Isomerism Flashcards
What is isomerism and what are the main different types?
Isomerism is the existence of 2 or more molecules composed of the same atoms in the same proportions but with variations in the arrangement of these atoms.
The main types are:
1. Constitutional aka Structural (including tautomers)
2. Stereoisomers (including optical, geometric, cis-trans, and racemic mixtures)
What is a structural isomer and give an anaesthetic example?
Structural isomers* have identical chemical formulae but the order of atomic bonds varies.
Isoflurane and enflurane are a good example of a structural isomer, they have the same chemical formula but the location of the fluorine atoms varies.
Isoflurane has less CV effects than enflurane.
Structural isomers are the same as constitutional isomers.
What is a tautomer?
Tautomerization is a form of dynamic structural isomerism where 2 structural isomers exist in equilibrium with each other.
A tautomer is an organic compound that is interconvertible by a chemical reaction involving the migration of a hydrogen atom accompanied by the switch of a single bond and adjacent double bond.
Thiopental is an example of an anaesthetic drug which is a tautomer.
What are aromatic isomers?
Aromatic isomers refer to molecules with a benzene ring and when a functional group is bonded to different positions on the benzene ring.
An example of an aromatic isomer is Nitric oxide modified aspirin (NO-ASA)
What is a stereoisomer and what are the main different types?
A stereoisomer has the same atoms connected in the same order but have different 3D arrangements of the atoms.
A carbon atom with 4 different groups attached to it satisfies this condition. There are two basic types of stereoisomer:
a) Optical
b) Geometric
What is an optical isomer?
Optical isomers are a type stereoisomer, in which the 2 isomers (which are known as enantiomers) are mirror images of each other.
They are also known to be chiral. Any molecule which consists of an atom (usually carbon) bound to 4 different groups will be an optical isomer. The atom at the centre will be the optical isomer.
Enantiomers which have identical chemical and physical properties but rotate plane polarised light in opposite directions.
Chiral comes from the greek work cheir which means hand, if you place your hands open palmed they are mirror images of each other.
How are enantiomers classified?
Enantiomers can be classified by the distribution of the groups around the chiral centre:
* Left are S enantiomers
* Right are R enantiomers
OR
Enantiomers can also be classified by the direction plan polarised light rotates:
* Dextrorotatory enantiomer rotates it to the Right
* Levorotatory enantiomer rotates it to the Left
Can you give 2 examples of anaesthetic drugs which are optical isomers?
Bupivicaine. The S enantiomer of bupivicaine (e.g. levobupivicaine) is less cardiotoxic.
Ketamine is also an optical isomer. The S enantiomer of ketamine has less psychotic side effects.
What is a racemic mixture?
A racemic mixture is when there is an equal proportion of enantiomers.
Adrenaline is an example of this.
What are geometric isomers and what are the different types?
Are stereoisomers but unlike optical isomers they are not simple mirror images of each other.
The different types of geometric isomers are:
1. Cis-Trans
2. Conformers and atropisomers
When do cis-trans isomers occur?
Cis-trans isomers occur in relation to a C=C bond.
The presence of the second carbon bond prevents other groups attached to the carbon atoms from rotating so that only two potential fixed versions of the molecule exist.
Cis (both groups on the same side) or trans (groups exist further apart).
Give an example of an anaesthetic drug that is a Cis-Trans isomer?
Atracurium and Cisatracurium
As suggetsed by the name Cisatracurium is the Cis isomer.
Cisatracurium is associated with less histamine release and therefore increased CV stability
What are conformers or conformational isomers?
Conformers are a type of geometric isomers which are produced due to rotation around 1 single or more bonds.
Single conformers cannot be isolated. Butane is an example.
What is an atropisomer?
Atropisomers are a group of unusual stereoisomers which result from hindered rotation are a single bond.
This impaired rotation allows you to isolate atropisomers. Vancomycin is an example
What type of isomerism do the following drugs exhibit:
+Enflurane/Isoflurane
+Bupivicaine/Levobupivicaine
+Vancomycin
+Ketamine
+Midazolam
+Atracurium/Cistatracurium
Enflurane and Isoflurane are examples of structural isomers.
Levobupicaine is the S enantiomer of bupivicaine. It is a form of an optical isomer.
Vancomycin is an example of an atropisomer.
Ketamine is an optical isomer. The S enantiomer of Ketamine has less psychotic side effects.
Midazolam is an example of a tautomer* it becomes water soluble at low pH (open ring midazolam)
Atracurium and Cistatracurium is an example of a Cis-trans geometric isomer.
*strictly speaking midazolam is not an isomer as when the ring closes a H2O molecule is eliminated.
