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
- Structural (constitutional) isomers differ by the way their atoms are connected
- Stereoisomers (spatial isomers) differ in the way their atoms are arranged in space
Name 3 types of structural Isomers
- Structural Isomers e.g. difference in the chain/skeleton
- Functional Isomers - same molecular formula but different functional groups present e.g. Alcohol and Ether
- 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 ?
- Enantiomers - two non superimposable molecules, which are mirror images of each other
- 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
- Assign priorities to each substituent at the double bond
- If the two groups of higher priority are on the Epposite sides = E (Entgegen)
- 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 ?
- Eclipsed - maximally close
- Anti/staggered - maximally apart
- 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.