atom economy tcm4 670357

Atom Economy


Learning outcomes


Atom economy is derived from the principles of green
chemistry.



Atom economy is a measure of the proportion of
reactants
that become useful products.
% atom economy =



Mass of desired product(s)
x 100
Total mass of reactants


What is
green chemistry?


The sustainable design of chemical products and
chemical processes.



It minimises the use and generation of chemical
substances that are hazardous to human health or
the environment.


Green chemistry
principles


Better to prevent waste than to treat it or clean it up.



Chemical processes should aim to incorporate all
reactants in the final product.



Chemical processes should aim to use and generate
substances with minimal toxicity to human health and
the environment.


The green chemical
industry


Modern chemists design reactions with the highest
possible atom economy in order to minimise

environmental impact.



Chemists achieve this by reducing raw material and
energy consumption.


Percentage yield
% yield =

Actual yield
Theoretical yield

x 100



Historical method for evaluating reaction efficiency.



Measures the proportion of the desired product
obtained compared to the theoretical maximum.



Gives no indication of the quantity of waste
produced.

Atom economy


In an ideal reaction, all reactant atoms end up within
the useful product molecule. No waste is produced!



Inefficient, wasteful reactions have low atom economy.



Efficient processes have high atom economy and are
important for sustainable development. They conserve
natural resources and create less waste.


Atom economy
% atom economy =

Mass of desired product(s)
x 100
Total mass of reactants



A measure of the proportion of reactant included in the final useful product.



A reaction may have a high percentage yield but a low percentage atom economy, or vice versa.


High atom economy

All reactant atoms included in the desired product.


Low atom economy

Some reactant atoms not included in the desired
product.


Example 1
What is the percentage atom economy for the following reaction for
making hydrogen by reacting coal with steam?

C(s)

+

12 g

2H2O(g) → CO2(g)
2(2 + 16) g

+

[12 + (2 × 16)] g

2H2(g)
2(2 × 1)

g
12 g

36 g

44 g

4 g
Total mass of reactants
= 12 + 36 = 48 g

Mass of desired product
= 4 g


Example 1 (contd)
% atom economy = mass of desired product × 100
total mass of reactants
=

4 × 100
48

= 8.3%
This reaction route has a very low atom economy and is

an inefficient method of producing hydrogen.


Example 2
Calculate the percentage atom economy for the reaction
below.
CH3
H3C

C

acid

H3C

CH CH2

CH3

C6H12
Total mass of reactants
= [(6 × 12) + (12 × 1)]
1)]
= 84 g

CH3
C

H3C

C
CH3

C6H12
Mass of desired product
= [(6 × 12) + (12 ×
= 84 g


Example 2 (contd)
% atom economy = mass of desired product × 100
total mass of reactants
=

84 × 100
84
= 100%

This reaction route has a very high atom economy as all
reactant atoms are incorporated into the desired
product.


Example 3
Hydrazine (N2H4) is used for rocket fuel. Calculate the
atom economy for hydrazine production.
2NH3

+

NaOCl

N2H4

NH3
2 mol

NaOCl
1 mol

N2H4
1 mol

NaCl
1 mol

H2O
1 mol

34 g

74.5 g

32 g

58 g

18 g

Total mass of reactants

= 34 + 74.5 = 108.5 g

+

NaCl

+

H 2O

Mass of desired product
= 32 g


Example 3 (contd)
% atom economy = mass of desired product × 100
total mass of reactants
=

32 × 100
108.5

= 30%
This reaction route has an atom economy of 30%. The
remaining 70% is waste product (NaCl and H2O).


Catalysts


Have a crucial role in improving atom economy.



Allow the development of new reactions requiring fewer
starting materials and producing fewer waste products.



Can be recovered and re-used.



Allow reactions to run at lower temperatures, cutting
energy requirements.