1. Imagine the alkene as 2 pieces
each containing one of the sp2 C
2. Assign the priority (high = 1, low = 2) to each group on each sp2 C based on atomic number
3. Determine the relative position of the 2 higher priority groups
4. If they are on the same side then it is a Z-alkene (German; zusammen = together)
5. If they are on opposite sides then it is an E-alkene (German; entgegen = opposite)
The two substituents are -CH3 and H, so since the atomic numbers C > H then the -CH3 group is higher priority. Therefore the two high priority groups are on the opposite side, then this is E-2-butene.
NB: The term stereogenic has superceded the term chiral.
1. Identify the stereogenic centers
(most commonly an sp3 C with 4 different groups attached)
2. Assign the priority to each group (high = 1, low = 4) based on the atomic number of the atom attached to the stereogenic center
3. Position the lowest priority group away as if you were looking along the C-(4) sigma bond. If you are using a model, grasp the lowest priority group in your fist
4. For the other 3 groups, determine the direction of high to low priority (1 to 3)
5. If this is clockwise, then the center is R (Latin: rectus = right)
6. If this is counter clockwise, then it is S (Latin: sinister= left)
The chirality center is easy to spot, and the four attached groups are I (purple), Cl (green), F(brown) and H (white), listed in priority order, highest to to lowest. Rotate the JMOL image below so the you are looking along the C-H bond and the H is away from you, then determine the direction of high to low priority. It decreases clockwise, so this is the R enantiomer.
|© Dr. Ian Hunt, Department of Chemistry, University of Calgary|