Question

Strawberry growers have used large quantities of methyl bromide (b.p. 4 °C) to sterilize the soil before planting their crops. Like some of the freons, methyl bromide can diffuse up into the stratosphere, where it damages the protective ozone layer. Agricultural chemists have suggested using methyl iodide (b.p. 43 °C) as a replacement for methyl bromide. Why is methyl iodide likely to be more toxic to agricultural pests (and people) than methyl bromide? Why is methyl iodide less likely to reach the stratosphere than methyl bromide?

Accepted Answer

Step 1: Discuss the chemical reactivity of methyl iodide compared to methyl bromide. Methyl iodide contains iodine, which is a larger and more polarizable atom than bromine. This increased polarizability makes methyl iodide more reactive, allowing it to interact more effectively with biological molecules, potentially making it more toxic to pests and humans. Step 2: Explain the boiling point difference between methyl iodide (43 °C) and methyl bromide (4 °C). The higher boiling point of methyl iodide indicates stronger intermolecular forces, such as van der Waals interactions, due to the larger size and mass of the iodine atom compared to bromine. Step 3: Relate the boiling point to volatility. Methyl bromide, with its lower boiling point, is more volatile and can more easily evaporate and diffuse into the atmosphere. Methyl iodide, being less volatile, is less likely to reach the stratosphere. Step 4: Discuss the environmental implications. Methyl bromide's ability to diffuse into the stratosphere contributes to ozone layer depletion. Methyl iodide, due to its reduced volatility, poses a lower risk of atmospheric diffusion and subsequent ozone damage. Step 5: Highlight the trade-offs. While methyl iodide may be less environmentally damaging in terms of ozone depletion, its increased toxicity requires careful handling and consideration of its impact on human health and ecosystems.

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Key Concepts

Toxicity of Organohalides

Volatility and Boiling Points

Atmospheric Lifetime and Degradation