isomerism & stereochemistry Flashcards
What is stereochemistry? Where is it prevalent?
Stereochemistry is the chemistry of molecules in three dimensions
Deals with various chiral molecules and stereoisomers
Influence chemical reactivity and pharmacological actions
Study of complex biological molecules and drug molecules
What is stereoisomerism?
Stereoisomers are compounds with identical molecular formulae, atoms
bonded in the same order, but in a different spatial arrangement (3D)
What is isomerism?
isomers are compounds with the
same molecular formula, but atoms are bonded in different orders
What are the two major types of stereoisomers?
*conformational isomers
*configurational isomers
What are configurational isomers?
optical isomers (enantiomers and diastereomers)
geometrical isomers
What is the number of conformers?
No conformers: number of single bonds and number/size of the flexible rings
What happens to conformers in room temp in solution + drug transportation
Conformers rapidly interconvert at room temperature and in solution
A drug being transported in plasma will exist as a variety of different conformational isomers
What are the four conventional conformational isomers like?
*ball and stick
*sawhorse
*wedge and broken line
*newman projection
How do configurational isomers differ from each other?
Differ from each other only in the arrangement of their atoms in the space
Cannot be converted from one into another by rotations of single bonds
What can chiral molecules not be? + chiral carbon
Chiral molecules (cannot be superimposed on their mirror images): i.e.
carbohydrates and proteins
Chiral carbon (sp3 C) or an asymmetric carbon or asymmetric centre or stereogeni
centre
What is a chiral carbon?
Chiral carbon, Four different groups/atoms
present.No plan of symmetry
What is an Achiral carbon?
Achiral carbon.At least 2 same groups/atoms
present. Yes plan of symmetry
What are enantiomers?
Same physical/chemical properties
Differ in activities with plan polarised light
that gives rise to optical isomerism, and also
in their pharmacological actions
Can an achiral molecule be imposed?
Achiral molecule
compound and its mirror
image are the same, can
be superimposed
What do chiral molecules do? + what does a mixture contain?
Chiral molecules rotate plane-polarised light: this is called optical activity
A mixture that contains more of one enantiomer than the other is optically active
What is a racemic mixture?
a pair of enantiomers
What are the two types of rotations called?
Rotation clockwise direction: dextrorotatory, (+)
Anticlockwise rotation: levorotatory, (−)
What are the two systems for configurational isomers?
*D and L systems
* + = D-(whatever molecule name)
* - = L- (whatever molecule name)
What is the cahn-ingold-prelog system?
Cahn–Ingold–Prelog system: describe configuration of enantiomers more precisely
What is the CIP system rules? 1&2
Rules are applied for designating any enantiomer to R or S:
1) Priority: from 1 to 4. Assigned on the basis of atomic number.
Higher atomic number gets higher priority
Pentan-2-ol
2) Substituents: if no priority, the next set of atoms in the unassigned groups is
examined
What is the CIP system rules (3&4)?
3) Double or triple bonds: assigned priorities as if both atoms were duplicated or
triplicated
4) Arrange (rotate) the molecule: group 4 is pointing away from the viewer
What are Diastereomers?
isomers of a compound
that are non-identical
and not mirror images
of each other
How can you tell how many possible stereoisomers are in a drug?
Number of possible stereoisomers in a drug: 2n (where n= number of chiral
centres)
The exception to this is when meso forms exist.
What are geometric isomers?
stereoisomers that arise due to restricted rotation within a molecule
(Only if two different groups at each end of doublebond)
What is cis vs trans nomeclature of configurational isomers?
Only applies when the two ends of the double bond have the
same groups attached:
TRANS + identical groups are on different sides
CIS = identical groups are on the same side
Configurational isomers: E/Z nomenclature
E = higher priority groups on different sizes
Z = higher priority groups on the same size
Why is stereoisomerism important?
Around 30-50% of drugs has stereocentres
One enantiomeric form of a drug may be active, and the other may be inactive, less
active, having a different activity or even toxic
