isomerism & stereochemistry

Question 1

What is stereochemistry? Where is it prevalent?

Answer 1

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

Question 2

What is stereoisomerism?

Answer 2

Stereoisomers are compounds with identical molecular formulae, atoms
bonded in the same order, but in a different spatial arrangement (3D)

Question 3

What is isomerism?

Answer 3

isomers are compounds with the
same molecular formula, but atoms are bonded in different orders

Question 4

What are the two major types of stereoisomers?

Answer 4

*conformational isomers
*configurational isomers

Question 5

What are configurational isomers?

Answer 5

optical isomers (enantiomers and diastereomers)
geometrical isomers

Question 6

What is the number of conformers?

Answer 6

No conformers: number of single bonds and number/size of the flexible rings

Question 7

What happens to conformers in room temp in solution + drug transportation

Answer 7

Conformers rapidly interconvert at room temperature and in solution
A drug being transported in plasma will exist as a variety of different conformational isomers

Question 8

What are the four conventional conformational isomers like?

Answer 8

*ball and stick
*sawhorse
*wedge and broken line
*newman projection

Question 9

How do configurational isomers differ from each other?

Answer 9

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

Question 10

What can chiral molecules not be? + chiral carbon

Answer 10

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

Question 11

What is a chiral carbon?

Answer 11

Chiral carbon, Four different groups/atoms
present.No plan of symmetry

Question 12

What is an Achiral carbon?

Answer 12

Achiral carbon.At least 2 same groups/atoms
present. Yes plan of symmetry

Question 13

What are enantiomers?

Answer 13

Same physical/chemical properties
Differ in activities with plan polarised light
that gives rise to optical isomerism, and also
in their pharmacological actions

Question 14

Can an achiral molecule be imposed?

Answer 14

Achiral molecule
compound and its mirror
image are the same, can
be superimposed

Question 15

What do chiral molecules do? + what does a mixture contain?

Answer 15

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

Question 16

What is a racemic mixture?

Answer 16

a pair of enantiomers

Question 17

What are the two types of rotations called?

Answer 17

Rotation clockwise direction: dextrorotatory, (+)
Anticlockwise rotation: levorotatory, (−)

Question 18

What are the two systems for configurational isomers?

Answer 18

*D and L systems
* + = D-(whatever molecule name)
* - = L- (whatever molecule name)

Question 19

What is the cahn-ingold-prelog system?

Answer 19

Cahn–Ingold–Prelog system: describe configuration of enantiomers more precisely

Question 20

What is the CIP system rules? 1&2

Answer 20

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

Question 21

What is the CIP system rules (3&4)?

Answer 21

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

Question 22

What are Diastereomers?

Answer 22

isomers of a compound
that are non-identical
and not mirror images
of each other

Question 23

How can you tell how many possible stereoisomers are in a drug?

Answer 23

Number of possible stereoisomers in a drug: 2n (where n= number of chiral
centres)
The exception to this is when meso forms exist.

Question 24

What are geometric isomers?

Answer 24

stereoisomers that arise due to restricted rotation within a molecule
(Only if two different groups at each end of doublebond)

Question 25

What is cis vs trans nomeclature of configurational isomers?

Answer 25

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

Question 26

Configurational isomers: E/Z nomenclature

Answer 26

E = higher priority groups on different sizes
Z = higher priority groups on the same size

Question 27

Why is stereoisomerism important?

Answer 27

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