compounds with the same molecular formulae but different structural formulae
species with the same structural formula but with a different arrangement of the atoms in space. There are 2 types of stereoisomerism: optical and E/Z isomers.
- A chiral carbon is a carbon atom attached to four different atoms or group of atoms.
- Optical isomers are mirror images of each other.
- they cannot be superimposed.
- Optical isomers are chemically identical. However, optical isomers rotate plane-polarised light in different directions, one in clockwise and the another anit-clockwise. They are optically active.
- A mixture containing equal amounts of each isomer is known as a racemic mixture.
- A racemic mixture has no effect on plane-polarised light because the rotations cancel each other out.
It applies to:
- alkenes and other organic compounds that contain C=C bonds
- cyclic alkanes.
- only occurs to double-bonds as rotatoin is restricted
- the higher priority group at the one end of the double bond and the higher priority group at the other end of the doublebond are on the same side (Z, from German zusammen = together)
- or the opposite sides (E, from German entgegen = opposite) of the double bond
- also called cis and trans isomers:
Optical activity and biochemistry
Optical activity is important in biological systems where frequently only one of the optical isomers is bioloigxally active. In plants and animals only one of the optical isomers is synthesised naturally, and only one will interact with an enzyme due to the stereospecific nature of enzymes.
💊💊Pharmacological activity is the benefical or adverse effects of a drug on living matter. Bilogical molecules have complex three-dimensional structures that bind to a drug molecule in only on possible way. The three dimesional structure of the drug dtermines the pharmacological activity of a drug, and whether a pharmaceutical will have the desired therapeutic affect or not.
The production of a single isomer with the correct pharmacological activity presents significant advantages to pharmaceutical companies;
- Risks from undersible effect are reduced
- Drug doses are reduced
- when a drug containing a mixture of two optical isomers is prescribed, half of the drug is often wasted in the body because only one of the isomers has the desired therapeutic effect.
To prepare a chiral compound in the lab, complicated seperation techniques are needed to isolate the pharmacologically active isomer. Seperation is difficult because optical isomers usually have the same or very similar physical properties - melting points, boiling points and solubilities. Seperation techniques include the use of enzymes, electrophoresis and chromatography. These take a significant amount of time and cost.
A number of methods are currently being used or are being developed to prepare single chiral isomers;
- Using enzymes as biological catalysts- drug manufactures can use purified enzymes or enzyme-containing microorganisms to biocatalyse any reaction step that produces a chiral carbon.
- Chiral pool synthesis- pool of naturally occuring chiral molecules within the synthetic route. The chirality of the orginal molecule can lead to the formation of a product that is a single, pure optical isomer. Common starting materials used in chiral synthesis include natural alpha-amino acids and sugars.
- Use of transition element complexes- transition metal complexes to produce chiral catalyst, these can then transfer their chirality to synthesis a single isomer product.
Drafted by Eunice (Chemistry)