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Flashcards in Org 2 - Stereochemistry Deck (22)
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Define Stereochemistry

The study of the arrangement of atoms in a molecule, in 3 dimensions


What are Isomers ? 

Two different molecules with the same number and type of atoms (e.g. the same molecular formula)


Explain the two main categories of Isomers 

  1. Structural (constitutional) isomers differ by the way their atoms are connected 
  2. Stereoisomers (spatial isomers) differ in the way their atoms are arranged in space 


Name 3 types of structural Isomers 

  1. Structural Isomers e.g. difference in the chain/skeleton 
  2. Functional Isomers - same molecular formula but different functional groups present e.g. Alcohol and Ether
  3. Positional or RegioIsomers are structural Isomers where the functional group changes position on the parent structure


What are the two classifications of Spatial/Stereo Isomers ? 

  1. Enantiomers - two non superimposable molecules, which are mirror images of each other 
  2. Diastereomers - any pair of stereoisomers that are not enantiomers 


Why do Cis/Trans Isomers occur ? 

Carbons that are in a ring or double bond structure are not able to freely rotate resulting in cis and trans compounds. 


Explain Cis and Trans Isomers, and describe which is more stable 

  • When the substituents are on the same side of the ring or double bond, it is CIS. When they are on opposite sides, it is TRANS 
  • In TRANS isomers, the substituents are further apart, electron shell repulsion is minimised and the molecule is more stable


Explain Alkene E-Z notations for geometric stereoisomers 

Particularly useful for Isomeric compounds with 4 different substituent groups bonded to the double bond, and more complex molecules 


Explain how to assign E-Z notation to geometric/spatial stereoisomers 

  1. Assign priorities to each substituent at the double bond
  2. If the two groups of higher priority are on the Epposite sides = E (Entgegen)
  3. If the two groups of higher priority are on the Zame side = Z (Zusammen)


What are StereoIsomers ? 

  • Different compounds with the same structure, but differing in the spatial orientation or configuration of the atoms 
  • They can be Enantiomers or Diastereomers 


What is a Chiral molecule ? 

A molecule that is non-superimposable on it's mirror image 


What is a Chiral atom ?

An atom that is bonded to four different substituent groups 


What are Enantiomers ?

  • Enantiomers come in pairs, they are two non superimposable mirror images of each other.
  • To have an Enantiomer, a molecule must be chiral


What are the physical properties of Enantiomers ? 

Enantiomers have the same chemical and physical properties, the only difference is their interaction with other chiral molecules, and the rotation of PPL. 


What is a Fischer projection ? 

  • Used mainly for Carbohydrates 
  • A 2-D way of observing 3-D structures 


What is a racemic mixture ? 

A 50:50 mixture and L and D Enantiomers, it will show no rotation of Plane Polarized Light. 


What are Meso compounds ? 

Achiral (optically inactive) diastereomers of chiral stereoisomers. 

An internal plane of symmetry exists by drawing a line that will cut the molecule in half (MeSo = Mirror of Symmetry)


What are Ligands ? 

Atoms or groups attached to a central carbon 


What are Conformational Isomers ? 

Isomers which differ by rotation about single bonds, they are not true Isomers as they are different spatial orientations of the same molecule. 


Name 3 types of Conformational Isomer ? 

  1. Eclipsed - maximally close 
  2. Anti/staggered - maximally apart
  3. Gauche Formation - anywhere in between ! 



Explain which is the most stable type of Conformational Isomer, and why 

Anti formation is the most stable, as it minimises electron shell repulsion.