INTRODUCTION TO SYNTHETIC STRATEGIES IN CHEMISTRY:POROUS MATERIALS

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Chapter - 1

INTRODUCTION TO SYNTHETIC STRATEGIES IN CHEMISTRY

Prof. B. Viswanathan

INTRODUCTION

One of things that make chemistry unique among the sciences is the synthesis. Chemists

make things, new pharmaceuticals, food additives, materials, agricultural chemicals,

coatings, adhesives, and all sorts of useful new molecules. The chemists prepare them

from simpler more readily available starting materials. There are two aspects to organic

synthesis first the development of a synthetic strategy or plan of action and the second the

actual implementation of that plan in a chemical laboratory.

Synthesis of molecules and materials are important aspect of the development of

science. In the past, synthesis is based on some of the Name reactions in organic and

Inorganic chemistry.

The attempts to synthesis new molecules and materials have

always depended on the experience and extrapolation of the existing knowledge to new

situations and new synthesis of molecules.

However in the last two decades, the

synthesis has become a well established science going beyond the recollection and

adoption of the existing procedures. The normal procedures so far adopted for the

synthesis of materials are

(i)

precipitation based on solubility principle ( based on thermodynamic

quantities)

(ii)

Bond formation and bond breaking usually brought about by reagents,

participating species and leaving entities (these are often kinetically

controlled); processes depending on the strength of binding already existing

or being formed.

(iii)

simple solid state reactions often controlled by diffusion

(iv)

ligating or binding species

These procedures have been satisfying the normal curiosity of the chemists for new

molecules.

However today, Chemistry is the study of molecules, materials of

functionalities and hence they have to be synthesized, built, and architectured and

designed

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Introduction to Synthetic Strategies in Chemistry

Today the molecules and materials are for many applications.. Every sector of life

requires molecules/materials with functionalities, stability, as well as durability under

adverse conditions. Molecules of highly active functionalities with stability, durability

and appropriate stress- strain relationship (global properties of dissolution, digestion,

attachment to species and others) are the demands of today.

Most often the materials

required have to be in the meta stable state but should be stable enough to be useful in

devices.

The exploitable properties must be able to be controlled by Size, shape,

orientation and morphology.

Over the last thirty years, synthetic chemists have

developed an assortment of routes to obtain optically pure compounds. Many strategies

include the use of pure starting materials with chiral centres. In addition, the utilization of

chiral auxiliary groups have been implemented to achieve an increase in stereo-selectivity

and to simplify the purification process. Lastly, asymmetric catalysis using enzymes or

inorganic catalysts has also been used to afford the desired stereochemistry.

At this stage the important questions to be faced are:

1. In the case of organic chemistry how can one build the desired carbon skeleton?

2. How does one introduce the necessary functional groups?

3. How does one control the regio- and stereochemistry of reactions?

These questions can be expanded to any extent depending on the need for molecules

and materials.

The chemical synthesis of complex organic molecules is integral to many advances

that enhance the quality of life, such as novel disease treatments, agrochemicals with

improved properties and advanced materials for high performance technology and

biotechnology.

The methods for synthesizing complex organic molecules have

traditionally pieced molecules together in a linear manner, gradually building complexity

into the molecule. This can be a very time consuming process, and synthetic routes to

molecules can end up being incredibly long, requiring extensive resources and

manpower.

Since 2000 pioneering approaches to synthesis that combines two-directional

synthesis and tandem reactions. Linear symmetrical trifunctional compounds are

synthesized through use of two-directional synthesis and a range of tandem reactions are

then applied to generate a range of diverse structures from these simple substrates. Two-

Synthetic Strategies in Chemistry

1.3

directional synthesis, when used in combination with tandem reactions, can lead to

significantly faster strategies for the synthesis of complex molecules.

The recognition of the strong dimensionality-dependent physical-chemical properties

of inorganic matter at the nanoscale has stimulated efforts toward the fabrication of

nanostructured materials in a systematic and controlled manner. Surfactant-assisted

chemical approaches have now advanced to the point of allowing facile access to a

variety of finely size- and shape-tailored semiconductor, oxide and metal nanocrystals

(NCs) by balancing thermodynamic parameters and kinetically-limited growth processes

in liquid media. While refinement of this synthetic ability is far from being exhausted,

further efforts are currently made to provide nanocrystals with higher structural

complexity as means to increase their functionality. By controlling crystal miscibility,

interfacial strain, and facet-selective reactivity at the nanoscale, hybrid nanocrystals are

currently being engineered, which consist of two or more chemically different domains

assembled together in a single particle through a permanent inorganic junctions.

POROUS MATERIALS

This is one class of materials which has seen a tremendous advancement in the synthetic

strategies. Porous solids have high scientific and technological interest. They are able to

interact with atoms, ions and molecules at surfaces and throughout the bulk of material.

Distribution of sizes, shapes and volumes of the void spaces in porous materials are

directly related to their ability to perform desired function in particular application.

