Topics in Stereochemistry. Chiral Carbon Atoms A sp 3 hydbridized carbon atom with four different substituents is chiral. Chiral carbon atoms are also.

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Presentation transcript:

Topics in Stereochemistry

Chiral Carbon Atoms A sp 3 hydbridized carbon atom with four different substituents is chiral. Chiral carbon atoms are also referred to as a stereogenic centers. 1 chiral C Carbon is chiral even though stereochemistry is not specified 0 chiral C

Chiral Molecules If a molecule contains one or more chiral carbon atoms, and it does not have a plane of symmetry, it is a chiral molecule. Achiral molecule Meso compound Chiral molecule Meso compounds contain chiral centers, but have a plane of symmetry so are achiral.

Enantiomers Enantiomers are stereoisomers that are non-superimposable mirror images of each other. (R)-Carvone(S)-Carvone

Physical Properties of Enantiomers are Identical Except… Identical Enantiomers have identical NMR, IR and UV spectra, densities, melting points and boiling points. Different Enantiomers rotate plane polarized light in opposite directions of each other. (+): Rotation to the right (-): Rotation to the left You can not assign absolute configuration based on optical rotation alone. (R)-enantiomers can be (+) or (-) and (S)-enantiomers can be (+) or (-).

Diastereomers are stereoisomers that are not enantiomers. Diastereomers have different physical properties from each other. Diasteromers Ephedrine mp 40 °C Pseudoephedrine mp 117 °C Diastereomeric compounds have two or more chiral centers.

On a Side Note… Pseudoephedrine and Ephedrine Are No Longer OTC Racemic methamphetamine (R)-methamphetamine OTC decongestant Vicks Vapor Inhaler Little or no CNS activity “Levmetamfetamine” listed as active ingredient (S)-methamphetamine FDA approved for ADHD, obesity CNS stimulant (euphoria at high doses) Enantiomers often have different biological activities since enzymes and other receptors are chiral. Pseudoephedrine Original formula No longer OTC

Configuration and Conformation Configuration: Changing the configuration of a molecule requires bonds to be broken and reformed. A different configuration is a different molecule. Conformation: Changing the conformation of a molecule means rotating about single bonds. Conformations are interconvertible and are the same molecule. (R)-configuration(S)-configuration (R)-configuration in different conformations

L -Dopa Single enantiomer drug Parkinson’s disease D -Dopa Toxic Some Drugs Marketed as Racemic Mixtures, Other Drugs Sold as Single Enantiomers Ibuprofen NSAID Racemic mixture A racemic mixture is a 50:50 mixture of two enantiomers.

Celexa Antidepressant / antianxiety medication Selective Serotonin Reuptake Inhibitor (SSRI) Lexapro Mixture of enantiomers Single enantiomer 1)Lower dose (only active enantiomer) 2)Potential for fewer side effects 3)More expensive Single Enantiomer Drugs