organics- optical isomerism Flashcards
what are optical isomers
they are stereoisomers that have mirror images that are non-sumperimposable
what is a chiral carbon/chiral centre
A carbon atom that has four different atoms or groups of atoms attached to it
what do two chiral compounds exists as
two optical isomers which are also known as enantiomers
how do two optical isomers differ
Their physical and chemical properties are identical but they differ in their ability to rotate plane polarised light- they can rotate the plane of light - and are said to be optically active
how can optical isomers rotate the plane of polarised light
one enantiomer rotates it in one direction and the other rotates it in the other direction by the same amount
what is a mixture containing a 50/50 mix of the two isomers called
a racemate or racemic mixture
what effect does a racemate have on the plane of light
no effect - it is optically inactive - as each enantiomer cancels out one and other
what systems of nomenclature are in use for optical isomers
D/L or +/- - old ones
R/S - new ones
how can the rotation of a planes of light be used to determine the identity of an optical isomer
pass plane polarised light through a sample containing one of the two optical isomers of a single substance
Depending on which isomer the sample contains, the plane of polarised light will be rotated either clockwise or anti-clockwise by a fixed number of degrees
how to identify a chiral center
To be successful you need to differentiate the carbon atoms and determine one of the following:
Whether a particular carbon is bonded to four different atoms or groups of atoms and therefore is chiral
Whether a particular carbon is bonded to two of the same atoms or groups of atoms and therefore cannot be chiral
draw out structure as condensed structural or displayed so can see the bonds
how do you represent a chiral carbon
Chiral centres are marked with an asterisk (*)
how do you draw an optical isomer
You need to use stereochemical drawing conventions to represent optical isomers
In the convention:
a solid line is a bond in the same plane as the paper
a dotted line is a bond receding behind the plane of the paper(this can also be hatched or shaded wedges)
a solid wedge is a bond coming out of the paper
For example, suppose you are asked to draw the optical isomers of the amino acid alanine, CH3CH(NH2)COOH
Start by drawing a vertical dotted line to represent a line of symmetry in the centre of your page
Next draw the chiral carbon with four bonds in a tetrahedral arrangement
Make sure two bond lie in the plane of the paper, one comes out and one recedes
Add the four groups, but be careful to show the mirror image sequence of atoms
how are racemic mixtures used in drugs
In the pharmaceutical industry it is much easier to produce synthetic drugs that are racemic mixtures than producing one enantiomer of the drug
Around 56% of all drugs in use are chiral and of those 88% are sold as racemic mixtures
Separating the enantiomers gives a compound that is described as enantiopure, it contains only one enantiomer.
This separation process is very expensive and time consuming, so for many drugs it is not worthwhile, even though only half the of the drug is pharmacologically active
For example, the pain reliever ibuprofen is sold as a racemic mixture
what are some reasons why only one enantiomer may be produced
The other enantiomer could be harmful
The drug is very expensive to synthesize so you don’t want to waste materials and resources to produce a drug that is only 50% effective
what medical example can be used to show the dangers of a racemic mixture
Thalidomide was a drug introduced in the late 1950s as a sedative and treatment for morning sickness in pregnant women
A few years later, children of mothers who had taken thalidomide were born with missing limbs and it was later linked to the presence of the other enantiomer that caused birth defects
Fortunately the testing and regulation of drugs these days means it is very unlikely that such a tragedy could be repeated
The irony is that even if the drug had been sold as one enantiomer, some birth defects may still have occurred as we now know that some drugs an invert from one optical isomer to the other during metabolic reactions in the human body
