Chemical Informatics

Introduction

In the last few decades, there has been a constant and vigorous debate between Development and Conservation of the environment. Man has been consuming natural resources at an alarming rate leaving behind toxic byproducts which are hazardous to the environment. Today, there is a general acceptance that there should be a balance between development and conservation.

Overview of Principles of Green Chemistry

The 12 Principles of Green Chemistry [1-3] provide a blueprint for sustainable development.

1) The generation of waste products should be minimized. It is better to prevent the formation of waste rather than clean it up after its generation.

2) Atom economy of a process should be so designed that maximum amount of reacting material is converted into the desired product.

3) The use and generation of toxic materials should be avoided.

4) Non toxic chemical substances should be used as starting materials in reactions.

5) The use of auxiliary substances like solvents and separating agents should be minimized .Green solvents like water should be used in preference to halogenated organic solvents. Green Chemistry aims at solvent free reactions.

6) Energy consumption in reaction should be minimized and ideally reactions should be carried out at ambient temperature.

7) Renewable feedstock like biomass should be used as starting materials instead of non renewable feedstock like crude oil.

8) Reactions should be carried out with minimum number of steps. Steps like derivatization, protection and de-protection of functional groups in molecules should be avoided as far as possible.

9) Catalysts should be used to increase reaction rates as they are superior to stoichiometric reagents. Catalysts increase the atom economy and yield of reactions

10)The products of a reaction should degrade easily to harmless substances and should not pose a threat to the environment.

11) The generation of hazardous substances should be monitored by sensitive analytical methods.

12) Chemically safer substances with minimum potential of accidents should be used. Solids and non volatile liquids have less risk of accidents compared to gases and volatile liquids.

Methods

1)Improvement in Atom Economy

A catalyst should improve the atom economy of a reaction

Atom economy of a reaction is a theoretical measure of the amount of starting material that ends up as the desired final product.Atom economy is the most widely used metrics to measure the greenness of a reaction.

% Atom Economy = [Mass of desired product/Total mass of all products] X 100

Thus greater the % atom economy ,more green (efficient) is the reaction.ie 100% atom economy means 0% waste.

Atom economy can be maximized by designing the synthesis so that the final desired product contains the maximum proportion of the starting material and few or no atoms are wasted.This can be achieved by use of a suitable catalyst.

2) Optimization of reaction yield

A catalyst should improve the yield of a reaction.

Discussion

A catalyst is a substance which accelerates the rate of a reaction but remains chemically unchanged at the end of the reaction ie. its chemical composition and amount remains unchanged. A catalyst takes an active part in the reaction but does not appear in the end product of the reaction.

A catalyst lowers the minimum energy required for the reaction to take place and hence increases the rate of reaction. This energy threshold called Energy of Activation (E) is related to the absolute temperature T by the Arrhenius equation, k=A e-E/RT where k is rate constant, R is the gas constant and A is a constant. Temperature has the most profound effect on rate of reaction.

Figure 1: Effect of catalyst on rate of reaction

E and E’ are the energies of activation of the uncatalyzed and catalyzed reactions respectively .

Since E’ < E, the rate of the catalyzed reaction k’ > rate of uncatalyzed reaction k.

Mechanism of catalysis

The catalyst forms an unstable short lived intermediate with the reactant.The intermediate then decomposes to give the product and the catalyst comes out unchanged.

Reactant + Catalyst ---------→ Intermediate

Intermediate ----------→ Product + Catalyst

Example : Friedel Craft’s reaction using anhydrous AlCl3 as catalyst

CH3COCl + an.AlCl3 --------→ CH3COCl.AlCl3

Acetyl chloride Intermediate

CH3COCl.AlCl3 + C6H6 -------→ CH3COC6H5 + HCl + AlCl3

Acetophenone

CH3COCl + C6H6 -------→ CH3COC6H5 + HCl

Some industrial catalysts used on a large scale are :

1) Aluminosilicates (Al2O3 + SiO2) in cracking of petroleum to give smaller fractions,

2) Fe along with Mo as promoter in the manufacture of ammonia by Haber’s process,

3) V2O5 in the manufacture of sulphuric acid by Contact Process

4) Raney nickel in hydrogenation of ethylene to ethane and propene to propane.

References

1. Paul T Anastas ,Williamson TC (Eds.) (1996) ACS Symposium Series 626,American Chemical Society,Washington DC.
2. Paul T Anastas , John C Warner(1998) Green Chemistry :Theory and Practice,Oxford University Press,New York.
3. Collin Baird,Michael Cain(2005)Environmental Chemistry (3rd edn.),W.H.Freman and Company.New York.
4. Asim K Das (2010) Environmental and Green Chemistry,Books and Allied (P)Ltd.,Kolkata.
5. Murthy BS,Shankar P,Baldev Raj,Rath BB,James Murday (2012) Textbook of Nanoscience and Nanotechnology,(Series in Metallurgy and Materials Science),United Press-IIM,Hyderabad,2012.
6. Shivani S.Dhage,Amita A.Dalvi and Damodar V.Prabhu(2012)Reaction kinetics and validity of BOD test for domestic wastewater released in marine ecosystems,Environment Monitoring and Assessment 184:5301-5310.    DOI10.1007/s10661-011-2341-y