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Organic Chemistry Sem-IV Chem VIII DSC-D4 Stereochemistry

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Organic Chemistry Sem-IV Chem VIII DSC-D4 Stereochemistry 1 Chem.-VIII Bsc-II sem-IV (DSC D4) Notes by Dr. M.M.Mane Stereochemistry It is branch of chemistry which deals with study of spatial arrangement of atoms or groupsIn a molecule. The isomers which differ in the properties due to different arrangement of atoms or groups in space are called stereoisomers. Types of stereoisomerism There are three types of stereoisomerism, i) Optical isomerism ii) Geometrical isomerism iii) Conformational isomerism i)Optical isomerism The stereoisomers which have same physical and chemical properties but differ in optical rotation of plane polarized light, are called optical isomers and the phenomenon is called optical isomerism. e.g. lactic acid OH OH C C COOH HOOC H H CH3 H3C (-) lactic acid (+) lactic acid in sour milk in muscles ii|)Geometrical isomers The stereoisomers in which atoms or groups are held in different position due to restricted rotation about some bond are called as geometrical isomers and this phenomenon is called geometrical isomerism. e.g. Two different spatial arrangements of methyl groups about a double bond in 2-butene give rise to the following geometrical isomers. i.e., cis-2-butene and trans-2-butene H H H CH3 H C H H3C CH3 3 cis trans iii)Conformational isomers Stereoisomers which can be interconverted by rotation are known as conformation isomers Projection formula Projection formula is used to represent three dimentional arrangement in molecule. 1. Dotted line wedge formula Formulae are drawn with dotted line to show bonds or grs. Going away behind the Plane of paper, while wedge line is used to show bonds coming out of plane towards us. Normal line indicates bonds staying on the plane of paper. e.g HOOC H C 5 2 CH3 H RBM College Chandgad 2 Chem.-VIII Bsc-II sem-IV (DSC D4) Notes by Dr. M.M.Mane 2) Fischer projection formulae Fischer Projections are used often in drawing sugars and hydrocarbons, because the carbon backbone is drawn as a straight vertical line, making them very easy to draw The molecule is written with carbon chain coming vertically downward with more oxidisable gr. lying at the top. The horizontal line indicates bonds coming towards us out of Plane, while vertical lines indicates bonds going away from us below the plane of paper. A crossing between two lines represent a carbon. HOOC COOH H OH H OH HO H HO H CH CH CH3 2 3 3)Sawhorse projection formulae Here side projection is obtained by holding the central C-C bond slightly inclined to the plane of paper. The grs. attached to the carbons are shown by lines. H H H H H H H H H H H H 4) Newman Projection formulae Here central C-C bond is hold perpendicular to the plane of paper and molecule is viewed along the C-C bond axis from the front. The rear (back) carbon is shown by circle and grs. attached to that carbon are shown by dotted/ normal lines starts from periphery of circle, while front carbon is shown by point at the center of circle and grs. Attached is shown by lines drawn from that point. Conformational analysis in Ethane The different momentary arrangements of atoms or groups in molecule which can be interconvert by rotation are known as conformations. The study of physical and chemical properties of molecule in relation with its various conformations is known as conformational analysis. In Ethane if we kept one carbon fix and we rotate other carbon step by step, then we get following different conformations. 1.Eclipsed conformation In Ethane at dihedral angle 00 , 1200 , 2400 and 3600 we get conformations in which all H atoms of rear carbon are exactly eclipsed (overlap) with H atoms of front Carbon. This state is called eclipsed conformation. RBM College Chandgad 3 Chem.-VIII Bsc-II sem-IV (DSC D4) Notes by Dr. M.M.Mane H H H H H H In eclipsed conformation all bonds of front carbon and bonds of back carbon are at closest distance, which cause bond opposition (torsional) strain, further H atoms of front and back Carbons are at distant (2.3 A0 ) just less than sum of vanderwaal radius (2.4 A0 ), which causes steric strain. Thus due to these two strains P.E. of ethane in eclipsed state is higher and it is least stable conformation of Ethane. 2.Staggered conformation: In Ethane at dihedral angle 600, 1800 and 3000 we get conformations in which all H atoms of rear Carbon are at maximum distant apart from H atoms of front Carbon. This state is called staggered conformation. H H H H H H In staggered conformation all bonds of rear carbon and front carbon are at maximum distant apart, this release bond opposition strain, further H atoms of rear and front carbon are also separated by longest distance and this release steric strain. Thus steric and torsional strains are minimum in staggered state, hence P.E. of staggered state is minimum and it is most stable conformation of Ethane. 