Green Chemistry

Green reagents, Green catalysts, Ionic solvents : 

Green reagents, Green catalysts, Ionic solvents

Contents:: 

C ontents: What is green chemistry? P rinciples involved in green chemistry. Green reagents -Dimethyl carbonate -polymer supported reagents G reen catalyst -acid catalysts -oxidation catalysts -basic catalysts -polymer supported catalysts Ionic solvents

What is green chemistry?: 

What is green chemistry? Green Chemistry is the utilization of a set of principles that reduces or eliminates the use or generation of hazardous substances in the design, manufacture and application of chemical products. Green chemistry in short is defined as “Environmentally benign chemical synthesis”. T he synthetic processes are designed in such a way that there is least effect on the environment. As on today, maximum environmental pollution is caused by numerous chemical industries.

Twelve Principles Of Green Chemistry: : 

Twelve Principles Of Green Chemistry : Prevention of waste or by-products. Maximum incorporation of reactants into the final product. Prevention/minimization of hazardous products. Designing of safer chemicals. Energy requirement of any synthesis should be minimum. Selection of appropriate solvent. Selection of appropriate starting materials. Use of catalyst is preferred wherever possible. Products obtained must be bio degradable. Use of protecting group must be avoided wherever possible. Strengthening of analytical techniques to control hazardous compounds. 12. Inherently Safer Chemistry for Accident Prevention.

Selection of starting material:: 

Selection of starting material: Starting materials which can be obtained from agricultural or biological products are referred to as renewable starting materials and are non-hazardous. Till now most of the synthesis use petrochemicals as starting materials which are non-renewable and require more energy. Therefore use of petrochemicals must be reduced by using alternate starting materials which are of biological/agricultural origin.

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Choice of Catalysts: Certain reactions are much faster at lower temperature with the use of catalyst. Heavy metal catalyst should be avoided. Certain biocatalysts can also be used for various steps. Non–hazardous and non–toxic Preferably of biological origin, i.e., biocatalysts (enzymes) It must be Renewable

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Choice of Solvents Most of the common solvents cause hazards. CFCs were used up to as cleaning agent, but it causing depletion of the ozone layer. A versatile solvent co2 is used as liquid co2 or super critical co2 fluid. Safe, non-toxic and non-hazardous Cheaper and cost effective. Non-carcinogenic and non–mutagenic. It is ideal to carry out the reaction in aqueous phase if possible. Another way to carry out the reaction without the use of solvent.

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Safe and non – hazardous solvent should be selected, if not avoidable. Use of super critical fluids, such as super critical co2 will be cheaper and non – hazardous. Use of Renewable Ionic liquids is beneficial. Use of aqueous solvent (Water as a solvent) is the best both safety wise and economy wise. Highly volatile solvents should also be avoided. Solid - supported and solid state reactions whenever possible are the mostly preferred in Green Reactions. Selection of Appropriate Solvents:

GREEN REAGENTS:: 

GREEN REAGENTS: In order to carry out the transformation of selected feedstock into the target molecule the criteria of… Efficiency Availability and effect of reagent used must. Here we discuss about: 1. DMC 2. Polymer supported reagents Dimethylcarbonate(DMC) : DMC is produced through a green catalytic process by Enichen (Italy). Its low toxicity, the absence of irritating or mutagenic effects associated with it and its biodegradability -- qualify DMC as an environmentally friendly chemical.

DMC Production:: 

DMC Production: The most attractive route for industrial synthesis of DMC is found to be oxidative carbonylation of methanol in the presence of a suitable catalyst. In a green synthesis, DMC can act as:- 1. Carboxylating agent 2. Oxidizing agent 3. Methylating agent

How it act as a methylating agent?: 

How it act as a methylating agent? Conventional methylation reactions employ methyl halides or methyl sulphates. The toxicity of these compounds and their environmental consequences render these synthesis undesirable. TUNDO developed a method to methylate active methylene compounds selectively using DMC in which no inorganic salts are produced.

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However, operating at higher temperatures (> 160 0 C), DMC can be used as an excellent methylating agent.

How DMC act as a Carboxylating agent?: 

How DMC act as a Carboxylating agent? DMC is a Carboxylating agent when reacted with nucleophiles at reflux temperature (90 0 C) in the presence of base.

