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Lecture 3

Lecture 3. Symmetry elements :. One example when C n is present but the molecule is chiral. The molecule is chiral. Designation of R/S :. One simple way :. Covert the 3D structure into the Fischer projection Assign priority sequence

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Lecture 3

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  1. Lecture 3

  2. Symmetry elements : One example when Cn is present but the molecule is chiral. The molecule is chiral

  3. Designation of R/S : One simple way : • Covert the 3D structure into the Fischer projection • Assign priority sequence • By exchange in a group three, bring the group of lowest priority • to one of the vertical position • 4) Now check the R / S Groups occupying the vertical positions are away from the observer.

  4. Plane containing X and Y is orthogonal to that containing A and B The molecule will be chiral if A is not identical with B And X , Y are not identical Are the following molecules Chiral?

  5. Cycloalkanes Heat of combustion per CH2 Cyclopropane 697 KJ/ mole Cyclobutane 686 KJ/ mole Cyclopentane 664 KJ/ mole Cyclohexane 658.7 KJ/ mole Cycloheptane 662.4 KJ / mole Cyclooctane similar

  6. Origin of strain in cyclopropane / cyclobutane : Cyclopropane : Bayer’s strain : Angle strain : = normal angle – actual angle = (109.50 - 600)/ 2 = 24.750 + torsionalstrain : adjacent H’s eclipsed. MO theory : Angle strain exists because the sp3 orbitals of the carbon atoms can not overlap effectively. the bonds are called “bent” or “banana” bonds.

  7. conformations of cyclohexane : chair form is the most stable form. analysis of chair form Axial bonds : perpendicular to the average plane. Equatorial bond : lies around the perimeter of the ring. C-1, C-3, C-5 in one plane C-2, C-4, C-6 in another plane.

  8. At each carbon in cyclohexane, One axial bond, one equatorial bond One a - bond, one b – bond. Flipping in flipping axial becomes equatorial equatorial becomes axial But a remains a and b remains b. The two chairs look like mirror images.

  9. Some Important Rules: 1. bulky substituents tend to occupy equatorial positions. Putting them in axial positions incorporates what is called “ 1,3 – Diaxial Interactions” “ 1,3 – Diaxial Interactions” 2. i) if two substituents on different atoms are both a or both b, the compound will be a cis compound. ii) if the susbstituents are a and b or b and a it will be called a trans compound.

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