1. Green Chemicals

The increasing local and global concern for environmental pollution offers incentive to explore new green materials. They can be characterized in three parts:

I. Green Reagent

II. Green Catalyst

III. Green Solvent

I. Green Reagents

In order to carry out the transformation of selected feedstock into the target molecule the criteria of efficiency, availability and effect of the reagent used must be kept in mind. Commonly used green reagents are:

i) Dimethyl carbonate (DMC)

ii) Polymer supported reagent

i) Dimethyl carbonate (DMC): DMC is a non toxic, environmentally safe reagent that can be used in organic synthesis as a green substitute.

e.g. in methylation of aromatic amine

DMC is also used to methylate active methylene compounds in which no inorganic salts are produced.

ii) Polymer supported reagents: Polymer supported reagents are those in which ordinary reagents are bound to polymer support.

e.g. polymer supported peracid is used for epoxidation of alkenes.

II. Green Catalyst: Catalysts play a major role in establishing the economic strength of chemical industry and developing clean chemical processes and products. Following types of catalysts are used³:

i. Acid Catalyst

ii. Oxidation Catalyst

iii. Basic Catalyst

iv. Photocatalyst

v. Phase transfer Catalyst

vi. Polymer supported catalyst

vii. Biocatalyst

i) Acid Catalyst: One of the most widely used epoxidizing reagent is meta-chloroperoxybenzoic acid, or m-CPBA, because of its mild conditions and high yield. However, it also produces chlorinated waste and is potentially explosive. It can be replaced by green catalyst- the bead polymers. The bead polymers are used for epoxidation of cyclohexene.

Boric acid is also considered as a green reagent. It is used in amide synthesis from acid and amine. It is the first successful report of boric acid, which has been used as lewis acid catalyst for the preparation of symmetrical N,N'-alkylidenebisamides. The remarkable advantages offered by this method are use of green catalyst, mild reaction condition, simple procedure, fast reaction and moderate to good yield of products.

ii) Oxidation catalyst: A large number of reagents have been used in the liquid phase partial oxidation of organic substrates by the use of molecular sieves (titanium and vanadium). Use of titanium silicates (TS-1) in hydroxylation of phenol gives mixture of hydroquinone, catechol and resorcinol.

iii) Basic catalyst: 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. There are many industrial applications of basic catalysts.

e.g. Alkylation of phenol:

iv) Photo catalyst: Photo catalysis is a phenomenon, where free radical pair is generated on exposing semiconducting materials to light of suitable energy. Thus, the chemical reactions that occur in the presence of a semiconductor and light are termed as photo catalytic reactions. Photo catalytic degradation of chloral hydrate in aqueous semiconductor suspension involves ˙OH radicals as oxidizing species.

v) Phase transfer Catalyst (PTC): The basic function of PTC is to remove the interface between two immiscible reactants. It involves the transfer of ion or organic molecule between two liquid phases or a liquid and solid phase. It works as a transport shuttle. PTC has several advantages relevant to green synthesis: higher productivity; higher selectivity, ease of product separation, use of less hazardous solvents. It is used in organic synthesis like

Commonly used catalysts for solid-liquid systems are crown ether and polyglycol ether e.g. esterification

vi) Polymer supported Catalyst: The catalyst which is linked to a polymer backbone and used in catalyzing different reactions in homogenous phase are known as polymer supported catalyst.

e.g. polystyrene aluminium chlorides, polymeric super-acid catalyst, polymer supported phase transfer catalyst.

Polystyrene aluminium chloride is used for formation of ether from alcohol.

Polymer supported phase transfer catalyst is used for formation of nitrile from haloalkane.

vii) Biocatalyst: Usually chemical reactions are carried out in vigorous condition, while biocatalyst offers the possibility of carrying the reaction without formation of toxic and carcinogenic ingredients. There are many valuable compounds made in higher volume by biocatalyst. These include lactic acid, malic acid, L-aspartic acid and other amino acids. L-Malate is made by hydration of fumarate using a fumarase from Breviabacterium flavum.

Allyl alcohol is oxidized to acrolein with aldehyde dehydrogenase.

Green Solvents: The use of classical solvents in organic synthesis, such as acetone, THF, benzene, diethyl ether and others produce a large amount of environmentally toxic waste. Now-a-days following solvents are emerging as potential green solvents :

Strategies to reduce waste solvent production in chemical processes:

1. Use of supercritical fluids as solvent e.g CO₂

2. Use of water as a solvent

3. Use of ionic liquids as solvent

i. Supercritical CO₂: It can be used in the reaction process as ecofriendly alternative and its solubility limitation can be overcome by using non toxic ammonium carboxylate perfluoropolyether as surfactant. Supercritical CO₂ has been used to replace volatile organic compounds (VOCs) in spray paints. Under supercritical conditions, carbon dioxide flows like a gas, and dissolves like a liquid, but behaves with chemical properties unlike gases or liquids. e.g. Asymmetric catalytic hydrogenation reactions are carried out in supercritical CO_2.

Esterification of phthalic anhydride with methanol is reported in supercritical CO₂

Advantages of Supercritical CO₂

a) It is generally regarded as safe

b) It is inexpensive

c) It is easily available

d) It is non-toxic, non-flammable and inert to most materials

e) It is available in high purity

f) It is ideal substitute for many hazardous and toxic solvents

g) It is easy to separate and it does not damage the solutes of the product

ii. Water: Water promotes many reactions at temperature higher than 100⁰ C. These include

a) Diels-Alder reaction

b) Benzoin condensation

c) α-Cyano-β-arylnitroethenes can be prepared in high yields at room temperature by a one-pot reaction of aryl aldehydes with nitroacetonitrile in aqueous medium.

d) Preparation of conjugated nitroalkenes from alkenes in aqueous-organic media using sodium nitrite and iodine in the presence of ethylene glycol is reported.

