Configurational Isomers

Constitutional Isomer Different compounds with the same molecular formula (we’ve done this one!) Examples C6H10

Stereoisomers Differ only in the spatial orientation of atoms

Enantiomers is one of two stereoisomers that are non-superposable complete mirror images of each other, much as one's left and right hands are "the same" but opposite.

Diastereomers Are stereoisomers that are not enantiomers In another definition diastereomers are pairs of isomers that have opposite configurations at one or more of the chiral centers but are not mirror images of each other

Diastereomers

Chirality Chiral – have one carbon with four different groups bonded to it. (optically active) Possess “handedness” – mirror images which are not superimposave

Achiral Have no plane of symmetry, they are not mirror images which are superimposable

How do we decide? R/S System E/Z System We must learn a system to assign priorities to the atoms or groups of atoms attached to the carbon atom

Cahn, Ingold and Prelog Priority Assign sequence priorities to the four substituents by looking at the atoms attached directly to the chiral center. 1.  The higher the atomic number of the immediate substituent atom, the higher the priority. For example, H–  <  C–  <  N–  <  O–  <  Cl–. (Different isotopes of the same element are assigned a priority according to their atomic mass.) 2.  If two substituents have the same immediate substituent atom, evaluate atoms progressively further away from the chiral center until a difference is found. For example, CH3–  <  C2H5–  <  ClCH2–  <  BrCH2–  <  CH3O–. 3.  If double or triple bonded groups are encountered as substituents, they are treated as an equivalent set of single-bonded atoms. For example, C2H5–  <  CH2=CH–  <  HC≡C–

R/S Once the attached groups are arranged by priority we point the one with the least (4) priority away from you and then decide if the orientation of 123 is clockwise (R) or counterclockwise (S) .

R/S Convention

Fischer Projections

E/Z Orientation across a double bond Better than cis/trans

Fischer Projections

Exchange Method