Stereoisomerism

Nanoplasmonic Research Group

Organic Chem-istry

Chapter 5

Chirality (general)

Chirality (organic molecules)

It is important that you use stereo-chemical terminology correctly.

Here are the proper terms for describ-ing each of the following

An Atom- An atom with four different groups attached is a stereocenter- Stereocenters are also called chirality centers, asymmetric centers, and stereogenic centers- Absolute configuration of a stereocenter is assigned using R/S nomenclature

A Molecule- Achiral or Chiral (optically active)- Achiral molecules that contain stereocenters are called meso compounds- Optically active molecules can be labelled (+/-) or (d/l)

Related molecules- Enantiomers (non-identical mirror images)- Diastereomers (any stereoisomers that are not enantiomers)

Samples of Molecules- Optically pure (only one enantiomer present in sample)- Racemate or racemic mixture (mixture containing equal amounts of each enantiomer)- Racemic mixtures are not optically active- Mixtures in between optically pure and racemic are described by their optical purity or enantiomeric ex-cess

Note:It is important not to confuse experimentally derived labels (+/- or d/l) with structurally derived labels (R/S). They are not related!!!

Assigning R/S Stereochemistry (Cahn-Ingold-Prelog)

• Every stereocenter can be assigned as R or SAssign each group a priority (1 = highest)a) Highest atomic number has priorityb) Heavier isotopes have priorityc) In a tie, move along the chain to the first point of differenced) With multiple bonds, break each pi-bond and duplicate the atoms at each end

Put the lowest priority group (usually H) in back and view along the bond from carbon to group 4Draw an arrow from 1 to 2 to 3

a) Clockwise = Rb) Counterclockwise = S (sinister means left in Latin)

You can imagine atomic ar-rangement in space by

looking at the Newman pro-jection!!!

How to determine R/S form for the given structure ?

Let’s try with Example 5.5 and Problem 5.10

Don’t forget mesocompounds when consid-ersing two or more stereocenters !!!!

What about compounds with two or more stereocenters ?

The point is whether the molecule of interest does have a plane of symmetry

or not!!!!

Summary for Stereochemistry

Stereochemistry and Chemical Reactions (I)- Addition to Alkene -

Achiral to Chiral transition!!

Then, how chiral molecules come out ?- Addition to Alkene -

Resolution of a Racemic Mix-ture

• A Racemic mixture (Racemate)• A 50:50 mixture of two chiral com-

pounds that are mirror images does not rotate light