Isomers Flashcards
(19 cards)
Term: Isomers
Definition: Molecules with the same molecular formula but a different arrangement of atoms.
Two major categories of enantiomers
Structural isomers – different connectivity of atoms
Stereoisomers – same connectivity, different spatial arrangement
What defines structural isomers? Give an example of a structural isomer
They have different atomic arrangements (chemical structure) despite the same molecular formula. E.g. Isoflurane and enflurane—structural isomers with different physical and pharmacological properties.
Term: Tautomerization
Definition: A form of dynamic structural isomerism where two structural isomers exist in equilibrium.
Describe how thiopental undergoes tautomerization
In an acidic environment, the –SNa group becomes a –SH (thiol), which tautomerises to a thione (–C=S), increasing lipid solubility.
Does midazolam undergo true tautomerism?
No—it undergoes ring closure with the elimination of water, so it is not true tautomerism.
What are the two main types of stereoisomers?
Geometric (cis/trans)
Optical (enantiomers & diastereoisomers)
Term: Geometric isomerism.
Give an example of a geometric isomer
Definition: Occurs in molecules with C=C double bonds that prevent free rotation, leading to cis (same side) and trans (opposite side) forms.
Mivacurium—contains geometric isomers affecting its clinical properties.
Term: Optical isomers (enantiomers)
Definition: Molecules that are non-superimposable mirror images of each other.
What structural requirement must be met for a molecule to have optical isomers?
The presence of a chiral centre (a carbon with 4 different groups attached).
What’s the difference between levorotatory and dextrorotatory enantiomers?
Levorotatory: Rotates plane-polarised light to the left
Dextrorotatory: Rotates light to the right
Term: Racemic mixture
Definition: An equal mixture of both enantiomers; it has no net rotation of plane-polarised light.
How are R and S stereoisomers assigned?
Prioritize atoms attached to chiral centre by atomic number
View with lowest-priority group (usually H) pointing away
Clockwise = R (Rectus), Anticlockwise = S (Sinister)
Term: Diastereoisomers
Give 3 examples of drugs that are both diastereoisomers and enantiomers.
Definition: Stereoisomers that are not mirror images (e.g. molecules with multiple chiral centres, but not enantiomers).
Atracurium
Tramadol
Methohexital
Why do different isomers of a drug have different pharmacological effects?
They interact differently with chiral receptors, affecting:
Potency
Intrinsic activity
Side effect profile
What are the clinical advantages of using single isomers?
More predictable effects
Reduced side effects
Enhanced potency
Why is cisatracurium preferred over other isomers in atracurium?
3× more potent
Fewer autonomic side effects
Less histamine release
Reduced laudanosine (toxic metabolite)
Produced as a single enantiomer
Name local anaesthetics that exist as optical isomers.
Mepivacaine
Prilocaine
Bupivacaine
Ropivacaine
What are the benefits of using the S-isomer in local anaesthetics?
Increased vasoconstriction → longer action, less systemic absorption
Reduced cardiotoxicity
Reduced motor block
