MC- molecular shape :Configuration

importance of shape , structural isomers , stereoisomers and cahn ingold priority rules

Why does shape matter?

• interaction of a drug with a receptor ( action)

• interaction of a drug with an enzyme( metabolism and action)

• recognition of shape = vital

• more positive interactions = good drug molecule = binds strongly 

What is a structural isomer ( constitutional isomers)?

• same molecular formula

• can have different functional groups

• differences - atoms/ groups are attached differently 

What is a stereoisomer?

• same molecular structure , atoms are connecte din te same way , not structural isomers 

• same functional groups 

• atoms orientated differently in space

What are the different types of stereoisomers?

• enantiomers ( optical isomers) - chiral centres

• Stereoisomers- cis- trans isomers

What are the key processes/factors to consider for distinguishing between types of isomerism ?

• same molecular formulas

• are they bonded in the same order

• are they non- super impossable mirror images

What are the properties for enantiomers ?

• sp3 hybridized carbons - tetrahedral in shape

• carbon has 4 sigma bonds

• the carbon needs to have a chiral centre - which means there are 4 different groups/atoms attached to it

How do you draw enantiomers?

-tetrahedral shape

-dashed = away from you

-wedged= towards you

• some are mirror images of each other

What is an enantiomer?

• two forms that cannot be interchanged without breaking a bond - configurations 

• non super imposable , one cannot be rotated to be identical to the other 

• the central carbon atom is a chiral centre * - the molecules are called chiral molecules 

• the rotate polarised light differently 

What is a racemate ?

• when chiral compounds exit as a 50:50 mixture of both mirror images for,s a racemate

• but many important biological molecules are only active in one form

How do we get only one form of the enantiomer or use a racemate in medications ?

• purify drugs , separate mixture or prove that one enantiomer doesn’t do harm

• mixtures - cheaper no purification needed

• one form of the enantiomer= could be harmful

• when you rotate each an enatniomer they wont’ be the same

What is a molecule that doesn’t have a chiral centre called ?

Achiral 

• molecules can be super imposed on their mirror images 

• eg- glycine 

How do you distinguish the configurations of an enantiomer ?

• use CIP rules 1.

1. assign high , low priorities to each substituents based on atomic numbers

2. is atoms have the same priority move along to the main chain to the next atom

3. arrange molecules such that the lowest priority substituents go away from you

4. label the groups

5. if the go clockwise = R

6. if they go anti-clockwise = S

note- if the hydrogen isn’t away ( lowest then do normally rather than redrawing and then flipp the answer )

what is the lowest priority group isn’t away from you ?

• assign the priorities

• work out configurations

• then reverse the answer

What is the configuration of amino acids  ?

• all aa are S apart from cysteine 

• Cysteine - R  because of the sulphur substituent 

Whta is a diastereoisomer ?

• two forms that cannot be interchanged without breaking a bond 

• not mirror images

• different chemical properties 

eg- geometrical isomer/ cis- trans isomers

How do you distinguish between diastereomers ?

• also known as E , Z 

• use CIP RULES 

1. assign a priority to each carbon 

2. if the two high priorities are on the same side. of the molecule - Z 

3. if the two high priorities are on different sides -E  

Are diastereoisomers just for compounds with double bonds ?

No

• cyclic compounds - substituents can exist on either side of the ring

• look at the side with higher priority ( split th emolecule in half )