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 compoundIf they are superimposable, they are

not stereoisomersIf 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: CH3CH2CHClCH2CH3

Cl

C-C-C-C-C

This compound has no chiral centers.

Why?

Practice

CH3(CH2)CCHClCH2CH3

Cl Chiral Carbon

C-C-C-C-C-C

C bonded to 1. H 3.C-C

2. Cl 4. C-C-C

PracticeDraw 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…

CH3CH2CHBrCH2CH2Cl3-ethylhexaneCH2FCH2CHOH(CH2)2CH3

CH3(CH2)2CClF(CH2)2CHOCH3CH2CO(CH2)2CH3

Drawing Enantiomers

Cahn-Ingold-Prelog Naming SystemWe 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

ClEx: 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: CH3CH2CHCl(CH2)2CH3

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: CH3CH2CHClCHCH2

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: (CH3)2CHCHClCHCH2