Lecture 7: Stereoisomers & Optical Activity PDF

## Lecture 7 **Stereo isomers:** molecules that have the same molecular formula and the same type of atoms but differ in stereo form. **Optical Active isomers** / **Enantiomers:** one type of stereo isomers. **Chiral Center:** Asymmetric Center - atom that is bonded to different groups. >* Chiral...

## Lecture 7 **Stereo isomers:** molecules that have the same molecular formula and the same type of atoms but differ in stereo form. **Optical Active isomers** / **Enantiomers:** one type of stereo isomers. **Chiral Center:** Asymmetric Center - atom that is bonded to different groups. >* Chiral center may be (Carbon, C or Nitrogen, N) >* All compounds have one chiral atom are optically active. **Optical Activity** * All compounds have no chiral atom are optically inactive except atropes. * All compounds have one chiral atom are optically active. * All compounds have more than one chiral center may be optically active. **Number of Stereoisomers = 2^n where n= number of chiral center** **Enantiomers:** mirror-image isomers / pairs of compounds that are nonsuperimposable mirror images. **Chiral Atom:** To determine the arrangement of the groups around the chiral atom, we label the group with the smallest number of atomic number with 4. For example, hydrogen will always have a number 4, followed by assigning numbers 1, 2, and 3 to the remaining groups in decreasing order according to atomic number from the largest to the smallest. **Chiral "handed" vs a chiral "not handed" ** **Elements of Symmetry** * **Plane of symmetry:** A line that divides the molecule in half, where each half is a mirror image of the other. * **Center of symmetry:** A point at the center of the molecule, where every point on one side of the molecule has a mirror image on the other side. **Fisher Projection (3D) to (2D) Conversion** 1. Look for the atom that is at the center of the 3D projection. The atom that is at the center will be put on the horizontal line on the Fisher projection with the groups that are facing forward being on the right and the groups that are facing backward being on the left. **For compounds Containing Two chiral Carbons** * **Fisher Projection** * **Sawhorse Projection** * **Newman Projection** **Absolute Configuration (R and S) Using Fisher Projection Rules** 1. Atoms around the chiral carbon are arranged in priority from 1 to 2 to 3 according to their atomic number. >* The priority order of atoms is I>Br>Cl>S>F>O>N>C>D>H 2. If atom 4 (smallest atom) is vertical and the order of arrangement from 1 to 2 to 3 is anti clockwise, then the configuration is **S**. 3. If atom 4 (smallest atom) is vertical and the order of arrangement from 1 to 2 to 3 is clockwise, then the configuration is **R**. **Example** * CH3 * H * OH * C2H5 **Important Note:** Try to keep group 1 on the horizontal line. By switching the places of two groups, you are getting the enantiomer of the original molecule. If you swap the places of two groups, you are getting the enantiomer. Switching two groups twice is the same as switching four groups. You should always make sure that you are doing a pair switching.