“Green Star”: a holistic Green Chemistry metric for evaluation of teaching laboratory experiments

Figures & data

Table 1. The Twelve Principles of GC.

GC principle	Description
P1 – prevention	It is better to prevent waste than to treat or clean up waste after it has been created.
P2 – atom economy	Synthetic methods should be designed to maximize the incorporation of all materials used in the process into the final product.
P3 – less hazardous chemical synthesis	Wherever practicable, synthetic methods should be designed to use and generate substances that possess little or no toxicity to human health and the environment.
P4 – designing safer chemicals	Chemical products should be designed to effect their desired function while minimizing their toxicity.
P5 – safer solvents and auxiliary substances	The use of auxiliary substances (e.g. solvents, separation agents, etc.) should be made unnecessary wherever possible and innocuous when used.
P6 – increase energy efficiency	Energy requirements of chemical processes should be recognized for their environmental and economic impacts should be minimized. If possible, synthetic methods should be conducted at ambient temperature and pressure.
P7 – use renewable feedstocks	A raw material or feedstock should be renewable rather than depleting whenever technically and economically practicable.
P8 – reduce derivatives	Unnecessary derivatization (use of blocking groups, protection/deprotection, and temporary modification of physical/chemical processes) should be minimized or avoided if possible, because such steps require additional reagents and can generate waste.
P9 – catalysts	Catalytic reagents (as selective as possible) are superior to stoichiometric reagents.
P10 – design for degradation	Chemical products should be designed so that at the end of their function they break down into innocuous degradation products and do not persist in the environment.
P11 – real-time analysis for pollution prevention	Analytical methodologies need to be further developed to allow for real-time, in-process monitoring and control prior to the formation of hazardous substances.
P12 – safer chemistry for accident prevention	Substances and the form of a substance used in a chemical process should be chosen to minimize the potential for chemical accidents including releases, explosions, and fires.

Table 2. Risks to human health and environment of substances involved.

Risks	Hazard symbols	Score (S)
Health	C – corrosive	3
	T – toxic
	T+ – very toxic
	Xi – irritant	2
	Xn – harmful
	No indication	1
Environment	N – dangerous for the environment	3
	No indication	1

Table 3. Risks of potential chemical accident due to substances involved.

Risks	Hazards symbols	Score (S)
Health	C – corrosive	3
	T – toxic
	T+ – very toxic
	Xi – irritant	2
	Xn – harmful
	No indication	1
Flammability	F – highly flammable	3
	F+ – extremely flammable
	No indication	1
Reactivity	E – explosive	3
	O – oxidizing agent	3
	No indication	1

Table 4. Degradability and renewability characteristics of substances involved.

Characteristics	Criteria	Score (S)
Degradability	Not degradable and may not be treated to render the substances degradable to innocuous products	3
	Not degradable but may be treated to render the substances degradable to innocuous products	2
	Degradable and breakable to innocuous products	1
Renewability	Not renewable	3
	Renewable	1

Table 5. Criteria and scores (S) to construct the GS.

GC principle	Criteria	S
P1 – prevention	Waste is innocuous (S=1, Table 1)	3
	Waste involves a moderate risk to human health and environment (S=2, Table 1, for at least one substance)	2
	Waste involves a high risk to human health and environment (S=3, Table 1, for at least one substance)	1
P2 – atom economy	Reactions without excess of reagents (<10%) and without formation of by-products	3
	Reactions without excess of reagents (<10%) and with formation of by-products	2
	Reactions with excess of reagents (>10%) and without formation of by-products	2
	Reactions with excess of reagents (>10%) and with formation of by-products	1
P3 – less hazardous chemical synthesis	All substances involved are innocuous (S=1, Table 1)	3
	Substances involved have a moderate risk to human health and environment (S=2, Table 1, for at least one substance)	2
	At least one substance involved has a high risk to human health and environment (S=3, Table 1)	1
P5 – safer solvents and auxiliary substances	Solvents and auxiliary substances are not used, but if used are innocuous (S=1, Table 1)	3
	Solvents or/and auxiliary substances are used but have a moderate risk to human health and environment (S=2, Table 1, for at least one substance)	2
	At least one solvent or auxiliary substance has a high risk to human health and environment (S=3, Table 1)	1
P6 – increase energy efficiency	Room temperature and pressure	3
	Room pressure and temperature between 0 and 100°C when cooling or heating is needed	2
	Pressure different from room pressure and/or temperature>100°C or less than 0°C	1
P7 – use renewable feedstocks	All substances involved are renewable (S=1, Table 3)	3
	At least one substance involved is renewable, water is not considered (S=1, Table 3)	2
	None of substances involved are renewable, water is not considered S=3, Table 3)	1
P8 – reduce derivatives	Derivatizations are not used	3
	Only one derivatization or similar operation is used	2
	More than one derivatization or similar operations are used	1
P9 – catalysts	Catalysts are not used and if used are innocuous (S=1, Table 1)	3
	Catalysts are used but have a moderate risk to human health and environment (S=2, Table 1)	2
	Catalysts are used and have a high risk to human health and environment (S=3, Table 1)	1
P10 – design for degradation	All substances are degradable and break down to innocuous products (S=1, Table 3)	3
	All substances not degradable may be treated to render them degradable to innocuous products (S=2, Table 3)	2
	At least one substance is not degradable nor may be treated to render it degradable to innocuous products (S=3, Table 3)	1
P12 – safer chemistry for accident prevention	Substances used have a low risk to cause chemical accidents (S=1, Table 2)	3
	Substances used have a moderate risk to cause chemical accidents (S=2, Table 2, for at least one substance)	2
	Substances used have a high risk to cause chemical accidents (S=3,Table 2, for at least one substance)	1

