Question

In the following three instances, which choice is greener in each situation? Explain. (a) Benzene as a solvent or water as a solvent. (b) The reaction temperature is 500 \(\mathrm{K}\) or 1000 \(\mathrm{K}\) . (c) Sodium chloride as a by-product or chloroform \(\left(\mathrm{CHCl}_{3}\right)\) as a by-product.

Short answer

In summary, the greener choices in each situation are: (a) water as a solvent due to its non-toxic nature and renewable resource; (b) a reaction temperature of 500 K as it consumes less energy; and (c) sodium chloride as a by-product because it has a lower environmental impact compared to the toxic chloroform.

Step-by-step solution

(a) Comparing benzene and water as a solvent

To determine whether benzene or water is the greener option, consider the environmental impacts of each solvent. Benzene is a toxic and carcinogenic compound that poses health risks. In contrast, water is a non-toxic and renewable resource. Therefore, using water as a solvent is the greener choice due to its lower environmental impact.

(b) Comparing reaction temperatures at 500 K and 1000 K

To evaluate the greener option between the reaction temperatures of 500 K and 1000 K, consider the energy consumption in each case. Higher temperatures typically require more energy to be maintained, and therefore, result in higher energy consumption and environmental impact. Consequently, a reaction temperature of 500 K is greener compared to 1000 K due to lower energy consumption.

(c) Comparing sodium chloride and chloroform as by-products

To determine which by-product, sodium chloride (NaCl) or chloroform (CHCl3), is greener, consider the environmental impacts and potential hazards associated with each compound. Sodium chloride is a commonly found salt, relatively harmless to the environment in small quantities. On the other hand, chloroform is a toxic substance and is suspected of being a human carcinogen. Furthermore, chloroform is a greenhouse gas, which contributes to climate change. Therefore, sodium chloride (NaCl) is the greener by-product than chloroform (CHCl3), due to its lower environmental impact and toxicity.

Key concepts

Solvent Selection

When tackling chemical reactions, the choice of solvent can significantly influence the environmental footprint of the process. Solvents are substances that dissolve a solute (a chemically distinct liquid, solid, or gas), resulting in a solution. The ideal solvent in green chemistry practices is non-toxic, non-volatile, and renewable.

For instance, comparing benzene with water as solvents, benzene is recognized to have detrimental health and environmental effects. It is a hazardous air pollutant that can cause serious health issues, including cancer. Water, on the other hand, is a far more benign option. It's safe, non-toxic, abundantly available, and doesn't contribute to air pollution. Naturally, choosing water as a solvent is a prime example of a greener, safer chemical practice that aligns with the 12 principles of green chemistry, particularly with regard to reducing the use and generation of hazardous substances.

Reaction Temperature

The temperature at which a reaction is conducted is another key element with substantial eco-impact. High temperatures often require significant energy inputs, which may come from non-renewable energy sources such as fossil fuels. This increases both the carbon footprint and the cost of the chemical process.

Comparing reaction temperatures, one at 500 K and another at 1000 K, the reaction at the lower temperature is inherently greener, assuming it achieves the same yield and rate. It is often more energy-efficient, produces less waste heat—reducing thermal pollution—and aligns with the principle of increased energy efficiency. Hence, lower reaction temperatures are generally preferred in green chemistry, provided that they effectively facilitate the desired chemical transformation.

By-product Environmental Impact

By-products from chemical reactions can range from benign to hazardous, significantly affecting the environmental footprint of the process. In green chemistry, the objective is to design synthetic methods that generate substances with little or no toxicity to human health and the environment.

For example, the generation of sodium chloride (table salt) as a by-product is preferable to chloroform, which is a potent environmental pollutant. Sodium chloride is widely used in various industries and poses minimal risk to ecosystems in controlled amounts. Chloroform, contrarily, is not only toxic but also contributes to greenhouse gas emissions, representing a double threat to health and environmental sustainability. Prioritizing chemical reactions that minimize or eliminate toxic by-product formation is a crucial aspect of sustainable chemistry.