Isomers Flashcards
Geometric isomers
Enantiomers
Non-superimposable mirror images with the same physical properties except rotation of polarized light.
Different configurations at every stereocenter and therefore rotate polarized light opposite ways.
Geometric isomers
Racemic mixture
An equal mixture of two enantiomers. Because of the opposite rotation of polarized light, this mixture will have no optical activity.
Geometric isomers
Meso compounds
A type of stereoisomer which is superimposable on its mirror image. Because of this symmetry, it is achiral and not optically active.
Geometric isomers
Diastereomers
Non-superimposable, not mirror images. Some but not all chiral centers are different.
An epimer differs at exactly one stereocenter.
Geometric isomers
Cis-trans isomerism
A type of diastereomer.
Cis- = groups on the same side of the carbon chain
Trans- = opposite sides.
Exist on compounds with double bonds or rings.
Stereoisomers
Conformational isomer forms
Conformational isomers are stereoisomers that can be interconverted through rotation around a bond
Stereoisomers
2n rule
The number of possible stereoisomers is 2n, where n is the number of chiral carbons (central carbons with 4 DIFFERENT surrounding groups).
Stereoisomers
Cyclohexane bonds
Equatorial: Parallel to the cyclohexane ring.
Axial: Perpendicular to the ring.
Fischer projections
Horizontal lines represent bonds coming out of the page (wedge) while vertical lines represent bonds going into the page (dash).
Fischer projection manipulations
90o rotation inverts the stereochemistry while 180o rotation maintains the stereochemistry.
Switching two substitutents results in inversion of stereochemistry.
Isomer configurations
Relative configuration
Denoted by the letters D/L. Molecules have the same relative configuration when one substituent is different but others are in the same position.
Assigned with reference to a standard molecule like glyceraldehyde.
Isomer configurations
Absolute configuration
Denoted by the letters R/S. Refers to each individual stereocenter within a compound (as opposed to relative configuration which refers to a whole compound).
To determine the configuration, use the Cahn-Ingold-Prelog priority rules:.
1) Rank functional group priority based on highest atomic number.
2) Reorient the molecules so that the group with lowest priority sits behind the page.
3) Draw a circle from groups 1-2-3; a (R) stereocenter will be clockwise and (S) will be counterclockwise
Isomer configurations
E/Z geometric isomers
Use the Cahn-Ingold-Prelog priority rules and assign priority based on highest atomic number.
(E) alkenes will have the highest priority groups on different sides and (Z) will be on the same side
Mnemonic: zame side
Types of insomers
Isomers have the same – but different –.
Isomers have the same molecular formula but different physical properties
Types of insomers
Stereoisomers vs constitutional isomers
Stereoisomers have the same bonds between atoms whereas constitutional isomers have different connectivity.
Types of insomers
Configurational (aka geometric) isomers vs conformers
Conformers can convert forms without breaking bonds whereas configurational isomers cannot.
Types of insomers
Types of configurational isomers
Two types: enantiomers which are mirror images and diastereomers which are not
Types of insomers
Tautomers
Tautomers are constitutional isomers that easily interconvert through movement of a H+ and double bond.
Common example: keto/enol forms of aldehydes and ketones.
