mass
amount
molar mass
concentration
solution volume
gas volume
molar gas volume
Avogadro
constant, L
number of
entities, N
R2. THE ATOM ECONOMY - SEARCHING FOR SYNTHETIC EFFICIENCY
R2.1 RISING GLOBAL POPULATION & SUSTAINABLE DEVELOPMENT
Population in the world currently exceeds 7¾ billion (2020) and is growing at a rate of around 1.05 % per year. It is expected to increase by about another 3 billion over the next 50 years.
To maintain or improve living standards like education and healthcare in the face of an increasing population, the world’s economy needs to grow, particularly the economies of developing nations. However, environmental pollution problems are often linked to economic growth. A challenge for industry, agriculture and commerce is to develop in a fashion that meets the needs of the present generation without causing significant environmental damage and wasting limited resources.
So-called ‘sustainable development’ will become increasingly critical as the world population rises.
R2.2 ORIGINS, PRINCIPLES & PRACTICE OF GREEN CHEMISTRY
Green Chemistry is a relatively new, emerging field that strives to work at the molecular level to achieve sustainability. The field has received widespread interest in the first quarter of c.21st due to its ability to harness chemical innovation to meet environmental and economic goals simultaneously. The Green Chemistry movement has been propelled by Paul Anastas (Yale) who, from the early 1990s, conceived a framework and helped develop a set of twelve cohesive principles for making a greener chemical, process, or product. He has defined Green Chemistry as 'the design of chemical products and processes to reduce or eliminate the use and generation of hazardous substances.'
12 Principles of Green Chemistry
PREVENTION
ATOM ECONOMY, A
e
LESS HAZARDOUS CHEMICAL SYNTHESES
DESIGNING SAFER CHEMICALS
SAFER SOLVENTS & AUXILIARIES
DESIGN FOR ENERGY EFFICIENCY
USE OF RENEWABLE FEEDSTOCKS
DECREASED USE OF DERIVATIVES
CATALYSIS
DESIGN FOR DEGRADATION
REAL-TIME ANALYSIS
FOR POLLUTION PREVENTION
INHERENTLY SAFER CHEMISTRY
FOR ACCIDENT PREVENTION
By generally accepting the principles and adopting the practice of ‘Green Chemistry’ the chemical industry is working towards sustainable development which aims to prevent pollution and the production of hazardous materials rather than producing them in the first place and subsequently clearing them up.
Green Chemistry attempts to
• be safe
• conserve raw materials and energy
• be more cost effective than conventional methods.
Three ways to make chemical processes ‘greener’ include:
• re-design of production methods to use different, less hazardous starting materials
• use of milder reaction conditions, better catalysts and less hazardous solvents
• use of production methods with fewer steps and higher ATOM ECONOMY, A
e
R2.3 ATOM ECONOMY - aka ATOM EFFICIENCY - FOR A REACTION
The practice of atom economy is essentially pollution prevention at the molecular level. The idea of making it a primary criterion for improvement in chemistry is a central theme of the philosophy of Green Chemistry.
A as a cornerstone of synthetic efficiency rose to pre-eminence in 1990 after Barry Trost (Stanford) proposed one of the simplest methods for calculating it. The A concept is realizing the use of raw materials so that the desired product(s) contain(s) the maximum proportion of constituent atoms from the reactant(s).
e
e
e
%
A is one of the most widely used metrics for measuring the “greenness” of a process or synthesis. It expresses the conversion efficiency of a chemical process in terms of all atoms – raw materials – involved, when the amount of desired products is compared with the amount of waste produced, and is a measure of the amount of starting materials that end up as useful products.
e
Ideally A = 100 where a reaction would incorporate all the atoms of the reactants. Reactions that generate only
a single substance as product have a maximum A = 100 providing there are no complications from an enantiomeric
e
%
mixture where only one of the isomers can be utilised for the intended purpose, e.g., pharmaceutical applications may require an enantio-pure drug formulation. Important and long-standing industrial processes utilising A = 100 reactions include:
e
%
e
%
HABER PROCESS (ammonia synthesis)
CONTACT PROCESS (ultimately H2SO4)
CATALYTIC HYDRATION OF ETHENE
While there are other equivalent expressions, if molar mass is used in the numerator and denominator then it must be converted to a stoichiometric mass by a consideration of the relevant amount ratios.
Molar mass, M, is an intensive quantity while mass, m, is an extensive quantity; they should be treated as such in these calculations. So, expressions like
are generally wrong. By chance, they produce the correct numerical answer for reactions where the chemical equation contains favourable stoichiometric coefficients, i.e., where they are all unity.
R2.4 DISTINCTION BETWEEN ATOM ECONOMY & % YIELD FOR A REACTION
R2.5 MECHANISTIC IMPLICATIONS FOR REACTIONS THAT POSSESS HIGH Ae
R2.6 IS A HIGH YIELD OR A HIGH ATOM ECONOMY PREFEREABLE ?
