3.1b Stereochemistry Flashcards
Unit 3: Organic Chemistry
What is an Isomer?
These are molecules with the same molecular formula but different structural formulae
What is a structural isomer?
These are isomers in which the atoms are bonded in a different order in each isomer
What are stereoisomers?
These isomers are molecules in which the order of bonding is the same, but the spatial arrangement of the atoms is different in each isomers.
What are the two different types of stereoisomers?
-Geometric isomers
-Optical isomers
What is a geometric isomer?
Geometric isomerism occurs when there is a lack of free rotation inside molecules
Why may geometric isomers have restricted rotation?
-a carbon to carbon double bond
-a carbon to carbon sing bonded ring structure
As well as restricted rotation what else must there be for geometric isomers?
Different groups attached to each of the carbon atoms that make the bond of restricted rotation
What is a cis isomer?
The groups are located on the same side of the bond with restricted rotation
What is a trans isomer?
The groups are located on different sides of the bond with restricted rotation
What is the difference between the melting and boiling points in cis and trans isomers in non polar isomers?
Trans isomers pack more efficiently in the solid structure allowing more intermolecular forces. The U shape of the cis isomer doesn’t allow such tight packing so less energy is required to break bonds as there are fewer intermolecular forces so there is a lower melting point
What is the difference between the melting and boiling points in cis and trans isomers in polar isomers?
The boiling points in cis isomers are higher than trans isomers as the cis isomer will have a dipole so there are PD-PD interactions. The trans isomer also has polar bonds, but these cancel each other out so the molecule is not polar.
What is an optical isomer?
This occurs in compounds where four different groups are attached to a central tetrahedral carbon. The resulting isomers are mirror images of each other, and cannot be syperimposed on each other
What is the name given to the central carbon atom?
Chiral carbon
What are two optical isomers called?
Enantiomers
What is the exception for optical isomers physical properties?
All there physical properties are identical with the exception of their effect on plane polarised light
What happens when light is passed through a sample of enantiomers?
They can change the direction of the light, and are described as optically active. One enantiomer will rotate the light one way and the second will rotate in the opposite direction by the same amount
What sign is given to isomers that rotate the polarised light right (clockwise)?
+ (or R)
What sign is given to isomers that rotate the polarised light left (anti-clockwise)
- (or S)
What happens to the light if there is 50/50 mixture of the two enantiomers
The light will be rotated by the same amount in both directions so there will be no overall rotation
What is the name given to an equal mixture of optically active isomers?
Racemic mixture
What happens when there is an unequal % of each enantiomer in the mixture?
The light will be rotated in the direction of the most abundant enantiomer, by the difference in the percentages
Optical isomers have identical chemical properties except under which circumstances?
In a chiral environment
In what environment is optical isomers not having identical properties significant?
In biological systems shape is a critical, most active biological molecules interact with each other by binding to a molecule with a complementary shape
In biological systems, only one optical isomer is generally present, what is the significance of this?
This ensures that the “correct” enantiomers fit to the receptor is present, and energy is not wasted making molecules that cannot be used (or work differently). Regulation of biological systems is dependent on molecules interacting in the correct way that is needed.