3.Skew conformation Arrangements of atoms in between staggered and eclipsed state are called skew conformations. As skew has arrangement of atoms in between staggered and eclipsed their P.E and stability is also in between staggered and eclipsed. H H H H H H Thus P.E. curve for ethane conformers is as shown below, Thus stability order of different conformatios of ethane is , Staggered > Skew > eclipsed The energy difference between staggered and eclipsed conformation is 12 kJ/mole .At room temp energy of ethane molecule is about 63-84 kJ/mole,Which is more than sufficient to overcome the energy barrier of 12 kJ/mole.Hence at room temp. different conformations of ethane are interconverting and it becomes impossible to separate different conformation. RBM College Chandgad 4 Chem.-VIII Bsc-II sem-IV (DSC D4) Notes by Dr. M.M.Mane Conformational analysis of butane Butane can be considered as derivative of ethane, where H atoms of front and rear Carbons are replaced by methyl (- Me) group.In butane if we rotate certral C-C bond( C2-C3 ) step by step then we get different conformaions, out of these two conformations are eclipsed and two are staggered.These conformations are, 1.Synperiplaner At dihedral angle 00 and 3600 we get eclipsed conformation in which Methyl group of front and rear carbons are eclipsed, this is called synperiplaner. CH3 CH3 H H H H In synperiplaner all bonds of front and rear carbons are at closest distanace, which cause Tortional strain, further Methyl group of front and rear carbons are at distance less than Sum of vanderwaal radii, which cause steric starin. Due to these two types of starins P.E. of Synperiplaner is largest and it is least stable conformation of n-Butane. 2.Anticlinal At dihedral angle 1200 and 2400 we get another eclipsed conformation in which H atom of rear carbon is eclipsed with Methyl gr.of front carbon OR Methyl of rear carbon is eclipsed with H atom of front carbon. This is called Anticlinal conformation. H CH H3 CH3 H H In anticlinal all bonds of front and rear carbons are at closest dist. which cause torsional strain, but in anticlinal conformation methyl grs. of front and rear carbons are at 1200 away from each other , this release steric strain. Hence P.E. of anticlinal conformation is less than synperiplaner and hence it is more stable conformation than synperiplaner. 3.Antiperiplaner At dihedral angle 1800 we get staggered conformation in which two Methyl grs. of front and rear carbons are exactly opposite to each other, this is called antiperiplaner or anti conformation. CH3 H H H H CH 3 In antiperiplaner all bonds of front and rear carbons are at maximum distance apart, which release torsional strain, further two methyl grs. of front and rear carbons are also at max. distance apart, which release steric strain. Thus these two types of strains are minimum in antiperiplaner conformation , hence P.E. of antiperiplaner is minimum and it is most stable conformation of n- Butane. RBM College Chandgad 5 Chem.-VIII Bsc-II sem-IV (DSC D4) Notes by Dr. M.M.Mane 4. Synclinal ( Gauchi ) At dihedral angle 600 and 3000 , we get another staggered conformation in which two methyl grs. Are at angle 600 away from each other, this is called synclinal OR Gauchi conformation. H H3C CH3 H H H In Gauchi conformation all bonds are at max. dist. apart hence torsional strain is minimum, but two methyl grs. of front and rear carbons are at internuclear distance less than sum of vanderwaal radii, which cause steric strain, hence P.E. of Gauchi conformation is 4.0 kJ /mole more than antiperiplaner, hence Gauchi conformation is less stable than antiperiplaner. PE. Curve for n-Butane is as shown below, Thus stability order of different conformations of buthane decreases in following order, Antiperiplaner > synclinal > anticlinal > synperiplaner P.E. difference between synperiplaner and antiperiplaner conformation is about 18-25 kJ/ mole. But at room temp energy of butane is about 84 kJ/mole, which is much more than to overcome barrier between diff. conformations. Hence at room temp diff. conformations of butane are in constant interconverting and hence it is impossible to isolate diff. conformations. Angle strain (Baeyer strain) In alkanes, optimum overlap of atomic orbitals is achieved at 109.5° at which bonds are at maximum distance apart and have least torsional strain and is called stable angle. Any deviation from this angle introduce strain and makes molecule unstable. In 1885 Baeyer introduce strain theory, to explain the relative stabilities of cyclic molecules.
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