1.6 Isomerism Flashcards
Define structural isomerism
same molecular formula, different structural formula
define chain isomerism
isomers have different arrangements of the carbon chain
define positional isomerism
isomers have the same carbon skeleton but the functional group is in different positions in the molecule
define functional group isomerism
same molecular formula,, different functional groups
define stereoisomerism
same molecular formula, but atoms occupy different positions in space
define geometrical isomerism
molecules with the same structural formula but different arrangement of atoms due to the presence of one or more C=C bonds, around which there is restricted rotation for the two different groups bonded to each carbon atom
define optical isomerism
isomers exist as non-superimposable mirror images due to the presence of one or more asymmetric centres
What are the two main types of isomerism?
structural isomerism
steroisomerism
name types of structural isomerism
chain isomerism
positional isomerism
functional group isomerism
name types of stereoisomerism
geometrical isomerism
optical isomerism
Why is geometrical isomerism found in molecules with a C=C bond?
it is an energy barrier to free rotation
what is required in geometrical isomerism
a C=C bond where each carbon in the bond is attached to two different groups
How are priorities assigned in geometrical isomerism?
- On each carbon the groups are labelled 1 or 2 depending on priority - the atom with the highest atomic number bonded to the carbon is assigned 1.
- If highest priority groups are on the same side they are known as Z (zusammen)/cis, and if on opposite sides they are known as E (entgegen)/trans
define chiral/assymetric centre
a carbon atom with four different atoms or groups bonded to it
define optical isomers/enantiomers
molecules which exist as non-superimposable mirror images
define optically active
a sample that rotates the plane of plane polarised light
When drawing optical isomers, what is the chiral centre labelled with?
an asterisk
how can optical isomers be distinguished?
The optical isomers of a compound will have the same physical properties and can only be distinguished by their ability to rotate the plane of plane polarised light
what are the two types of optical isomer?
- D (dextrorotatory) - rotates plane of plane polarised light to the right
- L (laevorotatory) - rotates plane of plane polarised light to the left
define plane polarised light
visible light, or other forms of electromagnetic radiation, whose vibrations occur in a single plane
define racemic mixture/racemate
a 50:50 mixture of two optical isomers
Describe the optical activity of a racemate
it will be optically inactive. This is due to the optical isomers in the mixture rotating the plane of plane-polarised light by equal amount in opposite directions.
Name two examples of optical isomers in medicine
thalidomide
citalopram
describe thalidomide
- Introduced in 1957 to treat morning sickness during pregnancy
- Withdrawn after it was found to cause severe birth defects
- Contains an asymmetric centre that gives rise to two optical isomers - R-thalidomide and S-thalidomide
- R-isomer - effective in treatment of morning sickness, plus a mild sedative
- S-isomer - none of the therapeutic effects of the R-isomer and was responsible for limb deformities and other severe birth defects
Describe citalopram
- An effective treatment for depression - administered in the form of tablets containing a racemic picture of optical isomers.
- The S isomer has a shape that allows it to bond with receptor sites in the body, which then signals an increase in the production of serotonin.
