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Understanding The Stereochemistry Of Organic Compounds Environmental Sciences Essay

Stereoisomers are defined as molecules of identical atomic compositions (molecular formulas), but with different bonding arrangements of atoms or orientation of their atoms in space. Based on this definition, several types of isomerism are possible including constitutional, configurational, and conformational isomerism. Constitutional isomers (also called structural or positional isomers) are molecules with the same atomic composition but different bonding arrangements between atoms, as illustrated by theexamples of catechol, resorcinol, and hydroquinone . All of these compounds have the same atomic composition (C6H6O2), but different bonding arrangements of atoms and are thus distinct chemical entities with different chemical and physical properties.

Stereochemistry (from the the greek stereos,meaning solid) refers to chemistry in three dimensions.

Since most molecules are three dimension , stereochemistry ,infact, prevade all chemistry.

It is not so much a branch of the subject as a point of view, and wheather one choose to take this point of view in any given situation depends on the problem one wants to solve and on the tools one has available to solve it.

We factorize stereochemistry into its ststic and dynamic aspects.STATIC stereochemistry (better called stereochemistry of molecules) deals with the counting of stereoisomer, with their structure, with their energy and with their physical and most of their spectral property.

DYNAMIC stereochemistry(stereochemistry of reaction) deals with the stereochemical requirement and stereochemical outcome of chemical reactions, including interconversion of conformational isomerms.

If we represent the following imaginary molecule with no stereochemistry, there is nothing apparently special about it:

However, if we represent it fully, including the position of atoms in space, it becomes apparent that the description above is vague, as it encompasses two molecules; those below:

This is what stereochemistry is we can define compounds into many way and can also originate many new compouds.

So stereochemistry is chemistry that studies the property of isomers.

HISTORY

Historically the origins of stereochemistry stem from the discovery of plane polarized light by the french physicist MALUS.

In 1815 biot note that certain natural organic compounds rotate plane polarized light as

ARGO(1811), discovered that a quartz plate, cut at a right angle to its crystal axis ,rotates the plane polarized light through an angle proportional to the thickness of the plate. S ome quartz crystal rotate towards left while some to the right.

However in 1847 LOUIS PASTEUR find that equimolar solution of seprated mixture have equal but opposite optical activity.

In 1874 LEBEL and VAN'T-HOFF proposes that carbon with 4 attachment is tetrahedral and a molecule having a teahedral geometry will exist as pair of two isomer.

ISOMERISM

Isomers are defined as molecules of identical atomic compositions (molecular formulas), but with different bonding arrangements of atoms or orientation of their atoms in space. Based on this definition, several types of isomerism are possible including constitutional, configurational, and conformational isomerism. Constitutional isomers (also called structural or positional isomers) are molecules with the same atomic composition but different bonding arrangements between atoms, as illustrated by theexamples of catechol, resorcinol, and hydroquinone . All of these compounds have the same atomic composition (C6H6O2), but different bonding arrangements of atoms and are thus distinct chemical entities with different chemical and physical properties.

GEOMETRIC ISOMERISM

Geometric isomers have the same empirical formula or molecular formula and also the same structural formula, but have a different relative arrangement of the substituent groups. For example, the two geometric isomers of 1,2-dichloroethene (Figure 2) have the molecular formula of C2H2Cl2, and the same structural formula of Cl(H)C=C(H)Cl, but the relative position of the two chlorine atoms can either be the same side of the C=C double bond (i.e., cis, see Figure 2a) or on opposite sides of the C=C double bond (i.e., trans, see Figure 2b). The use of cis and trans is not limited to organic compounds such as olefins, but can also be used in metal complexes, e.g., Figure 3.

Figure 2: The two geometric isomers of 1,2-dichloroethene. Figure 3: Examples of (a) cis and (b) trans geometric isomers for metal complexes. When it is not possible to describe geometric isomers by the terms cis or trans, the terms facial (fac, Figure 4a) or meridinal (mer, Figure 4b) are commonly employed. Examples of (a) fac and (b) mer geometric isomers for metal complexes.

POLARIMETRY AND OPTICAL ROTATION

Polarized light is obtained by passing ordinary light through a polarizer , such as nicol prism.The orientation of the polarizer’s axis of polarization determines the plane of the resulting polarized light.

OPTICAL ACTIVITY:-

Unpolarized light from a suitable source is made to pass through a polarizer that usually consists of a pair of crossed Nichol prisms. The light leaving the polarizer is plane polarized and, in the diagram, the angle of the Nichol prisms has been adjusted to produce vertically polarized light. This light is then passed through a tube containing the sample. If the sample is optically active, the plane of the polarized light will be rotated, as shown in figure The extent of rotation will depend on the polarizability of the substance and its concentration. The light from the sample cell is then passed through another pair of crossed Nichol prisms called the analyzer. The analyzer prisms are then rotated so that the transmitted light is again vertically polarized. The angle through which the analyzer has been turned is called the angle of polarization. Clockwise rotation of polarized light is designated as (+) and anti clockwise rotation as ( ). The established method of defining rotation is that the (+) isomer is termed dextro and the ( ) isomer laevo.

