Tài liệu về stereoisomerism

Stereoisomerism C HA IN ISOMER ISM STRUCTURAL ISO MERISM POSITIO N ISOM ER ISM Same mol ecular formula but dif ferent structura l formulae FUNC TIO NA L GROUP ISOMER ISM GEO METRIC AL ISOMERISM STEREO ISO MERISM Oc curs due to the restricted rotati on o f C=C uble bo nds two forms… CIS and TRANS Same mo lecular formula but ato ms o ccupy dif ferent pos itio ns in spac e OP TIC AL ISOMERISM Oc curs when mo lecules have a c hiral centre Get two no n- superimpo s able mirror imag es Geometric isomers In alkenes CIS TRANS Groups/atoms are on the Groups/atoms are on OPPOSITE SIDES across the double SAME SIDE of the double bond bond RESTRICTED ROTATION OF C=C B ONDS Single covalent bonds can easily rotate What appears to be a different structure is not It looks like it but, due to the way structures are written out, they are the same ALL THESE STRUCTURES ARE THE SAME BEC AUSE C- C BONDS HAVE ‘FREE’ ROTATIO N RESTRICTED ROTATION OF C=C B ONDS C=C bonds have restricted rotation so the groups on either end of the bond are ‘frozen’ in one position; it isn’t easy to flip between the two This produces two possibilities The two structures cannot interchange easily so the atoms in the two molecules occupy different positions in space cis trans cis trans Isomerism • Constitutional Isomers: Same atoms but linked (bonded) together differently Spatial orientation not important hexane 3-methylpentane cyclohexane No, different molecular formulae!! Are these constitutional isomers of hexane? Are these constitutional isomers of cis but-2-ene? Not this one! It is 2-butene Cis / trans does not matter Stereoisomerism • Stereoisomers: Same molecular formulae, same connectivity; same constitutional isomer Different spatial orientation of the bonds Are these stereoisomers of cis but-2-ene? How does the connectivity differ between these two? Enantiomers and Diastereomers Two kinds of Stereoisomers – Enantiomers: stereoisomers which are mirror objects of each other Enantiomers are different objects, not superimposable – Diastereomers: stereoisomers which are not mirror objects of each other If a molecule has one or more tetrahedral carbons having four different substituents then enantiomers will occur If there are two or more such carbons then diastereomers may also occur Summary of Isomerism Concepts Isomers, contain same atoms, same formula Constitutional isomers, different connectivities, bonding Stereoisomers, same connectivity, different three dimensional orientation of bonds Enantiomers, mirror objects Diastereomers, not mirror objects Mirror Objects – Carbon with different substituents We expect enantiomers (mirror objects) Reflect! The mirror plane still relates the two structures Notice that we can characterize or name the molecules by putting the blue in the back, drawing a circle from purple, to red, to green Clockwise on the right and counterclockwise on the left Arbitrarily call them R and S Notice how the reflection is done, straight through the mirror! Arrange both structures with the light blue atoms towards the rear… These are mirror objects Are they the same thing just viewed differently ?? Can we superimpose them? We can superimpose two atoms but not all four atoms S R Recap: Tetrahedral Carbon with four Different Substituents Enantiomers Mirror objects Different, not superimposable Simple Rotation, Same Enantiomers Simple Rotation, Same But the reflection might have been done differently Position the mirror differently… Reflection can give any of the following… Again all three objects on the right are the mirror object of Can you locate theare mirror which would transform the structure above They different views of the theis original molecule each reflection mirror object? What common to eachinto of these enantiomer operations? A swap of two substituents is seen to be equivalent Into thea course of each twoatom substitutents are reflection at reflection, the carbon swapped The other two remain unchanged All three of these structures are the same, just made by different mirrors The structures are superimposable What rotations of the whole molecules are needed to superimpose the structures? Now Superimposable mirror objects: Tetrahedral Carbon with at least two identical substituents Reflection can interchange the two red substituents Clearly interchanging the two reds leads to the same structure, superimposable! Remember it does not make any difference where the mirror is held for the reflection This molecule does not have an enantiomer; the mirror object is superimposable on the original, the same object Polarized light vibrates in one plane only, in contrast to ordinary light, which vibrates in all planes What causes such a rotation of the plane of polarized liight? According to the van’t Hoff theory, such an effect on the plane polarized light is due to the presence of one or more chiral carbon atoms Fischer projection A two-dimensional method of indicating the structure of an enantiomer Horizontal lines indicate bonds extending forward from the paper and the vertical lines indicate bonds extending backward from the paper The formulas are always written with the aldehyde(or ketone) group)-the most highly oxidized-at the top CHO H C OH CH2OH convert to  a Fischer projection CHO H OH CH2 OH Glyceraldehyde (an aldotriose) H H O C C H C OH CH2 OH D (+) O HO C H CH2 OH L (-)