanamide process (see
calcium dicarbide), and the *Haber
process.
nitrogen monoxide (nitric oxide)
A colourless gas, NO; m.p. –163.6°C;
b.p. –151.8°C. It is soluble in water,
ethanol, and ether. In the liquid state
nitrogen monoxide is blue in colour
(r.d. 1.26). It is formed in many reactions involving the reduction of nitric acid, but more convenient
reactions for the preparation of reasonably pure NO are reactions of
sodium nitrite, sulphuric acid, and either sodium iodide or iron(II) sulphate. Nitrogen monoxide reacts
readily with oxygen to give nitrogen
dioxide and with the halogens to give
the nitrosyl halides XNO (X = F,Cl,Br).
It is oxidized to nitric acid by strong
oxidizing agents and reduced to dinitrogen oxide by reducing agents. The
molecule has one unpaired electron,
which accounts for its paramagnetism and for the blue colour in the liquid state. This electron is relatively
easily removed to give the nitrosyl
ion NO
+ , which is the ion present in
such compounds as NOClO 4 , NOBF 4 ,
NOFeCl 4 , (NO) 2 PtCl 6 and a ligand in
complexes, such as Co(CO) 3 NO.
In mammals and other vertebrates,
nitrogen monoxide is now known to
play several important roles. For example, it acts as a gaseous mediator
in producing such responses as dilation of blood vessels, relaxation of
smooth muscle, and inhibition of
platelet aggregation. In certain cells
of the immune system it is converted
to the peroxynitrite ion (
–
O–O–N=O),
which has activity against tumour
cells and pathogens.
nitrogen mustards A group of nitrogen compounds similar to *sulphur mustard. They were used as
chemotherapy agents in cancer treatment. Like sulphur mustard, they are
powerful blistering agents. Large
quantities were made during World
375
nitrogen mustards
n
www.AzShimi.ir www.AzShimi.com