Figure 1.  (a) GS of maximum greenness and (b) GS of minimum greenness.

Table 6. Risks for human health and for the environment for the synthesis of iron(II) oxalate dehydrate.

		Scores for substances used for construction of GS1 (sulfuric acid used)			Scores for substances used for construction of GS2–GS4 (ascorbic acid used)
Reagents used	Hazard symbols	Human health	Environment	Chemical accident	Human health	Environment	Chemical accident
Stoichiometric reagents
Iron(II) sulfate heptahydrate	Xn	2	1	2	2	1	2
Oxalic acid dehydrate	Xn	2	1	2	2	1	2
Auxiliary substances
Solvents
Water	–	1	1	1	1	1	1
Other auxiliary substances
Sulfuric acid	C	3	1	3	NU	NU	NU
Ascorbic acid	–	NU	NU	NU	1	1	1
Product
Iron(II) oxalate dehydrate	Xn	2	1	2	2	1	2
Waste
Sulfuric acid diluted	Xi	2	1	2	2	1	2
Oxalic acid in excess	Xn	2	1	2	2	1	2
Ascorbic acid	–	NU	NU	NU	1	1	1
Dehydroascorbic acid	–	NU	NU	NU	1	1	1
Note: NU, not used.

Table 7. Scores used to construct the GS for the synthesis of iron(II) oxalate dehydrate.

	GS1		GS2		GS3		GS4
	Sulfuric acid, boiling temperature, 76% excess of oxalic acid		Ascorbic acid, boiling temperature, 76% excess of oxalic acid		Ascorbic acid, room temperature, 76% excess of oxalic acid		Ascorbic acid, room temperature, 4% excess of oxalic acid
GC principle	Score	Explanation	Score	Explanation	Score	Explanation	Score	Explanation
P1 – prevention	2	Oxalic acid, Xn diluted sulfuric acid, Xi	2	As in GS1	2	As in GS1	2	As in GS1
P2 – atom economy (incorporation of atoms into the product)	1	Excess of oxalic acid formation of by-products	1	As in GS1	1	As in GS1	2	Excess of oxalic acid <10%
P3 – less hazardous chemical synthesis	1	Sulfuric acid, C	2	Xn – maximum hazard	2	As in GS2	2	As in GS2
P5 – safer solvents and auxiliary substances	1	Sulfuric acid, C	3	Water and ascorbic acid	3	As in GS2	3	As in GS2
P6 – increase energy efficiency	2	Temperature near 100°C	2	As in GS1	3	Room temperature	3	As in GS3
P7 – use renewable feedstocks	1	Substances not renewable	1	As in GS1	1	As in GS1	1	As in GS1
P8 – reduce derivatives	3	Without derivatives	3	As in GS1	3	As in GS1	3	As in GS1
P9 – catalysts	3	Without catalysts	3	As in GS1	3	As in GS1	3	As in GS1
P10 – design for degradation	1	Substances not degradable	1	As in GS1	1	As in GS1	1	As in GS1
P12 – safer chemistry for accident prevention	1	Sulfuric acid, C	2	Xn – maximum hazard	2	As in GS2	2	As in GS2

Figure 2.  Green Stars and mass metrics for iron(II) oxalate dihydrate synthesis*.

Figure 3.  Calculation of the GS area and GSAI.