Understanding The Stereochemistry Of Organic Compounds Environmental Sciences Essay

Understanding The Stereochemistry Of Organic Compounds Environmental Sciences EssayStereoisomers be defined as scraps of identical atomic writings (molecular laws), nalwaystheless with divergent adhere arrangements of atoms or orientation of their atoms in space. Based on this definition, several types of isomerism argon viable including constitutional, conditional, and patternal isomerism. radical isomers (also called structural or positional isomers) ar molecules with the uniform atomic composition but antithetical hold fast arrangements in the midst of atoms, as illustrated by theexamples of catechol, resorcinol, and hydroquin iodin . altogether of these compounds bewilder the homogeneous atomic composition (C6H6O2), but disagreeent trammel neting arrangements of atoms and atomic number 18 thus distinct chemical entities with different chemical and material properties.Stereochemistry (from the the greek stereos,meaning solid) refers to chemistry in one- third dimensions.Since most molecules atomic number 18 three dimension , stereochemistry ,infact, prevade all chemistry.It is non so much a break up of the subject as a point of view, and wheather one choose to agree this point of view in any given situation depends on the problem one wants to lap 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 grammatical construction, with their energy and with their physical and most of their apparitional prop.DYNAMIC stereochemistry(stereochemistry of chemical chemical 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, in that location is nonhing apparently special nearly itHowe ver, if we represent it fully, including the position of atoms in space, it becomes apparent that the description above is vague, as it encompasses twain molecules those belowThis is what stereochemistry is we washstand define compounds into many way and foot also originate many new compouds.So stereochemistry is chemistry that studies the property of isomers.HISTORYHistorically the origins of stereochemistry stem from the discovery of tack polarized lighthearted by the french physicist MALUS.In 1815 biot note that certain natural organic fertilizer compounds spread out plane polarized light asARGO(1811), discovered that a watch crystal plate, break up 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 smorgasbord receive correspond but opposite opthalmic a ctivity.In 1874 LEBEL and VANT-HOFF proposes that degree Celsius with 4 attachment is tetrahedral and a molecule having a teahedral geometry will exist as coupling of dickens isomer.ISOMERISMIsomers are defined as molecules of identical atomic compositions (molecular formulas), but with different attaching arrangements of atoms or orientation of their atoms in space. Based on this definition, several types of isomerism are possible including constitutional, bodal, and conformational isomerism. Constitutional isomers (also called structural or positional isomers) are molecules with the analogous atomic composition but different beating arrangements between atoms, as illustrated by theexamples of catechol, resorcinol, and hydroquinone . All of these compounds pee the same atomic composition (C6H6O2), but different perplexing arrangements of atoms and are thus distinct chemical entities with different chemical and physical properties.GEOMETRIC ISOMERISMGeometric isomers have t he same verifiable formula or molecular formula and also the same structural formula, but have a different carnal slamledge arrangement of the substituent congregations. For example, the two geometric isomers of 1,2-dichloroe paste (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 sack apiece be the same side of the C=C parallel 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 restrict to organic compounds such as olefins, but finish also be apply 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 comm completely em ployed. Examples of (a) fac and (b) mer geometric isomers for metal complexes.POLARIMETRY AND OPTICAL ROTATIONPolarized light is obtained by passing ordinary light through a polarizer , such as nicol prism.The orientation of the polarizers axis of polarisation determines the plane of the resulting polarized light.OPTICAL ACTIVITY-Unpolarized light from a suitable root word is made to pass through a polarizer that usually consists of a pair of crossed Nichol prisms. The light divergence the polarizer is plane polarized and, in the diagram, the angle of the Nichol prisms has been well- cognize(prenominal)ized to produce vertically polarized light. This light is then passed through a metro containing the sampling. If the sample is opticly active, the plane of the polarized light will be rotated, as shown in figure The extent of gyration will depend on the polarizability of the substance and its concentration. The light from the sample cell is then passed through other pair of c rossed 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 enantiomerA pair of enantiomer are distinguised by their ocular 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 isENANTIOMEREnantiomers 