In Section 21-16, we saw that Sevin insecticide is made by the reaction of 1-naphthol with methyl isocyanate. A Union Carbide plant in Bhopal, India, once used this process to make Sevin for use as an agricultural insecticide. On December 3,1984, either by accident or by sabotage, a valve was opened that admitted water to a large tank of methyl isocyanate. The pressure and temperature within the tank rose dramatically, and pressure-relief valves opened to keep the tank from bursting. A large quantity of methyl isocyanate rushed out through the pressure-relief valves, and the vapors flowed with the breeze into populated areas, killing about 2500 people and injuring many more.
(a) Write an equation for the reaction that took place in the tank. Explain why the pressure and temperature rose dramatically.
(b) Propose a mechanism for the reaction you wrote in part (a).

Verified step by step guidance

1. Step 1: Begin by identifying the chemical reaction that occurred in the tank. Methyl isocyanate (CH3NCO) reacts with water (H2O) to produce carbon dioxide (CO2), methylamine (CH3NH2), and heat. The reaction can be represented as: CH3NCO + H2O → CH3NH2 + CO2.
2. Step 2: Explain why the pressure and temperature rose dramatically. The reaction between methyl isocyanate and water is highly exothermic, meaning it releases a significant amount of heat. This heat causes the temperature inside the tank to rise, which in turn increases the pressure due to the expansion of gases and vaporization of methyl isocyanate.
3. Step 3: Propose a mechanism for the reaction. The mechanism begins with the nucleophilic attack of water (H2O) on the carbon atom of the isocyanate group (-NCO) in methyl isocyanate. This forms an unstable intermediate.
4. Step 4: The intermediate undergoes rearrangement, leading to the cleavage of the C-N bond and the formation of methylamine (CH3NH2) and carbon dioxide (CO2). The release of CO2 gas contributes to the pressure increase in the tank.
5. Step 5: Summarize the safety implications. The exothermic nature of the reaction, combined with the production of gaseous CO2, highlights the importance of controlling water exposure to methyl isocyanate in industrial settings to prevent catastrophic pressure buildup and release of toxic vapors.

Key Concepts

Reaction of 1-naphthol with Methyl Isocyanate

The reaction between 1-naphthol and methyl isocyanate is a nucleophilic substitution reaction where the hydroxyl group of 1-naphthol acts as a nucleophile, attacking the electrophilic carbon of the isocyanate. This reaction forms Sevin, an insecticide, through the formation of a carbamate structure. Understanding this reaction is crucial for writing the correct chemical equation and predicting the products formed.

Gas Laws and Pressure-Temperature Relationship

The rise in pressure and temperature in the tank can be explained by the ideal gas law, which states that pressure is directly proportional to temperature when volume is constant. When water was introduced to the methyl isocyanate, it likely caused a rapid exothermic reaction, generating gas and increasing both the temperature and pressure within the tank, leading to the release of toxic vapors.

Reaction Mechanism

A reaction mechanism outlines the step-by-step sequence of elementary reactions that lead to the formation of products. For the reaction between 1-naphthol and methyl isocyanate, proposing a mechanism involves detailing the nucleophilic attack, the formation of intermediates, and the final product. Understanding the mechanism is essential for predicting reaction pathways and the stability of intermediates involved.