Optical activities of enantiomer

A pair of enantiomer are distinguised by their optical activites because a pair of enantiomers rotates the plane of polarized light by equal amounts in opposite direction.

In a mixture of two enantiomers, each contribute to the optical rotation in proportion to its concentration.It follows that a sample containing equal amounts of two enantiomer must have an observed optical rotation of zero.

Enantiomer of tartaric acid is:

ENANTIOMER

Enantiomers are compounds that have the same ordering of atoms as each other, but that differ from one another when viewed in three dimensions. The compound drawn to the left is an enantiomer because it has the H3C group projecting up from the page toward the reader. The corresponding enantiomer would have the H3C group projecting below the page away from the reader. A racemic mixture refers to a 50:50 mixture of two corresponding enantiomers.

CHIRALITY

Chirality is a structural property of an object. An object is said to bechiral if its mirror image can not be superimposed with itself by the means of rotations. The most familiar example of a chiral object is our hand. To see this, we first have to convince ourselves that our two hands are mirror images of each other. Then we can spend hours rotating our two hands around and trying to superimpose them point-to-point, only to find out that it is impossible. Thus a human hand is a chiral object. An object is said to be achiral if its mirror image is its exact replica

DIASTEREOMER

They are different physical/chemical properties in chiral/achiral environments.

Stereoisomers that are not enantiomer are called diastereomers.

FISHER PROJECTION

Fischer projections are used to visually describe various isomers of the same compound in two dimensions. They are also used as a basic test for optical activity (or chirality). The Fischer projection looks like a cross, with the (invisible) asymmetric carbon located at the points where the lines cross. The horizontal lines are taken to be wedges, or bonds that project out of the plane of the paper. The vertical lines are taken to project away form the viewer, or back below the plane of the paper, as dashed lines.

CONFIGURATION

Configurational isomers are defined as molecules of identical atomic composition and bonding arrangements of atoms, but different orientations of atoms in space, and these different orientations cannot interconvert freely by bond rotation. Since these types of isomers differ only in relative spatial orientations of atoms, they are commonly referred to as stereoisomers. Configurational stereoisomers are subcategorized as optical isomers (enantiomers) or geometric isomers (Fig. 2), depending upon the hybridization state and geometry of the atoms that impart the properties of stereoisomerism and the overall structure of the molecule. Stereoisomers of this type are distinct chemical entities that may have different chemical and physical properties.

CONFORMATION

Conformational isomers (conformers) are stereoisomeric forms characterized by different relative spatial arrangements of atoms that result from rotation about sigma bonds. Thus, unlike configurational isomers, conformers are interconverting stereochemical forms of a single compound.

STEREOCHEMICAL CORRELATION

The absolute configuration of most organic compounds are determined instead by using chemical reaction correlate with other compounds of known absolute compounds is known as stereochemical correlation.

STEREOCHEMISTRY OF CHEMICAL REACTION

No chemical reaction can be planned without stereochemical details,and no chemical reaction can be planned without considering problems oe stereochemistry that might arise.

A.STEREOCHEMICHEMISTRY OF ADDITION REACTION

An addition reaction can occur in either of two stereochemically different ways,called syn addition and anti addition.

Stereochemistry of an addition can be determined only when the stereochemically different modes of addition give rise to stereochemically different products.

Syn and anti addition gives different products only when both carbons of the double bond becomes carbon stereocentre in the product.

STEREOCHEMISTRY OF SUBSITITUTION REACTION

A substitution reaction can occur in two stereochemically different ways, called retention of configuration and inversion of configuration then x and x’ have the same relative stereochemical position.

Substation rxn with retention of configuration is

It implies that if x and x’ have the same relative priorities in the R,S system then the carbon that undergoes subsititution will have the same configuration in the reactant and the product.

When the subsititution occur with inversion of configuration then x and x’ have different relative stereochemical positions.

Subsititution with inversion of configuration is:

They have same relative priorities in the R,S system, then the carbon that undergoes substitution must have opposite configuration in the reactant and the product.

Stereochemistry of  Reactions

Two products are formed when a chiral substrate that possesses an asymmetric, electrophilic carbon is applied in an  reaction. One of them has the same absolute configuration as the starting product (if, according to the CIP rules, the leaving group and the nucleophile have the same position in the priority order of the substituents), which is called retention. In contrast, the other product possesses the opposite absolute configuration, known as inversion. In reactions, the nucleofuge exits the substrate before the nucleophilic attack can ever occur. Thus, an intermediate carbocation is then formed. Due to the carbocation's trigonal planar shape, its two enantiotopic sides are susceptible to attack by the nucleophile with the same probability.