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 drifting up from the page toward the reader. The corresponding enantiomer would have the H3C group projecting below the page international from the reader. A racemic mixture refers to a 5050 mixture of two corresponding enantiomers.CHIRALITYChirality 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 induce 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 when 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 riposteDIASTEREOMERThey are different physical/chemical properties in chiral/achiral environments.Stereoisomers that are not enantiomer are called diastereomers.FISHER PROJECTIONFischer projections are apply 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 degree centigrade placed 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 dotted lines.CONFIGURATIONConfigurational 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 spacial orientations of atoms, the y 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.CONFORMATIONConformational 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.STEREO chemical CORRELATIONThe 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 REACTIONNo chemical reaction can be plan without stereochemical details,and no chemical reaction can be planned without considering problems oe stereochemistry that superpower a bound.A.STEREOCHEMICHEMISTRY OF ADDITION REACTIONAn sum reaction can fall out 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 REACTIONA 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 isIt implies that if x and x have the same relative priorities in the R,S system then the ca rbon 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 isThey 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 ReactionsTwo products are formed when a chiral substrate that possesses an asymmetric, electrophilic carbon is applied in an reaction. one and only(a) of them has the same absolute configuration as the starting product (if, correspond to the CIP rules, the leaving group and the nucleophile have the same position in the anteriority 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 carbocations trigonal planar shape, its two enantiotopic sides are susceptible to attack by the nucleophile with the same probability.Stereochemistry ofReactionsIf a pure enantiomer is applied to an reaction, three different stereochemical results are conceivableThe initial spatial arrangement of the reaction centers substituents remains (retention).The initial substituents spatial arrangement is anatropous (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 EliminationIn the E2 elimination reaction, the carbon- heat content sigma bond and the carbon-leaving group sigma bond must lie in the same plane. This allows the orbitals to sire 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 bondIn 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 to a greater extent stable than the eclipsed conformation required for syn elimination, anti elimination is favorite(a) in E2 reactionsSTEREOCHEMISTRY OF ALKANEConformationsAlkane conformers arise from rotation around sp3 hybridised carbon carbon sigma bonds. The smallest alkane with such a chemical bond, ethane, exists as an myriad number of conformations with respect to rotation around the C-C bond. Two of these are prize as energy minimum (staggered) and energy maximum (eclipsed) forms. The existence of particular proposition conformations is due to hindered rotation around sigma bonds, although a role for hyperconjugation is proposed by a competing theory.CONSTITUTIONAL ISOMERThere 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 olefineThe 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 ALKENEIf 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 bondCIS TRANSE-Z SYSTEMThe cis- / trans- style is based on the longest chain whereas the E/Z style is based on a set of precedency rules.You need to know both styles.Z SYSTEM cis-but-2-eneor (Z)-but-2-eneThe 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 APLLICATION1.Temperature modulation of the stereochemistry of enzymatic catalys.2.It is used in medical examination purpose for malarial prevention, control and research3.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 pu rposes like enzyme catalyst e.t.c.5. Penning ionisation is known to be one of the most important types of chemical reaction in aerospace.Steric shielding effect of methyl group an penning ionisation in subsitutional aniline.6. Stereochemistry is always used in discovering new compounds7. Stereochemistry is used to know the property and other new property of the existing compound or the compounds not known..REFRENCESHehre, W.J.(1975),J.AM.CHEMKARLE J.(1973),ELECTRON DIFRACTION IN NACHOD,F.C AND ZUKERMAN, VOL 5 P12.ELIEL L. ERNEST(2009),STEREOCHEMISTRY OF ORGANIC COMPOUNDS,WILEY STUDENT adaptationLOUDON MARC G.(2009),4th EDITON ,ORGANIC CHEMISTRY ,OXFORD UNIERSITY PRESSWEBSITEhttp//commons.wikimedia.org/wiki/FileExample_of_stereoisomershttp//www.google.co.in/imgres?imgurl=http//img121.imageshack.us/img121/5564/81111276.jpgimgrefurl=http//www.chemicalforums.com/index.WWW.CHEMISTRY.UKwww.wileyindia/chem.com