Stereochemistry Flashcards
What percentage of organic chemistry questions are based on content form this chapter?
1/3
What is stereochemistry?
The study of the relative spatial 3-D arrangement of atoms within molecules
What are isomers?
Two different molecules with the same number and type of atoms (the same molecular formula)
What are structural isomers?
They have different atoms and/or bonding patterns in relation to each other
What are conformational isomers?
Isomers which only differ by the rotation about single bonds.
As a result, substituents (ligands, attached atoms/groups) can be maximally close, apart, or anywhere in between
What is the most stable kind of conformation? Why?
Anti, because it minimises electron shell repulsion
What are geometric isomers?
They occur because carbons that are in a ring or double bond structure are unable to freely rotate.
Occur only as alkenes and cyclic compounds. This results in cis and trans compounds
What are cis compounds?
When the substituents (i.e. bromine) are on the same side of the ring or double bond
What are trans compounds?
When the substituents are on opposite sides.
This is more stable than cis, as the substituents are on opposite sides so electron shell repulsion is minimised
What are stereoisomers?
Stereoisomers are isomeric molecules that have the same molecular formula and sequence of bonded atoms (constitution), but that differ only in the three-dimensional orientations of their atoms in space.
May be further divided into enantiomers and diastereomers
What is a stereogenic centre?
An atom bearing attachments such that interchanging any two groups produces a stereoisomer.
If a molecule has n stereoisomers, then it can have up to 2^n different non-superimposable structures (enantiomers)
What is an enantiomer?
Two non-superimposable molecules, which are mirror images of each other. They come in pairs. Must be chiral molecules .
Enantiomers have the same chemical and physical properties. The only difference is with their interactions with other chiral molecules, and their rotation of plane polarised light
What is a chiral molecule?
Contain at least one chiral carbon, which is a carbon atom that has four different substituents attached.
For the purposes of GAMSAT, the concepts of chiral carbon, asymmetric carbon and stereocenter are interchangeable
What are diastereomers?
Conversely to enantiomers, the are any pair of stereoisomers that are not enantiomers.
Both chemically and physically different from each other
What is the R, S system?
Its purpose is to provide information about the absolute configuration of a molecule.
This is done by assigning a stereochemical configuration at each asymmetric (chiral) carbon in the molecule by using 4 steps
What are the 4 steps of the R, S system of nomenclature?
- Identify an asymmetric carbon, and the 4 attached groups.
- Assign properties to the 4 groups:
- Atoms of higher atomic number have higher priority
- An isotope of higher atomic mass receives higher priority
- The higher priority is assigned to the group with the atom of higher atomic number or mass at the first point of difference
- If the difference between the two groups is due to the number of otherwise identical atoms, the higher priority is assigned to the group with the greater number of atoms of higher atomic number or mass - View the molecule along the bond from the asymmetric carbon to the group of lowest priority (i.e. the asymmetric carbon is near and the low priority group is far away.
- Consider the clockwise or counterclockwise order of the priorities of the remaining groups. If they increase in a clockwise direction, the asymmetric carbon is said to have the R configuration. If they decrease in a clockwise direction, the asymmetric carbon is said to have the S configuration
How is a stereoisomer named?
By indicating the configurations of each of the asymmetric carbons
What is a Fischer projection?
A 2-D way of looking at 3-D structures
With Fischer projections, what is the arrangement?
All horizontal bonds project towards the viewer, while vertical bonds project away from the viewer.
Used mostly for carbohydrates. To determine if 2 Fischer projections are superimposable, you can: 1) Rotate 1 projection 180degrees or 2) keep one substituent in a fixed position and then you can rotate the other 3 groups either clockwise or counter clockwise. Either technique preserves the 3-D configuration of the molecule
How are the lines of a Fischer projection interpreted?
- Dotted lines extend away from the viewer
- Solid triangle line extends towards the viewer
- Straight line extends in the plane of the paper
What are optical isomers?
Enantiomers (and therefore stereoisomers) that differ by different spatial arrangements about a chiral carbon atom
What is plane polarised light? What happens to it when it passes through a sample of chiral substance?
Light whose waves only oscillate in one plane.
It will emerge vibrating in a different plane than when it started.
Optical isomers differ only in this rotation
If light is rotated in a clockwise direction, the compound is what? (Concerning optical isomers)
Dextrorotary, designated by a D or +
If light is rotated in a counterclockwise direction, the compound is what? (Concerning optical isomers)
Levrorotary, designated by an L or -
All L compounds have the same relative configuration as L-glyceraldhyde
What is a racemic mixture?
Shows no rotation of plane polarised light.
This is a consequence of the fact that a racemate is a mixture with equal amount of D and L forms of a substance
What is specific rotation (a) is what?
a = (observed rotation in degrees) / (tube length in dm) (concentration in g/ml)
Inherent physical property of a molecule
The observed rotation is the rotation of the light passed through the substance. The tube length is the length of the tube that contains the sample in question. The specific rotation is dependent on the solvent used, the temperature of the sample, and the wave length of the light
Is there a clear correlation between the absolute configuration (i.e. R, S) and the direction of rotation of plane polarised light
No
What are meso compounds?
Archiral (optically inactive) diastereoisomers of chiral stereoisomers
In a meso compound, an internal plane of symmetry exists by drawing a line that will cut the molecule in half.
MeSo = Mirror of Symmetry
What is E, Z notations?
The IUPAC preferred method for designating the stereochemistry of double bonds. Similar to cis/trans but more precise
How do E, Z notations work?
- To begin with, each substituent at the double bond is assigned a priority.
- If the two groups of higher priority are on opposite sides of the double bond, the bond is assigned the configuration E (from German word entgegen, means opposite).
- If the two groups of higher priority are on the same side of the double bond, the bond is assigned the configuration Z (from German word zusammen, means together)
