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 : 1)Covert the 3D structure into the Fischer projection 2)Assign priority sequence 3)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.

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?

Cycloalkanes Heat of combustion per CH 2 Cyclopropane 697 KJ/ mole Cyclobutane 686 KJ/ mole Cyclopentane 664 KJ/ mole Cyclohexane KJ/ mole Cycloheptane KJ / mole Cyclooctane similar

Origin of strain in cyclopropane / cyclobutane : Cyclopropane : Bayer’s strain : Angle strain : = normal angle – actual angle = ( )/ 2 = torsional strain : adjacent H’s eclipsed. MO theory : Angle strain exists because the sp 3 orbitals of the carbon atoms can not overlap effectively. the bonds are called “bent” or “banana” bonds.

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.

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

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