Chemistry > Organic - Optical Isomerism > Flashcards
Organic - Optical Isomerism Flashcards
What is the formula of alkenes?
RCH=CH2
What is the formula of alkynes?
RC≡CH
What is the formula of halogenoalkanes?
R-X
X is fluorine, chlorine, bromine or iodine
What is the formula of carboxylic acids?
RCOOH
What is the formula of anhydrides?
RCOOCOR’
What is the formula of esters?
RCOOR’
What is the formula of acyl chlorides?
RCOCl
What is the formula of amides?
RCONH2
What is the formula of nitriles?
RC≡N
What is the formula of aldehydes?
RCHO
What is the formula of ketones?
RCOR’
What is the formula of alcohols?
ROH
What is the formula of amines?
RNH2
What is the formula of arenes?
C6H5R
What are isomers?
Isomers are compounds with the same molecular formula but have different molecular structures or a different arrangement of atoms in space.
What is structural isomerism?
Structural isomers:
- have different functional groups (functional group isomerism) OR
- have functional groups attached to the main chain at different points (position isomerism) OR
- have a different arrangement of carbon atoms in the skeleton of the molecule (chain isomerism)
What is stereoisomerism?
Stereoisomerism is where two (or more) compounds have the same structural formula. They differ in the arrangement of the bonds in space.
There are two types:
- E-Z isomerism
- optical isomerism
What is optical isomerism?
Optical isomers occur when there are four different substituents attached to one carbon atom (asymmetrical carbon) within an organic molecule (chiral centre). This results in two isomers that are non-superimposable mirror images of one another, but are not identical.
What is chirality?
Optical isomers are said to be chiral and the two isomers are called a pair of enantiomers. The carbon bonded to the four different groups is called the chiral centre or the asymmetric carbon atom, and is often indicated on formulae by *. If there’s a double bond, you can’t have a chiral centre.
Which molecules are chiral?
- All alpha-amino acids, except glycine the simplest one, have a chiral centre.
- 2-hydroxypropanoic acid (lactic acid) is also chiral. Although the chiral carbon is bonded to two other carbon atoms, these carbons are part of different groups and you must count the whole group.
Why does optical isomerism happen?
Optical isomerism happens because the isomers have three-dimensional structures so it can only be shown by three-dimensional representations or by models.
What is optical activity?
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 motion of the light wave (transverse).
If the light passes through a special filter, called a polaroid, all the vibrations are cut out except those in one plane. The light is now vertically polarised and it will be affected differently by different optical isomers of the same substance.
How can optical rotation be measured using a polarimeter?
- 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 positive isomer.
- The other will rotate the plane of polarisation by the same angle, anticlockwise. This is the negative isomer.
What is a racemic mixture?
Many of the reactions used in organic synthesis to produce optically active compounds actually produce a 50:50 mixture of two optical isomers. This is called a racemic mixture or racemate and is not optically active because the effects of the two isomers cancel out. These two isomers are able to form in this way due to a nucleophilic addition reaction.
What is the synthesis of 2-hyroxypropanoic acid (lactic acid)?
2-hydroxypropanoic acid has a chiral centre, marked by * in the structure. The synthesis produces a mixture of optical isomers. It can be made in two stages from ethanal.
What is stage one of the synthesis of 2-hydroxypropanoic acid?
Hydrogen cyanide is added across the C=O bond to form 2-hydroxypropanenitrile. This is a nucleophilic addition reaction in which the nucleophile is the CN- ion.
2-hydroxypropanenitrile has a chiral centre, the starred carbon, which has four different groups (-CH3, -H, -OH, and -CN). The reaction used does not favour one of these isomers over the other (i.e., the -CN group could add on with equal probability from above or below the CH3CHO which is planar) so you get a racemic mixture.
What is stage two of the synthesis of 2-hydroxypropanoic acid?
The nitrile group is converted into a carboxylic acid group. The 2-hydroxypropanenitrile is reacted with water acidified with a dilute solution of hydrochloric acid. This is a hydrolysis reaction.
The 2-hydroxypropanoic acid that is produced still has a chiral centre - this has not been affected by the hydrolysis reaction, which only involves the -CN group. So you still have a racemic mixture of two optical isomers.
How do molecules produced synthetically and naturally differ?
It is often the case that a molecule with a chiral centre that is made synthetically ends up as a mixture of optical isomers. However, the same molecule produced naturally in living systems will often be present as only one optical isomer.
Amino acids are a good example of this. All naturally occurring amino acids (except aminoethanoic acid, glycine) are chiral, but in every case only one of the isomers is formed in nature. This is because most naturally occurring molecules are made using enzyme catalysts, which only produce one of the possible optical isomers.
How are optical isomers used in the drug industry?
Some drugs are optically active molecules. For some purposes, a racemic mixture of the two optical isomers will do. For other uses, only one isomer is required. Many drugs work by a molecule of the active ingredient fitting an area of a cell (called a receptor) or an enzyme’s active site like a piece in a jigsaw puzzle. Because receptors have a 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 is inactive. This is a problem and there are 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.
The over-the-counter painkiller and anti-inflammatory drug ibuprofen (sold as Nurofen and Calprofen) is an example.
Give an example of a case where one of the optical isomers is an effective drug and the other is toxic or has unacceptable side effects.
Naproxen has one isomer that is used to treat arthritic pain, whilst the other causes liver poisoning. In this case it is vital that only the effective optical isomer is sold.
Why doesn’t a racemic mixture have any effect on the plane of polarisation of plane polarised light?
It’s a 50:50 mixture of the two enantiomers (optical isomers). One of these is rotating the plane of polarisation anti-clockwise, and the other is rotating it by exactly the same amount clockwise. They cancel each other out, so there is no overall effect on the light.
What are the two different optical isomers called?
They are called enantiomers and differ in the way they rotate the plane of polarisation of polarised light - either clockwise (positive isomer) or anticlockwise (negative isomer). Each enantiomer causes the rotation of plane polarised light by 90 degrees in opposite directions.
How do nucleophilic addition reactions produce optical isomers?
In these reactions, nucleophiles are able to attack a molecule with a carbonyl group from above or below the carbon-oxygen double bond. This means the two possible products of the reaction are mirror images and therefore optical isomers.
