4. Stereoisomerism Flashcards
What is structural isomerism?
When compounds have the same molecular formula but differ in the arrangement of their atoms.
What are the three types of structural isomerism?
Chain isomerism- when the carbon chain is arranged differently
Positional isomerism- when the functional group is in a different position
Functional group isomerism- when the functional group is different
What is stereoisomerism?
When isomers have the same shortened formula but have a different arrangement of the atoms in space.
What are the two types of stereoisomerism?
E-Z isomerism
Optical isomerism
How does E-Z isomerism occur?
Occurs in alkenes as the π bond restricts rotation about the double bond. As a result, some alkenes can exist in two forms.
How can you identify if an alkene displays E or Z isomerism?
E- heaviest groups on opposite sides of the double bond
Z- heaviest groups on the same side of the double bond
How does optical isomerism occur?
When two isomers have different effects on plane polarised light.
What is a chiral centre?
An atom that is bonded to four different groups or atoms. The four different atoms or groups bonded to the centre can bond in two different ways, which are mirror images of each other.
What are enantiomers?
Stereoisomers that are non-superimposable mirror image forms of eachother.
Where does optical activity occur?
In molecules with chiral centres.
How does optical activity occur?
When a solution of an enantiomer is placed in a beam of plane polarised light, the enantiomer is able to rotate the beam.
What is used to measure optical activity?
A polarimeter.
What affects the extent of which the plane of polarised light is rotated?
The particular enantiomer
The concentration of the enantiomer in the solution
The length of the tube containing the solution through which light passes
The frequency of the light source used
The temperature
What is a racemic mixture?
An equimolar mixture of both enantiomers that produces no overall rotation of plane polarised light.
How can you make a racemic mixture?
Adding equal amounts of each enantiomer in a solution, whose rotations cancel each other out.
What is the danger of racemic mixtures?
Since biological systems usually respond to one enantiomer in the required way rather than the other, there is a danger that the ‘unwanted’ form could have serious and unwanted side effects.
Give an example of a product sold as a racemic mixture
Ibuprofen. When taken, one of the enantiomers is much more biologically active than the other, but fortunately the less active form is converted by an enzyme in the body into the other, more active isomer.
