Organic - Optical Isomerism Flashcards

Question

What is the synthesis of 2-hyroxypropanoic acid (lactic acid)?

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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: 1. Separate the two isomers - this may be difficult and expensive as optical isomers have very similar properties. 2. Sell the mixture as a drug - this is wasteful because half of it is inactive. 3. 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.

Answer 6

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.

Answer 7

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.

Answer 8

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.

Answer 9

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.