green reagents


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about green reagents


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GREEN REAGENTS Green reagent is one that reduces or eliminates the use or generation of hazardous substances in the design, manufacture and application of chemical products . Chemistry is undeniably a very prominent part of our daily lives. Chemical developments also bring new environmental problems and harmful unexpected side effects, as a result there is a need for ‘greener’ chemical products. A famous example for environmental pollutant is the pesticide DDT. Green reagents have minimal side effects , and low pollutability

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1.44 Å 1.34 Å 1.22 Å Dimethyl Carbonate- a green reagent

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Some industrial processes involving DMC Phenols Phenolic ethers Pharmaceutical intermediates Flavour and fragrances Amines Quaternarium ammonium compounds Surfactants Softeners Electronics Phenol Diphenylcarbonate Aromatic polycarbonates Methylisocianate production Allyl alcohol diols Allylcarbonates Optical organic glasses 1,6 hexanediol Aliphatic polycarbonate diols Polyuretans C12-C15-OH Oxoalcohol carbonates Synthetic lubricants

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Reactions of DMC As a methoxycarbonylating agent (substitute for phosgene ) e.g. RNH 2 + CH 3 OCOOCH 3 RNHCOOCH 3 + CH 3 OH instead of: 2 RNH 2 + COCl 2 + 2 NaOH RNHCOOR + 2NaCl + 2H 2 O As a methylating agent (substitute for dimethylsulfate and metal halides ) e.g. ArOH + CH 3 OCOOCH 3 ArOCH 3 + CO 2 + CH 3 OH instead of: ArOH + CH 3 X + NaOH ArOCH 3 + NaX + H 2 O ArOH + (CH 3 ) 2 SO 4 + NaOH ArOCH 3 + NaCH 3 SO 4 + H 2 O

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Dimethyl carbonate as a methylating reagent Conventional methylation reactions employ methyl halides or methyl sulfates. The toxicity of these compounds and their environmental consequences render these syntheses is somewhat undesirable. Tundo develped a method to methylate active methylene compounds using dimethylcarbonate , in which no inorganic salts are produced. R- CN + CH3-O-C-O-CH3 R- CH3OH+CO2

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Polymer supported reagents Besides DMC, there are a group of reagents which though are ordinary reagents , are bound to polymer support. The main advantage of using these reagents is that any excess of the reagent can be recovered by filtration and used again. Also, the isolation of the product is very easy. Some of these reagents are given as follows. ( e.g ) Polymer supported peracids These are used for epoxidations of alkenes in good yields. P -COOOH + -C=C- -C-C- P -COOH +

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Polymer supported chromic acid The polymer supported chromic acid ( Amberlyst A-26 , HCrO4 form) is commercially available and has been used to oxidise primary and secondary alcohols to carbonyl compounds and also oxidises allylic and benzylic halides to aldehydes and ketones . Cl -

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Polymeric Thioanisolyl Resin Polystyrene Methyl Sulphide on reaction with chlorine in the presence of triethylamine forms s- Chloro sulfonium chloride resin , which acts as a selective oxidant for the alcohols. OH(CH2)nOH HO(CH2) n-1 CHO(50.2%) + OHC(CH2) n-2 CHO(2.2%) P- P- Et 3 N Cl 2 s- Chloro sulfonium chloride

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Poly N – Bromo Succinimide It is an efficient polymer based brominating agent and is used as benzylic and allylic brominating agent. Thus , cumene on bromination yield α β β ’ tribromo cumene . However, bromination with NBS gives αβ dibromocumene and α bromocumeme . Cumene α , β , β ’ Tribromo cumene α , β , Dibromo cumene + Alpha bromo cumene N Br PNBS

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The polymer supported organotin dihydride reagent has been used for the conversion of aldehydes and ketone 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 the presence of other functional groups is generally superior to lithium aluminium hydride. This can also be used for the selective reduction of only one functional group of a symmetrical difunctional aldehyde ( terephthaldehyde ). Polystyrene carbodiimide Polystyrene carbodiimide is useful for the synthesis of anhydrides. It can also be used for the Moffat oxidation of alcohols to aldehydes and ketones . Even the prostaglandin intermediate(A) is readily converted to the desired aldehyde (B). Polymeric organo Tin Dihydride reagent as a reducing agent The use of polymeric organo tin dihydride reagent involves ease of opration and reaction work up and avoids toxic vapors , characteristic of tin hydride.

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Polystyrene anhydride Acetylation of aniline with polystyrene anhydride gives benzanilide in 90% yield. Similarly, ethyl benzoate is obtained in 90% yield by acylation of ethanol. Sulfonazide polymer It provides a route by which diazo group can be transferred to 1,3 dicarbonyl compounds very conveniently. P -SO2-N3 + Et3N + R-CO-CH2-CO-R1 R-CO-C(N2)-CO-R1 + P -SO2-NH2 At 25 Degree C

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Polystyrene Wittig reagent The polymeric Wittig reagent reacts with carbonyl compounds to give the usual products. Polymeric phenyl Thiomethyl Lithium reagent The polymeric phenyl thiomethyl lithium reagent is useful for lengthening of side chain of alkyl iodide in good yield. Polymer supported peptide coupling agent Ethyl 1,2 dihydro-2-ethoxy-1-quinoline carboxylate is used for forming peptide bond with no racemization . This reagent is now used as polymer supported EEDQ.

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Merits It minimizes the no of steps involved in the synthesis of a chemical compound. It reduces the no of by products released. It promotes the liberation of nontoxic compounds. Demerits It is not applicable for the synthesis of all chemical compounds.

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