Stereochemistry Unit 8

Stereochemistry Stereochemistry – the study of compound structures in 3 dimensions Stereoisomers – compounds that differ only in the spatial arrangement of atoms (isomers that differ in how they are connected to eachother)

Types of Stereoisomers 1. Diastereomers – stereoisomers that are not mirror images (like we did in Intro to Hydrocarbons) 2. Enantiomers – stereoisomers that are mirror images a.k.a. optical stereoisomers

Diastereomers vs.Enantiomers

Simple Enantiomers

Enantiomers You can have mirror images for any compound If they are superimposable, they are not stereoisomers If they are not superimposable, they are stereoisomers and enantiomers

Enantiomers A compound that has enantiomers is “chiral” and one that has no enantiomers is “achiral” Chiral = Greek for hand

Enantiomers To have enantiomers a compound must contain a carbon atom that is bonded to 4 different atoms/groups (asymmetric carbon) Chiral center (tetrahedral stereocenter) = a carbon atoms that bonds to 4 different atoms/groups

Practice Ex: CH 3 CH 2 CHClCH 2 CH 3 Cl C-C-C-C-C This compound has no chiral centers. Why?

Practice CH 3 (CH 2 ) C CHClCH 2 CH 3 Cl Chiral Carbon C-C-C-C-C-C C  bonded to 1. H3.C-C 2. Cl4. C-C-C

Practice Draw the following structures, tell if they do/don’t have a chiral center, and if so, list the 4 diff. groups that bond to the chiral center… CH 3 CH 2 CHBrCH 2 CH 2 Cl 3-ethylhexane CH 2 FCH 2 CHOH(CH 2 ) 2 CH 3 CH 3 (CH 2 ) 2 CClF(CH 2 ) 2 CHO CH 3 CH 2 CO(CH 2 ) 2 CH 3

Drawing Enantiomers Cahn-Ingold-Prelog Naming System We use R and S notation R= Rectus S=Sinister

Priorities for Drawing Enantiomers The priorites are an order for atoms that was established by chemists so the structures would be drawn the same

Priority Rules 1. If all 4 atoms that bond directly to the chiral carbon are different, priority is assigned by atomic number with highest atomic # having highest priority Cl Ex: F-C-H Order? Br

Priority Rules 2. If two or more atoms bonding directly to the chiral center have the same atomic #, look at the atoms they bond to and continue outward until you reach an atom of higher atomic # which makes that group a higher priority Ex: CH 3 CH 2 CHCl(CH 2 ) 2 CH 3 Priority for alkyl groups is decyl>9>8>…>1)

Priority Rules 3. A double bond is considered to be bonding to two atoms C=A is considered C-A A A triple bond is considered to be bonding to 3 atoms A C= A is C - A A

Priority Rules #3 example: CH 3 CH 2 CHClCHCH 2 4. A Real bonding to two atoms takes priority over a double bond / a real bonding to three atoms takes priority over a triple bond EX: (CH 3 ) 2 CHCHClCHCH 2