Advantages of water: Water is the best solvent among all the green solvents because of its many advantages such as

a. Environmental benefits

b. Safety

c. Synthetic efficiency

d. Simple operation

e. Potential for new synthetic methodologies

f. Cost

iii Ionic Liquids: It is the generic term for a class of materials, consisting entirely of ions and being liquid below 100⁰ C. The first known ionic liquid was ethylammonium nitrate which was discovered in 1914 by Walden.

IL Cations includes Pyridinium, Imidazolium, Pyrrolidinium, Thiazolium

IL Anions includes Hexafluorophosphate, Tetrafluoroborate, Chloroaluminate, Trifluoromethanesulfonate

Characteristics of ionic liquids:

a) These melt <100^oC; high thermal capacities

b) These are highly polar and non-coordinating

c) These are considered as excellent solvents for a wide range of inorganic, organic, and polymeric materials

d) These has broad liquid range

e) Ionic liquids have extremely low vapour pressure

f) These are non-flammable, renewable and reusable non-flammable

g) Ionic liquids may be water immiscible

h) These may exhibit Bronsted, Lewis and super-acidity

The ionic liquid-[bmim]⁺[BF₄]^-containing [Ru-BINAP] as homogenous catalyst, could selectively hydrogenate 2-arylacrylic acid enantio-selectively in high yields with 80% ee⁴.

Esterification of carboxylic acids with alcohols was carried out in Bronsted acidic ionic liquid 1-methyl-imidazolium tetrafluoroborate.

Advantages of ionic liquids:

a) Ionic liquids results in enhanced yields.

b) These results in enhanced selectivity.

c) These liquids enhance reaction rates.

d) Ionic liquids are renewable and reusable.

e) There is limited solvent loss due to evaporation because they have high boiling points.

Organic Synthesis in solid state: The use of solid support in reaction has been widely reported as an appropriate methodology for the reduction of waste and minimization of energy use, in line with the principle of green chemistry. The organic synthesis can be presented in two parts:

I. Solid phase organic synthesis without using any solvent: Condensation of substituted benzaldehyde derivatives and cyanoamides under solvent free condition, in the presence of piperidine catalyst yielded nitro derivatives in high conversion and purity.

II. Solid supported organic synthesis using solvent: Dieckmann condensation of diethyl adipate and pimelate occur in the presence of powdered tBuOK.

Green Methodology: Use of energy sources like light, microwave, ultrasound and electricity are more clean and efficient. Emerging techniques, in the overall development of “Green Chemistry”, can be categorised in following parts:

I. Photochemistry

II. Microwave Chemistry

III. Sonochemistry

IV. Electrochemistry

I. Photochemistry: A photochemical reaction occurs when an atom or molecule absorbs light and it must transfer all its energy to atom or molecule and promote it to higher energy state. The energy required for promotion from ground state to lowest excited state falls in ultraviolet and visible region of electromagnetic spectrum. e.g. Dithianes, benzyl ethers and related compounds have been cleaved by the use of visible light.

Acylhydroquinones can be produced from 1,4-benzoquinone and an aldehyde using light from a sunlamp. In contrast to the usual acylation with an acid chloride, this process produces no byproduct salt.

II. Microwave Chemistry: The use of microwave energy instead of conventional heating, results in good yields in a very short time. Microwave heats some substances not others due to selective absorption of microwave radiations by polar molecules and the non polar molecules. Reaction can manifest in several ways including:

1. Highly accelerated reaction rates

2. Improved yield

3. Reduction in side products

4. Limited amount of solvents needed

5. Successful product formation that fail under conventional conditions

6. Simplification and improvement of classical synthetic method

Reactions occurring in microwaves can be classified in two categories:

a) Reaction with solvent: In microwave some organic reactions can be carried out with the use of organic solvent like ethanol, methanol, hexane etc. e.g. Esterification of alcohol

Microwave assisted synthesis of Schiff bases can be done by using ethanol as a solvent to get the product in high yield.

b) Solvent free reactions: A novel and efficient synthesis of N-arylamines by the reaction of activated aryl halides with secondary amines in the presence of basic Al₂O₃ under microwave irradiation in solvent free conditions is reported.

Alkylation:

Synthesis of fused anthraquinones using clay under microwaves

III. Sonochemistry: The study of effects of ultrasound on chemical reactivity is termed as sonochemistry. The chemical reactivity of a system increases on irradiating it with power ultrasound. Ultrasound assisted organic synthesis gives excellent yields compared to other reactions. Combination of sonication with other techniques, e.g. microwave, photocatalysis etc. gives best results. Some important applications of ultrasound in chemical synthesis are:

i. Hydrolysis

ii. Substitution

iii. Strecker synthesis of aminonitriles

IV. Electrochemistry: Electrochemical synthesis is a well established technology for major processes such as aluminium and chlorine production. The possible green benefits of using electrochemical synthesis include: water based processes, mild operating conditions and atom-efficient. e.g. Synthesis of 3-bromothiophene provides an example of the obvious environmental benefits of the electrochemical route compared to conventional process.

Conventional route

Electrochemical route