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Medicinal Chemistry > Isomers (Chirality) > Flashcards

Isomers (Chirality) Flashcards

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1
Q

Define Isomers

A

Different compounds with the same molecular formula

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2
Q

Define Constitutional Isomers / Structural Isomers

A

Isomers whose atoms have a different connectivity.

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3
Q

Define Stereosiomers / optical / configurational

A

Isomers with the same molecular formula and same connectivity of atoms but different arrangement of atoms in space.

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4
Q

2 branches of Stereosiomerism:

A

Enantiomers (mirror images)

Diastereomers (not mirror images)

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5
Q

Geometric Isomerism

A

‘cis’ and ‘trans’

cis = same side (Z)
trans = opposite (E)
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6
Q

Optically inactive..

A

chiral

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7
Q

Racemic mixture

A

1:1 mixture of enantiomers, racemate

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8
Q

Geometric Isomerism

How do we assign priority group?

A

CIP rule

Based on highest atomic number = priority

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9
Q

Chiral means..

A

Assymetrical and mirror image are not superimposable.

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10
Q

In a chiral molecule, how do we differentiate them?

A

S or R rotation

S= anticlockwise 
R = clockwise
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11
Q

How do we assign S or R rotation?

A

Assign priority groups using CIP - number 1-4

and see the direction of priority groups

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12
Q

What should always be assigned priority group number 4?

A

Hydrogen - lowest priority.

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13
Q

What if Hydrogen is not on number 4?

A

Assign chirality then reverse

e.g. if its S with H at the front, answer becomes R.

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14
Q

What if Hydrogen is on a plane bond?

A

Rotate before assigning.

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15
Q

Do R and S enantiomers have the same physical and chemical properties? Why?

A

Yes because they have the same arrangement.

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16
Q

How do R and S enantiomers differ? (2)

A

Based on

rotation of plane polarised light

interaction with other chiral molecules

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17
Q

Is a racemic mixture optically active or inactive?

A

Inactive - equal amounts.

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18
Q

How to calculate light passing through a polarising filter? (equation)

A

a (Degrees) = measured rotation / C x L

c = conc g/ml

L = path length

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19
Q

What is enantiomeric excess?

A

When one enantiomer is present in higher concentration than the other.

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20
Q

Equation for enantiomeric excess

What if its in moles/grams?

A

observes rotation / rotation of pure. X 100

R - S / R+ S X100

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21
Q

Significance of Enantiomers in drugs
Ibuprofen

Whats it for?
Whats the significance here?

A

S enantiomer is responsible for the anti-inflammatory properties.

R = not harmful and there is an enzyme that converts R to S.

22
Q

Significance of Enantiomers in drugs
Fluoxetine

Whats it for?
Whats the significance here?

A

Depression

S = pharmacological activity

23
Q

Significance of Enantiomers in drugs
Warfarin

Whats it for?
Whats the significance here?

A

Reduce blood clots

S = more active than R

24
Q

Significance of Enantiomers in drugs
Thalidomide

Whats it for?
Whats the significance here?

A

Was for morning sickness but caused babies to be born without limbs.

Only one enantiomer had this effect.

25
Q

Describe Enantiomers

A

A single chiral centre, can either be R or S.

Optically Active

26
Q

Describe Diastereisomers

A

Multiple chiral centres.

Can be RR SR etc
and can use 2 n to find out all the possibilities.

where n is the number of chiral centres

27
Q

Describe Meso-compounds

A

Can have multiple chiral centres but are symmetrical (unlike the rest)

Optically INACTIVE.

28
Q

Can Enantiomers be separated?

A

No because of their identical properties but they can be converted to diastereoisomers with different physical properties.

29
Q

What is a classic method of ‘separating’ Enantiomers?

A

Resolution

Through the preparation of appropriate diastereoisomeric derivates.

30
Q

Can Diastereisomers be separated?

A

Yes because they have different physical properties and reactivity - melting point, boiling points, solubilities etc.

31
Q

Give an example of how we can separate Diastereoisomers: (3)

A

Use a solvent in which one diastereoisomer is very soluble in and the other is less.

Both will dissolve, less soluble one crystallises out and is filtered

other is evaporated

32
Q

Define Resolution

A

The separation of enantiomers from either racemic mixture or enantiomerically enriched mixture.

Via converting into diastereoisomers.

33
Q

Describe Chiral Chromatography

A

Racemic solution is passed over a chiral stationary phase giving rapid and reversible diastereotopic interaction.

34
Q

Classic Resolution

involving Salts

A

R- acid + S-acid enantiomers

converted into R,S salt and S,S salt diastereoisomers

Crystalise and separate into —–

R, S salt and S,S salt

Reacidify using HCL.
S-baseH+ and S-baseH+

becomes

R-acid and S-acid
separated

35
Q

Resolution by Esterification

A

Form covalent bond between chiral carboxylic acid and alcohol = diastereoisomers.

Separate them by crystallisation and then recover the separate enantiomers by ester hydrolysis.

36
Q

How many stereoisomer can Meso compounds have?

A

3 stereoisomers

assuming 2 identical chiral carbon atoms with no other stereo centres

37
Q

2 examples of pharmacological effects of chirality:

A

one may have strong binding to target, other may not

one may be active with no adverse effects, other may be toxic

38
Q

2 examples of biochemical effects of chirality:

A

if a chiral reactant is used, do the isomers form the same products or does each form a different product?

Are some stereoisomers formed more than others?

39
Q

Stereoselective reaction

A

Preferential formation of one stereoisomer over another

No prevention of formation but one predominates

40
Q

Stereospecific reaction

A

Each stereoisomeric reactant produces a different stereoisomeric product or a different set of products.

41
Q

Stereoselectivity reaction can… (2)

A
  • can show no stereoselectivity, results in racemisation

- can show by inverting stereochemistry, conserving it to preferentially reacting to form particular ones,

42
Q

Stereochemical outcomes are important in: (3)

A

metabolism of drugs

drug action at receptors

toxicity through undesirable activity of stereoisomers

43
Q

What is a concerted reaction?

A

Bond making and breaking happens in the same step. (SN2)

44
Q

Stereochemistry is either …… or …… in concerted reactions

A

inverted or conserved

45
Q

What is a non-concerted reaction?

A

Bond breaking and making occurs in different steps. (SN1)

Intermediates allow changes to stereochemistry.

46
Q

Do non-concerted reactions have stereoselectivity?

A

no stereoselectivity

47
Q

In non concerted reactions, from where can the planar intermediate be attacked?

A

can attack planar intermediate from above or below the plane equally easily.

Forms planar intermediate carbocation. Also planar radicals.

48
Q

What is syn addition?

A

Two substituents added to the same side of the double bond

similar to cis

49
Q

What is anti addition?

A

Two substituents added to opposite sides of the double bonds

similar to trans

50
Q

Enzymes can react with pharmaceuticals to…

A

destroy stereochemistry or to form chiral products with alternative activities which is why its important a drug is administered to the right place.

Medicinal Chemistry (2 decks)

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