stereochemistry of Butane

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Stereo chemistry of Butane

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stereochemistry of Butane

  1. 1. Topic:- conformational analysis of n- BUTANE Made by:- Sophia Mubashir 1
  2. 2. 2
  3. 3. Isomerism ‣Isomers are non-identical compounds with the same molecular formula. Types ‣The two main classes of isomers are called structural isomers and stereoisomers. ‣Structural (constitutional) isomers ‣Stereoisomers 3
  4. 4. ISOMERS Compounds with the same molecular formula Conformational Isomers rotation about single bonds with chiral centers Stereoisomers Meso Compounds Enantiomers Constitutional Isomers Cis,Trans (E,Z) Isomers (can be called diastereomers) Conformations rotation restricted different connectivity Diastereomers stereocenters but no chiral centers Enantiomers one chiral center m ore than one chiral center chiralachiral not mirror images mirror images Atropisomers same connectivity 4
  5. 5. Conformations or Conformers or Rotamers:- ‣Different spatial arrangements of a molecule that are generated by rotation about single bonds. Conformational analysis :- ‣Conformational analysis is the study of the effect of rotation on the properties of a molecule. 5
  6. 6. Andiron or Sawhorse Sawhorse Projections are very similar to Newman Projections, but are used more often because the carbon-carbon bond that is compressed in a Newman Projection is fully drawn out in a Sawhorse Projection. Newman Projection The molecule is viewed from front to back in the direction of bond linking the two carbon. The bond joining the two carbons is hidden. ). The front carbon atom is called proximal, while the back atom is called distal Representation:- 6
  7. 7. Sawhorse Projection 7
  8. 8. Newman Projection 8
  9. 9. Conformations  Torsional strain ◦ also called eclipsed interaction strain ◦ strain that arises when nonbonded atoms separated by three bonds are forced from a staggered conformation to an eclipsed conformation ◦ the torsional strain between eclipsed and staggered ethane is approximately 12.6 kJ (3.0 kcal)/mol +12.6 kJ/mol 9
  10. 10. Conformations  Steric strain (nonbonded interaction strain): ◦ the strain that arises when atoms separated by four or more bonds are forced closer to each other than their atomic (contact) radii will allow 10
  11. 11.  Angle strain: ◦ strain that arises when a bond angle is either compressed or expanded compared to its optimal value  The total of all types of strain can be calculated by molecular mechanics programs ◦ such calculations can determine the lowest energy arrangement of atoms in a given conformation, a process called energy minimization 11
  12. 12. Conformations  Dihedral angle the angle created by two intersecting planes 12
  13. 13. Anti Butane  Energy-minimized anti conformation ◦ the C-C-C bond angle is 111.9° and all H-C-H bond angles are between 107.4 and 107.9° ◦ the calculated strain is 9.2 kJ (2.2 kcal)/mol CH3 H H H H CH3 13
  14. 14. Eclipsed Butane ◦ calculated energy difference between (a) the non- energy-minimized and (b) the energy-minimized eclipsed conformations is 5.6 kJ (0.86 kcal)/mol H H H H H H 14
  15. 15. C C CH3 H3C H H H H HH CH3 H H H3C Staggered: anti C C CH3 H H3C H H H CH3H H H H H3C Staggered: gauche 3 KJ/mol 15
  16. 16. Butane Conformers C2-C3  Highest energy has methyl groups eclipsed.  Steric hindrance  Dihedral angle = 0 degrees totally eclipsed 16
  17. 17. Butane Conformers (2)  Lowest energy has methyl groups anti.  Dihedral angle = 180 degrees anti 17
  18. 18. Butane Conformers (3)  Methyl groups eclipsed with hydrogen  Higher energy than staggered conformer  Dihedral angle = 120 degrees eclipsed 18
  19. 19. Butane Conformers (4)  Gauche, staggered conformer  Methyl closer than in anti conformer  Dihedral angle = 60 degrees gauche 19
  20. 20. Conformational Analysis => 20
  21. 21. E2 is an anti-elimination. They are stereo specific. The hydrogen and the halogen must be on opposite sides of the molecule before the E2 elimination can take place. This makes sense as both the base and the leaving group are negatively charged. Therefore they would try to be as far apart as possible. In addition, the leaving group is large and there is more room for the removal of the adjacent proton if it is on the opposite side from the leaving group. Reactions If the anti-arrangement is not possible, syn-arrangement may take place. 21
  22. 22. Mechanism = elimination, bimolecular E2 100% anti-elimination! base: C X C H C C + H:base + :X RDS 22

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