Principles of green chemistry and solutions to its mathematical problems
Green Chemistry
Green chemistry is a branch of chemistry that focuses on researching chemical processes and production methods that cause less environmental pollution and reduce risks. Effectively, green chemistry is a research philosophy aiming to invent and adopt chemical methods that reduce the amount of industrial waste, decrease the use of hazardous chemical substances, and minimize energy waste. It is a newer branch of chemistry.
Principles of Green Chemistry
In 1991, US scientists Paul T. Anastas and John C. Warner proposed the concept of green chemistry. It consists of 12 main principles of green chemistry, which are:
- Prevention of waste
- Atom economy
- Less hazardous chemical syntheses
- Designing safer chemicals
- Safer solvents and auxiliaries
- Design for energy efficiency
- Use of renewable feedstocks
- Reduction of derivatives
- Catalysis
- Design for degradation
- Real-time analysis for pollution prevention
- Inherently safer chemistry for accident prevention
Examples
Here, a few examples explaining the application of green chemistry are discussed. In 2005, the Nobel Prize in Chemistry was awarded for the discovery of the “Metathesis Method in Organic Synthesis.” Using this method, many “smart” things can be produced following green chemistry principles. A substance called hydrazine is produced by this method.
However, hydrazine can also be produced using green chemistry with the following method—
In the second method, hydrogen peroxide is used instead of ammonia to produce hydrazine because no byproducts other than water are formed. Additionally, ozone (O3) and CFCs were used as “blowing agents” to produce polystyrene. Under green chemistry, supercritical carbon dioxide is now being used instead.
Supercritical Carbon Dioxide:
A special state of carbon dioxide below its critical temperature where it behaves somewhat like a liquid and somewhat like a gas. Supercritical carbon dioxide can be used as a green solvent to reduce environmental pollution.
Atom Economy
Atom economy is defined as the ratio of the total molecular mass of the desired product to the sum of the molecular masses of all reactants used in the reaction. That is—
(i) Atom Economy (AE) of reaction = 29.629 %
(ii) Atom Economy (AE) of reaction = 47.058 %
Applications / Benefits of Green Chemistry in Chemical Reactions
- Green Chemistry in Protecting Human Health:
- Reducing the amount of air pollution harmful to the human body.
- Reduced disposal of hazardous chemical wastes into drinking water.
- Removal of persistent toxic chemicals entering the food chain.
- Developing safer pesticides to replace harmful ones.
- Using pest-specific safe pesticides by removing toxic chemicals from the food chain.
General Questions and Answers
Mathematical Examples
1. Two methods of ethanol production are as follows—
i. C6H12O6 → 2C2H5OH + 2CO2
ii. C2H5Br + KOH → C2H5OH + KBr
Which method of ethanol production is compatible with green chemistry? Explain.
Solution:
The first reaction of the stimulus is:
i. C6H12O6 → 2C2H5OH + 2CO2
Here, the reactant is glucose (C6H12O6) and the desired product is ethanol (C2H5OH).
Therefore, total mass of reactant (C6H12O6) atoms = {(12 × 6) + (1 × 12) + (16 × 6)} = 180 and total mass of the desired product (C2H5OH) atoms = 2 × {(12 × 2) + (1 × 5) + 16 + 1} = 92.
The second reaction of the stimulus is:
ii. C2H5Br + KOH → C2H5OH + KBr
Here, the reactants are C2H5Br and KOH, and the desired product is ethanol (C2H5OH).
Therefore, total mass of reactant atoms = {(12 × 2) + (1 × 5) + 80} + {(39 + 16 + 1)} = 165 and total mass of the desired product (C2H5OH) atoms = {(12 × 2) + (1 × 5) + 16 + 1} = 46.
Conclusion: The principle of green chemistry is to integrate participating reactants to convert them into the maximum desired product. According to this, the first reaction yields the highest amount of desired product among the two methods. Therefore, the first method is compatible with green chemistry.
2. Arrange according to Green Chemistry:
(i) C2H4 + Cl2 + Ca(OH)2 → C2H4O + CaCl2 + H2O
(ii) C2H4 + 0.5O2 → C2H4O
(iii) CH2(OH) – CH2Cl + KOH → C2H4O + KCl + H2O
Answer: The higher the atom economy of a reaction, the greener it is, meaning it is more effective in green chemistry. Therefore, when arranged according to Green Chemistry, the order is (ii) > (iii) > (i).
[Answer Hints: Atomic masses of H, O, Cl, K, Ca are: 1, 16, 35.5, 39, 40 and formula: Atom Economy (AE) = (Total molecular mass of desired product / Total molecular mass of reactants) × 100%]
3. Titanium can be extracted from its ore by two different methods. Namely—
i. Use of more active metal: TiO2 + 2Mg → Ti + 2MgO
ii. Electrolysis of ore: TiO2 → Ti + O2
Using the concept of maximum presence of reactant atoms in the desired product, which of the above methods is greener? [Ti = 47.88, Mg = 24.3, O = 16]
4. Calculate the % atom economy for the reaction: C2H4 (Ethene) + H2 → C2H6 (Ethane).
Answer: 100%
[Answer Hints: Atomic masses of H and C are: 1 and 12, and formula: Atom Economy (AE) = (Molecular mass of desired product / Sum of molecular masses of reactants) × 100%]
5. Calculate the % atom economy for the reaction: C6H5OH (Phenol) + Zn → C6H6 + ZnO.
Answer: 48.9%
6. CH3CH2OH + CH3COOH → CH3COOCH2CH3 + H2O; What is the atom economy of the compound CH3COOCH2CH3?
Answer: 83%
7. X (78g) + Y (98g) → M (84g) (Product) + N (92g) (Waste)
Answer: 47%
8. CH3 – CH2 – Br + NaOH(aq) → CH3-CH2-OH + NaBr, what is the atom economy of this reaction?
Answer: 30%