How DMC acts as an oxidizing agent?: 

How DMC acts as an oxidizing agent? Dimethyl carbonate can also act as an organic oxidant. Cyclopentanone and cyclohexanone react with DMC and a base pottasium carbonate to yield adipic and pimelic methyl(ethyl) esters, respectively.

Polymer supported reagents:: 

Polymer supported reagents: The main advantages of using these reagents are… The excess of the reagent can be recovered by filtration and used again. The isolation of product is very easy. Rate of reaction is faster. Yield is increased.

Polymer supported reagents: 

Polymer supported reagents Some of them are: Polymer supported peracids Polymer supported chromic acids Polymeric thioanisolyl resin Poly-N-bromosuccinamide Polymeric organotin dihydride reagent as a reducing agent Polystyrene carbodiimide Polystyrene Wittig reagent Polymeric phenylthiometyhl lithium reagent Polymer supported peptide coupling agent

Polymer supported peracids: : 

Polymer supported peracids : These are used for epoxidation of alkenes in good yields. P---Polymer.

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Polymer supported chromic acid: It has been used to oxidize primary and secondary alcohols to carbonyl compounds. It also oxidizes allylic and benzylic halides to aldehydes and ketones. Poly -N- bromosuccinamide ( PNBS) : It is an efficient polymer based brominating agent. It is used as benzylic and allylic brominating agent. Eg: cumene bromination.

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How ever, bromination with NBS gives α , β -dibromo cumene and α –bromocumene. α , β , β '-Tribromocumene α , β -Dibromocumene α -Bromocumene

Polymeric organotin dihydride reagent as a reducing agent:: 

Polymeric organotin dihydride reagent as a reducing agent: This reagent involves ease of operation, reaction workup and avoids toxic vapours. Used for the conversion of aldehydes and ketones to alcohols in 80-90% yields and the reduction of halides to hydrocarbons. The use of organotin hydride for the reduction of alkyl and aryl halides in presence of other functional groups is generally superior to lithium aluminium hydride. Also can be used for selective reduction of only one functional group of a symmetrical difunctional aldehyde (terephthaldehyde).

GREEN CATALYSTS: 

GREEN CATALYSTS Chemists have found ways to remove the need for large quantities of reagents by the use of catalysts that would otherwise have contributed to the waste stream. Catalysts play an imp role in establishing the economic strength of the chemical industry. Different types of catalysts used are: 1. Acid catalysts 2. Oxidation catalysts 3. Basic catalysts 4. Polymer supported catalysts

ACID CATALYSTS:: 

ACID CATALYSTS: The traditional catalyst HF, an extremely corrosive, hazardous and toxic chemical used in the pdtn of LAB’S. It has been successfully replaced by a solid acid catalyst like flourided silica-alumina catalyst, which involves lower operating costs and obviates the need of an acid scrubbing system and waste disposal of calcium fluoride. Microencapsulated Lewis acids have replaced traditional corrosive monomeric Lewis acids in reactions like Michael, friedel crafts, mannich, iminoaldol reactions.

Friedelcraft’s reaction:: 

Friedelcraft’s reaction: [MCSc(OTf)3]—Microencapsulated scandium trifluoro methane sulfonate.

Mannich Reaction: 

Mannich Reaction

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Several industrial processes use heteropolyacid catalysts. Examples are hydration of isobutylene and polymerisation of tetrahydrofuran. Zeolites are widely used in petrochemical industry in acid catalysed processes

OXIDATION CATALYSTS:: 

OXIDATION CATALYSTS : A large number of supported reagents have been used in the liquid phase partial oxidation of organic substrates. Also called as molecular sieves (titanium and vanadium) which have use in commercial units.

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The most important application of TS-1 is hydroxylation of phenol, giving mixtures of hydroquinone and catechol.

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Vanadium silicate molecular sieves are capable of selectively oxidising 4-chlorotoulene to 4-chloro benzaldehyde using H 2 O 2 as the source of oxygen in acetonitrile solvent By using other different catalyst we can get the same product but yield is only 4%.

BASIC CATALYSTS:: 

BASIC CATALYSTS: In contrast to the solid acid catalysis and oxidative catalysis, the use of solid base catalysis in liquid phase reactions has not met the same level. The industrial applications of basic catalyst are… Alkylation of phenol. Side chain alkylation. Isomerisation reactions.

