Chapter 5

Chapter 5 - Chapter 5 Stereochemistry Chirality o For a...

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Unformatted text preview: Chapter 5 Stereochemistry Chirality o For a carbon to be chiral, it must have four different groups bonded to it Must be tetrahedral, but thats necessary for the above to be true anyway. chiral achiral C C CH 3 CH 3 Cl Br H H Br H chiral achiral C C CH 3 CH 3 Cl Br H H Br H Thus, CH 3 or CH 2 will never be chiral The groups do not have to be different atoms C This means that if a carbon has a methyl, ethyl, propyl, and butyl group coming off it, then it is chiral, even though it has four carbons attached to it. R/S absolute configuration o Assign priorities to all the atoms coming off the chiral carbon one atom at a time o 1 2 3 4 H Br 1 2 3 4 H Br Based on Mass, not electronegativity The heaviest atom is given priority #1 o If theres a tie, go out one more atom o Then, make sure your lowest priority atom is facing away from you o Make a circle from 1 to 2 to 3 1 2 3 4 H Br 1 2 3 4 H Br o If the circle was clockwise, then assign R o If counterclockwise, then assign S What do I do if the #4 group is coming forward? Br o Still assign priorities to the different groups 1 2 3 4 Br 1 2 3 4 Br o Find R/S as though #4 was going back. 1 2 3 4 Br 1 2 3 4 Br o Switch it. This one looks like S, so we have an R. What do I do if the #4 group is in the plane? 1 2 3 4 F Cl 1 2 3 4 F Cl o You have several options. o 1) Look at the molecule from another direction so that the #4 is facing away from you. This method seems to be easiest for those of you who like this chapter/ find this stuff easy. 1 2 3 4 look from over here F Cl 1 2 3 4 look from over here F Cl o 2) Rotate around a single bond to get the #4 group in the back. 1 2 3 4 1 2 3 4 F Cl F Cl 1 2 3 4 1 2 3 4 F Cl F Cl o 3) Switch two groups, find R/S for the new molecule, and then switch it back. This one seems to be easiest for those of you who hate this chapter. 1 2 3 4 looks like an S, so it's an R 1 2 3 4 F Cl Cl F 1 2 3 4 looks like an S, so it's an R 1 2 3 4 F Cl Cl F Enantiomers vs. Diastereomers o An enantiomer is a non-superimposible mirror image F Br F Br All the R/S designations will be changed in enantiomers If there is a double bond, then its stereochemistry (cis/trans) will remain the same. Br Br o A diastereomer is a non-superimposible non-mirror image. F Br F Br Some but not all of the R/S designations will change. Cis/trans double bonds make your molecules diastereomers as well. Br Br How do enantiomers differ from one another?...
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Chapter 5 - Chapter 5 Stereochemistry Chirality o For a...

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