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Stereoisomerism, of atoms, Compounds having different three-dimensional relative arrangement, , in space are callad, , stereoisomers. This phenomenon is called stereoisomerism. These compounds are said to have, ve, kinds:, different, of, the, Stereoisomerism is, following, different, , configurations., , () Optical isomerism, (i) Geometrical isomerism, (ii) Conformational isomerism., , 3.3, , OPTICAL ISOMERISM OR ENANTIOMERISM, Before taking up optical isomerism, let us understand the terms: plane polarised light, optical, , activity, , and, , specific, , rotation., , Plane polarised light, , Ordinary light has vibrations taking place at right angles to the direction of propagation otf, light spread in all the possible planes. If we pass ordinary light through nicol. prism vibrations in, , all planes except one are cut off. Thus light coming out of nicol prism has vibrations only in one, , plane. Such a light is called plane polarised light., , Optical activitv, Behaviour of certain substances is strange. When a plane polarised light is passed thrqugh tne, , solution of such substances, the, , light coming out of the solution is found to, , be, , in a, , different plane

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359, , STEREOCHEMISTRY OF ORGANIC COMPOUNDS, , The plane of polarised light is rotated. Such substances, which rotate the plane of plane polarised, light when placed in its path are known as optically active susbtances and the phenomenon is, , known as optical activity. The angle of rotation (oa) of plane polarised light is known as Optical, , rotation. The substances which rotate the plane of polarised light to the clockwise or right direction, are known as dextrorotatory or having positive (+) rotation and those which rotate the plane, polarised light to the anticlockwise or left direction are known as laevorotatory or having negative, rotation. Substances which do not rotate the plane of polarised light are said to be optically, inactive., The instrument used for measuring optical rotation is called, polarimeter. It consists of a light, source, two nicol prisms and in between a tube to hold the solution of, organic substance. The, schematic representation of a polarimeter is, in, 3.1., , given, , E, , Light, , First, , source, , Nicol prism, , (polariser), , Fig., , Eye, Sample, tube, , After, , Second, , rotation Nicol prism, , (analyser)

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Mirror, , L, , (1, , M, , M, , (o, , N, , N, , compound of formula MNO having four different groups L, M, N and 0 attached to a Cark, atom., , This, , compound, , can, , be, , represented by, , two models which look like mirror, , other., , images of edeac, , Van't Hoff, , Enantiomerism in nature, , It is important to note here that these two molecules cannot be, superimposed on each otner l, will, not, coincide, in, all, their parts. We may turn them in as, they, many ways as we like but we u, that though two groups of each may coincide, the other two do, not. Hence these must represent, isomers of formula CLMNO Lactic acid, ,H,CHCICH,, and, , exist, , as, , two, , optically, , CH,CHOHCOOH, , active 1somers which are, , sec-Butyl chloride C,H,CHCCn, of, enantiomers i.e. mirror imaE

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STEREOCHEMISTRY, CHEMISTRY OF ORGANIC COMPOUNDS, , COOH, , HO, , CH, , 361, , COOH, , OH, , H, , CH, , Enantiomers of Lactic Acid, , CH5, , CH, , Enantiomers, , H, , CH, , H, , CI, , Cl, , CH, , Enantiomers of Sec. Butyl Chloride, , each other. Mirror images of the two compounds are represented as above., , The carbon atom to which four different groups are attached,isknown as asymmetricor chiral, , Ccarbon atom or stereogenic.centre., , If two of the groups attached to carbon are same, we shall observe that it is possible to superimpose, , ue miTor images on each other. Such a compound will not show optical isomerism or enantiomerism., , Hence non-superimposability of the mirror images is responsible and essential for the type of, , Sercoisomerism known as enantiomerism., ne term optical isomerism is used for the existence of stereoisomers which differ in their, , aIOur, , enantiomeric molecules, towards the plane polarised light. Thus, , mirror images, pOsable, Chiral or asymmetric nature, , due, of symmetry and is, , nay, , are, , of mirror, of each other. The non-superimposability, , always nonimages arises, , of molecule. A molecule is said to be chiral if it has no plane, , refore non-superimposable on its mirror image., , be concluded with the remarks that, , chirality is the, , fundamental condition of, , nantiomerism or optical isomerism., , Chirality and dissymetry, chiral object. And, its mirror image, An, non-superimposable, which, is, Ohi Dject, Examples of chiral, object Cn superimposable its miror image is of hands, shoes,object., gloves, letters P, F, J, is called, , on, , an, , called achiral, , 1s, , on, , cluding letters from English alphabets, , Amples of achiral obiects and letters, , are:, , are:, , A, , etc., , pair, , Ball, sphere, letters A, O, M,, , etc.

