Ask A Scientist© Chemistry Archive

 >    name         Sarah
 >    status       student
 >    age          16

Question -   I am a grade 12 chemistry student doing an assignment on
the
 >Bhopal Disaster in India 1982.
 >A major component of the pesticides manufactured there was the
 >chemical methyl iso cyanate.  This had a huge reaction with water,
 >which produced a poisonous gas.
 >Would you be able to explain this reaction to me, and why the gas
 >produced respiratory problems? Thankyou

I answered almost this exact question a while ago, but I couldn't find it in
the archives.  So, here it goes again.

The problem in Bhopal was not that the reaction of methyl isocyanate with
water created a poisonous gas, but that the reaction allowed methyl
isocyanate to escape from its containment.  Methyl isocyanate is volatile,
irritating to the nose and throat, and toxic.  Although it is a liquid at
room temperature, methyl isocyanate has a boiling point near human body
temperature.  At normal room temperature, its vapor pressure is over a third
of an atmosphere.

The reaction of methyl isocyanate with water produces methylamine and carbon
dioxide,

        CH3NCO + H2O --> CH3NH2 + CO2

along with heat.  Methylamine and carbon dioxide are both gases at room
temperature.  The production of gas by the reaction raised the pressure in
the container;  the generation of heat raised the pressure still further.
This rise in pressure caused a rupture of the containment vessel, released
the MIC into the surrounding air.  The heat also made the methyl isocyanate
more volatile (whether or not it boiled, increasing the temperature raised
the MIC vapor pressure, meaning that anyone breathing the air got a larger
MIC dose).

The chemical plant in Bhopal was the primary source of jobs in the region,
so many people lived very close to it. About 2,000 people died quickly as a
result of the exposure, and 200,000 more were exposed to the vapor.

Richard E. Barrans Jr., Ph.D.
Assistant Director, PG Research Foundation
Darien, IL USA
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