Green
Chemistry
By
Prof.Nagendrappa
The following examples illustrate the ideas of
Green Chemistry embodied in the above principles.
Ibuprofen
Ibuprofen is a widely used anti-inflammatory
drug. Its synthesis has stimulated the development of many new
synthetic routes which tend to conform to the principles of Green
Chemistry. The original and the new routes given below clearly
demonstrate how resources can be saved by careful planning.
The new synthesis avoids MeCOCl, AlCl3, alkali, acid,
ethyl chloroacetate, hydroxylamine, etc., most of which do not
form part of the final product. This demonstrates how the original
process causes huge amounts of waste generation, resulting in
not only cost enhancement, but environmental pollution through
wrong use of resources.
In the new method, there is hardly any product that can be considered
as waste. The only by-product is acetic acid which can be converted
to acetic anhydride. All other chemicals are either catalysts
(Raney nickel, PdCl2-Ph3P, HF) which are
recoverable or reactants that form part of the product (Ac2O),
H2, CO), and solvents (MEK and others not shown in
the equation).
Clay as an acid-catalyst
Clay modified by suitably treating with protic
acids or salts of metal ions that act as Lewis acids behaves
like an acid. This enables it to be used as an acid catalyst
in place of Bronsted acid or Lewis acid in reactions that are
catalyzed by such acids, e.g., esterification, ether synthesis,
rearrangements, elimination reactions, etc.
The use of clays
confers the following advantages, as they are
1. Abundant and easily available
2. Economical
3. Easily modified to bestow them with desired properties
4. Environment-friendly
5. Reusable
To understand the catalytic function of
clay, let us very briefly consider the structure. Though there
are several classes of clay, a simplified gross structure can
be described as given below.
Structure
of Clay
The interstitial space contains protons, alkali
and alkaline earth metal ions as seen in the structure.. The
last two ionic species can be replaced by protons, aluminium,
magnesium, nickel, iron, copper, cobalt and many other metal
ions. This modification of clay makes it behave like a Bronsted
acid or Lewis acid and enables it to be used as an acid catalyst.
Because such catalysts can be used repeatedly or regenerated,
the chemical waste produced in such reactions will be considerably
reduced. |