Stereochemistry of Reactions

If a pure enantiomer is applied to an reaction, three different stereochemical results are conceivable:

The initial spatial arrangement of the reaction center's substituents remains (retention).

The initial substituent's spatial arrangement is inverted (inversion).

Retention, as well as inversion takes place. If retention and inversion occur to the same degree, the reaction yields a racemate (racemization).

Stereochemistry of E2 Elimination

In the E2 elimination reaction, the carbon-hydrogen sigma bond and the carbon-leaving group sigma bond must lie in the same plane. This allows the orbitals to begin to overlap to form the pi bond as the bonds to the hydrogen and the leaving group are broken. There are two possible planar arrangements of these bonds: both on the same side of the C-C bond (syn-coplanar); or on opposite sides of the C-C bond (anti-coplanar)

In syn the bond from the carbon to the leaving group (green) and the bond from the other carbon to the hydrogen (blue) are syn-coplanar. The dihedral angle between these bonds is zero degrees. This conformation is eclipsed about the carbon-carbon bond

In anti the bond from the carbon to the leaving group (green) and the bond from the other carbon to the hydrogen (blue) are anti-coplanar. The dihedral angle between these bonds is 180 degrees. This conformation is staggered about the carbon-carbon bond. Because this conformation is more stable than the eclipsed conformation required for syn elimination, anti elimination is preferred in E2 reactions

STEREOCHEMISTRY OF ALKANE

Conformations

Alkane conformers arise from rotation around sp3 hybridised carbon carbon sigma bonds. The smallest alkane with such a chemical bond, ethane, exists as an infinite number of conformations with respect to rotation around the C-C bond. Two of these are recognised as energy minimum (staggered) and energy maximum (eclipsed) forms. The existence of specific conformations is due to hindered rotation around sigma bonds, although a role for hyperconjugation is proposed by a competing theory.

CONSTITUTIONAL ISOMER

There are different types of isomer.

Isomers such as butane and isobutane that differ in the connectivity of their atom are termed as constitutional isomer.butane and isobutane are only constitutional isomer with the formulaC4H10 .

However ,more constitutional isomer are possibles for alkane with more carbon atoms.

STEREOCHEMISTRY OF ALKENE

The stereochemistry of halogen addition is unequivocally determined by the anti addition and the almost completely restricted rotation of the carbon-carbon bond of the halonium ion. Therefore, the bromination of cis-2-butene yields a racemate of (2R,3R)- and (2S,3S)-dibromobutane, whereas the bromination of trans-2-butene yields the meso compound.

CIS TRANS GEOMETRY OF ALKENE

If alkenes have two different substituents at each end of the C=C then they can exist as stereoisomers (as geometric isomers ).

This is because there is restricted rotation of the double bond due to the pi bond

CIS TRANS

E-Z SYSTEM

The cis- / trans- style is based on the longest chain whereas the E/Z style is based on a set of priority rules. 

You need to know both styles.

Z SYSTEM cis-but-2-eneor (Z)-but-2-ene

The E- and Z- style is more reliable and particularly suited to highly substituted alkenes, especially when the substituents are not alkyl groups.

STEREOCHEMISTRY:-FUTURE PROSPECTUS AND APLLICATION

1.Temperature modulation of the stereochemistry of enzymatic catalys.

2.It is used in medical purpose for malarial prevention, control and research

3.The HeI resonance line used for UPS was produced by DC discharge of pure helium gas.

4. Stereochemistry is highly used in biochemistry in various purposes like enzyme catalyst e.t.c.

5. Penning ionization is known to be one of the most important types of chemical reaction in aerospace.

Steric shielding effect of methyl group an penning ionization in subsitutional aniline.

6. Stereochemistry is always used in discovering new compounds

7. Stereochemistry is used to know the property and other new property of the existing compound or the compounds not known…..

REFRENCES

Hehre, W.J.(1975),J.AM.CHEM

KARLE J.(1973),’ELECTRON DIFRACTION ‘IN NACHOD,F.C AND ZUKERMAN, VOL 5 P12.

ELIEL L. ERNEST(2009),STEREOCHEMISTRY OF ORGANIC COMPOUNDS,WILEY STUDENT EDITION

LOUDON MARC G.(2009),4th EDITON ,ORGANIC CHEMISTRY ,OXFORD UNIERSITY PRESS

WEBSITE

http://commons.wikimedia.org/wiki/File:Example_of_stereoisomers

http://www.google.co.in/imgres?imgurl=http://img121.imageshack.us/img121/5564/81111276.jpg&imgrefurl=http://www.chemicalforums.com/index.

WWW.CHEMISTRY.UK

www.wileyindia/chem.com

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