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362, , CHEMISTRY FOR DEGREE, , STUDENTS, , P, , I, , (ORGANIC Cu, , CHEMISTR, , Chiral, , MI M, , Achiral, , Mirror, , Achiral, , objects, , Chiral and achiral, , Objects, , molecules possess a, plane of, two identical halves., Chiral objects or molecules do symmetry that can divide the molecule into, not possess a, divide the molecule into two, plane of symmetry that can, identical halves. The, is, or, , chirality also known as dissymetry while, achirality 1s also known as symmetry., Thus chirality or, dissymmetry is the property associated with certain, do not have a, of, plane symmetry and are, compounds which, Dissymmetry is an essential condition for thus non-superimposable on their mirror images., In the study of, optical, one chiral carbon, enantiomers containing, atom, we find that they do not have activity., a, of, in nature, for, plane, and, hence are dissymmetrie, symmnetry, example, lactic acid, sec-butyl chloride etc. We can, thus say that, carbon atom are, optically active in nature., compounds a chiral, This is not true,, however, for compounds with two or more, more isomers, chiral carbon atoms. Here, one or, may be optically inactive in, of, the, spite, for example, the case of, presence of chiral carbon atoms. Consider, tartaric acid, CH(OH) COOHCH, (stereogenic centres) marked and thus has four isomers as(OH) COOH. It has two chiral carbons, *, , shown below:, , COOH, , COOH, , H-C-0H, , HO-CH, , HO-C-H, , H-C- OH, , COOH, , of, , plane, , of symmetry, , COOH, , HCOH, , H, , optically, , COOH, , COOH, ***, , I and II are, , active due to the absence, , HO, , *1**, , -OH, , H, , HO, , COOH, , H, , OOH, , Isomers I and II are, optically active. But ll and IV have, , ll and IV, , optucally, the presgnce, planc of symmetry, (shown by dotted line), are, , inactive due, , to, , IV, , non-superimposable mirror images of each other, they are aissy, , optically inactive in nature., , In other words, optical activity, , not due to, , in, , chiral carbon atoms alone., , a, , plane, , of, , symmetry. Therefore, they, , organic compounds, , is due to the, , are, , syn, , etric and, , and, e t r ya n d, , presence, , of, , dissyu

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man and Sawhorse representations for the conformations of ethane, , Newma, , Alkanes can have an infinite number of conformations by rotation around carbon-carbon singe, , Alka, , atoms are linked by a, single bond and each carbon atom is further, Lnds. In ethane two carbonatoms., linked with, loked with three hydrogen otherIf one of the carbon atoms is allowed to rotate about carboncarbon stationary, an infinite number of arrangements of the, single bond keeping the, carbon, hwdrogens of one carbon, with respect to those of the other, are obtained. All these arrangements, bonds. In ethane, , car, , ydrogens of, , ements, , ere called, conformations (Bond angles and bond lengths remain the same)., ca, Newman reppresentation, , This can be easily understood with the help of Newman Projection formulae. The molecule is, viewed from front to back in the direction of carbon-carbon single bond. The carbon nearer to the, eve is represented by a point and three hydrogen attached to it are shown by three lines at an angle, of 120 to one another. The carbon atom away from the eye is represented by a circle and three, hydrogens attached to it are shown by shorter lines at an angle of 120° to each other., Out of infinite number of conformations, Newman Projection formulae for two extreme cases, are as shown below:, , H, , H, , H, , H, , HH, , H, , H, , H, Newman, , H, , H, , Staggered, Projection, , H, , H, Eclipsed, , Formulae for conformations of ethane.

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CHEMISTRY FOR DEGREE, , 392, , STUDENTS -I (ORGANIC CHEMISTR, , far apart as possible, is known, N, atoms of two carbons are as, The conformation in which the H, back carbon are iut, of, atoms, H, the, the conformation in which, as Staggered conformation and, These are converted into one, behind those of the front carbon is known a Eclipsed conformation., The other conformations, in between, rotation of one carbon against the other through 60°., another, , by, , these two, are known as skew conformations, Sawhorse representation, , from above and from the right and, then projected on the paper. The bond between two carbons is drawn diagonally and is a bit longer, In this, , representation, the, , molecule is visualised, , slightly, , for the sake of clarity. The lower left hand carbon is taken as front carbon and the upper right hand, carbon is taken as back carbon. The Sawhorse representation of staggered and eclipsed conformations, , of ethane are given, , below:, , H, , H, , H, , H, H, , H-, , Staggered, Sawhorse, , H, , H, , H, , representation, , -H, , H, , H, Eclipsed, , for conformations of ehtene