High SA/volume ratio provides a strong driving force to speed up thermodynamic

processes that minimize free energy

In high surface area materials the active sites are more isolated. Materials with uniform

pores can separate molecules on basis of size. They are also employed as

adsorbents,

catalyst supports, and electrode materials.

The porous solids can be classified according to the range of pore sizes available in them.

A simple classification is shown in Scheme 1.1.

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Introduction to Synthetic Strategies in Chemistry

Porous materials

macroporous

mesoporous

Microporous

eg:

ZSM-5,AlPO

MCM-48,SBA-15,CMK-n

photonic crystals

P.D

< 2 nm

2-50 nm

> 50 nm

300-500 m2g-1

1000-3000 m2g-1

10-500 m2g-1

SA:

Scheme 1.1 A simple classification of porous substances.

The porous architecture is normally built in various ways. One such methodology is

shown in Fig.1.

Schematic representation

Silica/surfactant =

1/0.27

calcination

As-synthesized MCM-41

(hexagonal structure)

MCM-41

Silica/surfactant =

1/0.60

calcination

As-synthesized MCM-48

MCM-48

(cubic structure)

J. S. Beck, J.C. Vartuli, W. J. Roth, M. E. Leonowiez, C. T. kresge, K. D. Schmitt, C. Chu, D. H. Oison, and E. W.

Sheppard. S. B. McCullen, J.L. Schlenker. J. Am. Chem. Soc., 114 (1992) 10834

Fig. 1.1. The formation of mesoporous Mobil composition of materials (MCM) formed as

a result of template mechanism (the details will be discussed in chapter 6)

Synthetic Strategies in Chemistry

1.5

Conventionally the template route is understood in terms of space filling mechanism as

shown in Fig. 2.2 by the template molecules, template molecular aggregates or structure

directing agents.

The role of quaternary directing agents

Small individual alkyl chain length quaternary directing agents generate the formation

of micro-porous solids

Long alkyl chain length quaternary directing agents self-assemble to supramolecular

Species which can generate the formation of mesoporous molecular sieves

Thomos J. Barton et al., Chem Mater., 11 (10) ( 1999) 2633

Fig. 1.2. Simple mechanism for the formation of porous solids

Generally the interactions in formation of materials can be understood in terms of

bonding interactions. One such classification is given in Scheme 1.2.

ORGANIC SYNTHESIS

Classic examples of synthesis can be found in the area of organic Chemistry. Strategies

and Tactics in Organic Synthesis provide a forum for investigators to discuss their

approach to the science and art of organic synthesis. Rather than a simple presentation of

data or a second-hand analysis, one is provided with stories that vividly demonstrate the

power of the human endeavour known as organic synthesis and the creativity and tenacity

of its practitioners. First hand accounts of each project tell us of the excitement of

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Introduction to Synthetic Strategies in Chemistry

conception, the frustration of failure and the joy experienced when either rational thought

and/or good fortune give rise to successful completion of a project. Synthesis is really

done and are educated, challenged and inspired by these stories, which portray the idea

that triumphs do not come without challenges. We also learn that we can meet challenges

to further advance the science and art of organic synthesis, driving it forward to meet the

demands of society, in discovering new reactions, creating new designs and building

molecules with atom and step economies that provide solutions through function to create

a better world.

These are usually termed as Name reactions in organic chemistry. These

aspects will be dealt with in separate chapters.

Type of interaction

Surfactant

Inorganic precursor

Notation

Examples

S+-----I-

Cationic

Anionic

M41S,

Ionic

M-MCM-41, 48

(Direct

pathways)

Cationic

Anionic

S------I+

M-M41S,

S+ X-I+

Cationic

+ Cationic

SBA, APM

Ionic

(Mediated

pathways )

Metal Oxides

S- M+ I-

Anionic

HMS

S0-----I0

Neutral

Hydrogen

Amine

bonding

(Neutral )

SBA

S0-----I0

Neutral

Polymer

Covalent

TMS

Neutral

S-----I

Scheme 1.2. Possible interactions in the Synthetic Strategies for Porous Materials

META STABLE STATE OF THE MATERIALS

For many device and other applications, one is required to make materials in the meta

stable state.

Meta stable state can be obtained in materials by a variety of routes. The

possible list of the methods can be:

Organization

Templating bonding and structural

Auto-thermal

Synthetic Strategies in Chemistry

1.7

Transport CVD, PVD

Field induced ( electro-deposition, T induced)

Moulding and blowing

Extrusion ( geometry control)

The list given is not complete but it gives an idea of what methods can generate the meta

stable of materials.

INORGANIC MATERIALS

Inorganic materials occupy a unique place and they can be synthesized by various

methods. Some well known methods include

Sol gel techniques

Co-ordination chemistry

Weak interactions

bonding interactions

These methods are described in detail in subsequent chapters. The purpose of the present

book is to assimilate available information on synthetic strategies in Chemistry. Though

one can not claim comprehensiveness on this topic, it is the endeavour to bring some of

the available knowledge in one place. It is hoped from that point of view, the synthetic

chemists may find this book useful.

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