# 17.4: Analysis of Reaction's Adherence to the Green Chemistry Principles

## Analysis of Reaction’s Adherence to Green Chemistry Principles

The reaction that will be analyzed is the preparation of S-methyl L-cysteine from L-cysteine using methyl iodide, sodium hydride, and methanol. The amounts used or produced in this reaction are listed below¹:

L-Cysteine: 0.24 g

Methyl Iodide: 105 μl (0.2394 g)

Sodium Hydride: 0.04 g

Methanol: 5 ml (3.96 g)

S-Methyl L-Cysteine: 0.22 g

Below are the 12 Principles of Green Chemistry and how this reaction relates to them.

• Prevention: This reaction is not the best in terms of preventing waste. There are 4 reactants, reagents, and solvents that are used to make one product.
• Atom Economy: The calculated atom economy is about 51.38%, which isn’t very efficient.
• Less Hazardous Materials: A lot of these materials can be considered hazardous. Excess cysteine can harm cells², while alkyl halides and sulfides (methyl iodide and S-methyl L-cysteine, in this case) can hurt the environment.³
• Designing Safer Chemicals: There are alternative methods to produce a sulfide without using a thiol. Thiols have a pungent smell, so using another reagent is preferred.⁴ In addition, the reaction can be safer by using less chemicals overall. For example, a sulfide can be made from a thiol without solvents or catalysts (even though a high temperature is required).⁵
• Safer Solvents and Auxiliaries: As mentioned before, this reaction can be done without solvents or catalysts, but that also means that more energy is “spent”.⁵ With that said, it’s difficult to find a safer solvent without sacrificing efficiency or creating more waste.
• Design for Energy Efficiency: The reaction is relatively efficient in terms of energy (unless the alternative method is used).
• Renewable Feedstock: L-cysteine (a reactant) is the bio-based chemical in this reaction, which means it comes from renewable and/or natural resources.⁶ .
• Reduce Derivatives: This reaction could be improved by using a strong basic hydroxide (like sodium hydroxide) in water instead of the combination of sodium hydride and methanol. Cysteine can also form a sulfide with the use of enzymes.²
• Catalysis: There must be a base catalyst for the reaction to work. However, there are ways to make the catalyst "greener", such as using a sodium hydroxide solution in water instead of sodium hydride in methanol.
• Design for Degradation:  The reaction isn’t reversible, but the product can be used in other reactions to produce helpful products.
• Real-Time Analysis for Pollution Prevention: Since the reaction is at room temperature, it is expected that the reaction will take some time, which allows the person to safely monitor for any unexpected or undesired changes, and properly react if something does happen. Since the reactants have a strong smell, the reaction would probably take place in a controlled fume hood, ensuring the safety of the people doing the experiment.¹
• Accident Prevention: Extra caution is needed if you're handling any substance in this reaction, especially the cysteine due to it's nasty smell.

## Green Chemistry Calculations

• Atom Economy Percentage⁶:  $\frac{135.19 g/mol}{121.16 g/mol +141.94 g/mol} * 100 = 51.38%$

• Process Mass Intensity¹: $\frac{0.24 g +0.2394 g +0.04 g +3.96 g }{0.22 g} = 20$

With a PMI of about 20, the reaction falls under the Fine Chemicals industry.

## References

¹Mirzahosseini, A.; Noszál, B. The Species- and Site-Specific Acid–Base Properties of Biological Thiols and Their Homodisulfides. Journal of Pharmaceutical and Biomedical Analysis 2014, 95, 184–192.

²Wróbel, M.; Bronowicka-Adamska, P.; Bentke, A. Hydrogen Sulfide Generation from L-Cysteine in the Human Glioblastoma-Astrocytoma U-87 MG and Neuroblastoma SHSY5Y Cell Lines. Acta Biochimica Polonica 2017, 64 (1).

³Hydrogen Sulfide. https://www.vdh.virginia.gov/environ...rogen-sulfide/ (accessed Apr 22, 2021).

⁴Batista, G. M.; de Castro, P. P.; dos Santos, J. A.; Skrydstrup, T.; Amarante, G. W. Synthetic Developments on the Preparation of Sulfides from Thiol-Free Reagents. Organic Chemistry Frontiers 2021, 8 (2), 326–368.

⁵Movassagh, B.; Soleiman-Beigi, M. Synthesis of Sulfides under Solvent- and Catalyst-Free Conditions. Monatshefte für Chemie - Chemical Monthly 2008, 140 (4), 409–411.

⁶12 Principles of Green Chemistry. https://www.acs.org/content/acs/en/g...chemistry.html (accessed Apr 22, 2021).

Joshua Thielen