Designing a green synthesis
Ans. ..... Green chemistry for chemical synthesis address our future challenge in working whit chemicls processes and products by inventing novel reactions that can maximize the desired products and minimize by products , designing new synthetic schemes and apparatus that can simply operations in Chemical productions,The bigning of green chemistry is frequently considerd as a response to the need to reduce the damage of the environment by man-mad materials and the process used to produce them. A quick view of green chemistry issue in the past decade demonstrates man methodologies that protect human helthvand the environment in an economically benifical manner.
Prevention of waste:
Manufacturers try to generate as little wast as possible, through reaction choice ,process designing and recycling. Industries aim to use chemical reactions and process that make the most effective use of available resources and generate the smallest possible amount of wast materials.But can prevention be assessed quantitatively.
One way of measuring the efficiency of a process is to calculate the yield which compares the expected products quantity with the actual amount obtained.
Eg... Is the manufacturing of phenol. It is used to be made from benzene using sulfuric acid and sodium hydroxide in a multi-satage
Process,which ,over all can be expressed as:
The chemicll equation shows that 1mol of benzene (78g)should yield 1 mol of phenol products is found to be about 77 g, givibg a yield of 82%,which may be regardes quit good.
(yield % = mass produced / mass expected x 100%)
However the calculation obscures the fact that the reaction also generates 1 mol (12 g) of sodium sulfite for each mol of phenol produced. This may be acceptable if there is enough demand for sodium sulfite,but if nit ,it presents a serious problem of wast management and adds significantly to costs ,meaning that this may not be the most suitable reaction for manufacturing phenol.
Maximum incorporation of the materials used in the process into the final product(atom economy)
Atom economy:--
The concept of atom economy as developed by Barry Tost consider how much of reactants in Chemical reaction ends up in tje final useful products.The goal of atom economy is to create synthesis in which most of the atoms of the reactants become incorporated into the desire final products leding to lower percentage of wast by products. Atom economy is the concept that evaulates the efficency of chemical transformation, and calculated as a ratio of total mass of atoms in the desired products to the total mass of atoms in the reactants.
Maximum incorporation of the reactants (atom economy) :--
Chemistry globally considers that if the yield of a reaction is abiut 90% , the reaction is good. The percentage of yield is calculated by
One of the fundamental and most important principle of green chemistry is that of atom economy
claisen rearrangement:--
atom economy %=(134.18/134.18)x100=100%Addition Reaction :
May atom economical reactions using various transition metal catalysts have been developed (Trost, 1983,1998-200,Trost et al., 200).
Atom economy of catalytic process:
Diels Alder reaction is 100% atom economy reaction as all the atoms of thr reaction are incorporated in the cyclo adduct
palladium catalyzed synthesis of methyl methylacrylate has 100 % atom economy
via green synthesis
The old method of synthesis has only 47% atom economy
Lesa hazardous Chemical syntheses
It is essential to design new synthetic
methods so as to use and generate substances that are not toxic to human life and health.E.g in manufacturing of polystyrene foam sheet, chlorofluorocarbons responsible for O2 depletion, global warming has now been replaced by CO2 as blooming agent
Monasto company has developed a method of synthesizing polyurethanes that totally eliminates the use of phosgene (walden and riley, 1994)
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