Alkylation of phenol:: 

Alkylation of phenol:

POLYMER SUPPORTED CATALYSTS:: 

POLYMER SUPPORTED CATALYSTS: The conventional catalyst which is normally used in the homogenous phase, is linked to a polymer backbone and is used in this form to catalyze different reactions. Some of PSC’S are: 1. Polystyrene aluminium chloride 2. Polymeric super acid catalysts 3. Polystyrene metalloporphyrins 4. Polymer supported photosensitizers 5. Polymer supported PTC

Polystyrene aluminium chloride:: 

Polystyrene aluminium chloride: Dicyclopropyl carbinol Di(dicyclopropylcarbinyl)ether Polystyrene aluminium chloride is a useful catalyst for synthetic reactions, which require both a dehydrating agent and a Lewis acid. Thus, acetals are obtained in good yield by the reaction of aldehyde, alcohol and polymeric aluminium chloride in an organic inert solvent.

Polymeric super acid catalysts:: 

Polymeric super acid catalysts: It is obtained by binding aluminium chloride to sulphonated polystyrene. Used for the cracking and isomerisation of alkanes at 357 c at atm pressure. Generally this process is carried out in the presence of a Lewis acid at high temperature and high pressure.

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Polystyrene metallomorphins: These catalysts are useful for the oxidation of thiols to disulphide in presence of a base and is rapid at room temperature .

Polymer supported photo sensitizers:: 

Polymer supported photo sensitizers: Photo sensitizers supply molecular oxygen in a photochemical reaction. Examples of usual photo sensitizers are Rose Bengal Eosin-y Fluorescein A polymer bound Rose Bengal photosensirtizer is commercially available. The efficiency of SENSITOX is about 60% of that of usual rose Bengal, but the high yields of the products and the ease of isolation more than compensate the slightly longer reaction periods.

PTC IN GREEN SYNTHESIS:: 

PTC IN GREEN SYNTHESIS: Definition: PTC are ionic substances, usually quaternary “onium” salts where size of the hydrocarbon group in the cation is large enough confer good solubility of salt in organic solvent. The cation must be highly lipophilic. Example: PTC of SN 2 reaction between sodium cyanide and alkyl halide: PTC is usually a quaternary ammonium halide. PTC transfer the nucleophile as an ion pair into the organic phase.

Mechanism of PTC:: 

Mechanism of PTC: Na + CN -- RX Aqueous phase Organic phase Here no reaction takes place bcoz the nucleophile cannot enter the organic phase to react with RX. Q + X -- R 4 N + X - -

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Q + X -- Q + X -- Na + CN -- Na + X -- Q + CN — Q + CN — RX RCN Aqueous phase Organic phase Here PTC transfer the cyanide ion (as Q + CN — ) into the organic phase where the reaction takes place rapidly.

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In the organic phase the nucleophile of the ion pair reacts with the organic substrate. The cation Q+ and the anion X- then migrate back into the aqueous phase to complete the cycle. This process continues until all the nucleophile and organic substrate has reacted. Last step is the rate determining step.

Ionic liquids as green solvents: 

Ionic liquids as green solvents Green solvents have the following advantages when compared to organic solvents they are…. Less temperature. Increase in the yield Less amount of solvent Ionic liquids are of two types: Simple salts Binary ionic liquids

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Simple salts include….

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Binary liquid medium includes the mixture of aluminium chloride and 1,3 dialkylimidazolium chloride. Ex: 1-ethyl-3-methyl imidazolium. Represented as….( emin)Cl-AlCl3.

Conclusion: 

Conclusion Use of green catalysts reagents and solvents is not a solution to all environmental problems but the most fundamental approach for preventing pollution. Green chemistry is used to reduce energy, hazardous chemicals and cost etc; As chemists, lets put our hands together for designing, manufacturing of a chemical synthesis that doesn’t cause any harm to the environment and livelihood.

References: 

References New trends in green chemistry- VK AHLUWALIA & M KIDWAI, 1 st edition, page no: 1-56 Green chemistry by V.K. AHLUWALIA, 1 st edition, page no:1-10, 90-116 www.wikipedia.com www.greenchemistry.com