Nomenclature and Isomerism Flashcards
When does optical isomerism occur?
When there are four substituents attached to one carbon atom. Therefore optical isomers are said to be chiral
What are non-superimposable isomers?
Two enantiomers/optical isomers which are mirror images of one another but are not identical.
What is the carbon bonded to four different groups referred to as?
- The chiral centre
- Asymmetrical carbon atom
INFORMATION ABOUT LIGHT?
- Light consists of vibrating electric and magnetic fields. You can think of it as waves with vibrations occurring in all directions at right angles to the direction of the motion of the light wave.
What happens when light passes through a special filter called a polaroid?
All the vibrations are cut out except those in one plane, for example, the vertical plane.
How to use a polarimeter to measure optical rotation and identify the pair of enantiomers?
- Polarised light is passed through two solutions of the same concentration, each containing a different optical isomer of the same substance
- One solution will rotate the plane of polarisation through a particular angle clockwise = this is the (+)-isomer.
- The other will rotate the plane of polarisation by the same angle anticlockwise = (-)-isomer
What does a polarimeter do?
It measures the rotation of the plane of polarisation of polarised light
What is an equal mixture of the pair of enantiomers referred to?
It is called a racemic mixture or racemate, and is not optically active because the effects of the two isomers cancel out.
Name steps through synthesis of 2-hydroxypropanoic acid (lactic acid) from ethanal (laboratory)
STAGE 1: HCN added across C=O to form 2-hydroxypropanenitrile through nucleophilic addition reaction (CN- is nucleophile)
This product has a chiral centre. The reaction used does not favour one of these isomers over the other. The -CN group could add on with equal probability from above or below the CH3CHO which is planar so would get a racemic mixture.
STAGE 2: The nitrile group is converted into a carboxylic acid group. The 2-hydroxypropanenitrile is reacted with water acidified with a dilute solution of HCl (this is a hydrolysis reaction). 2-hydroxypropanoic acid is formed along wih ammonium chloride. This product still has a chiral centre and has not been affected by the hydrolysis reaction which only involves the -CN group. Therefore we still have a racemic mixture of 2 optical isomers.
What happens when molecules are produced naturally in living systems?
There will typically be only one optical isomer present. Most naturally occurring molecules are made by using enzyme catalysts which only produce one of the possible optical isomers
What happens when molecules are produced naturally in living systems?
There will typically be only one optical isomer present. Most naturally occurring molecules are made by using enzyme catalysts which only produce one of the possible optical isomers. Specific in shape of enzyme’s active site
How does an optical isomer molecule act as a drug?
The drugs work by a molecule of the active ingredient filling an area of a cell (Called a receptor) or an enzymes active site. Because receptors have a specific 3D structure, only one of a pair of optical isomers will fit.
In some cases, one optical isomer is an effective drug and the other one is inactive, this is a problem… What are the three options?
- Separate the two isomers, this may be difficult and expensive as optical isomers have very similar properties
- Sell the mixture as a drug - this is wasteful because half of it is inactive
- Design an alternative synthesis of the drug that makes only the required isomer
What kinds of properties are non super-imposable isomers identical in?
- Solubility
- Melting point
- Boiling point
What does the R and + stand for in R+?
The letter refers to the 3d arrangement of atoms in space and the charge refers to the direction that isomer rotates the plane of polarisation of polarised light
