3.7 Optical Isomerisms Flashcards

1
Q

What property must a carbon atom have for the molecule to display optical isomerism about that carbon atom?

A

4 different substituents attached to one carbon atom

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2
Q

What are the similarities and differences between two optical isomers?

A

Same atoms and bonds, but they are non-superimposable mirror images of one another. NOT IDENTICAL in chemical properties necessarily. Differ in the way they rotate plane polarised light - rotate plane of polarisation by the same angle but in different directions.

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3
Q

What word is used to describe optically active

molecules?

A

Chiral

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4
Q

What are the pair of isomers called?

A

Enantiomers

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5
Q

What is the chiral centre?

A

The carbon that has four different substituents attached to it

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6
Q

How is the chiral centre denoted?

A

C*

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7
Q

Give two examples of chiral molecules.

A

All alpha amino acids, except glycine.

Lactic acid / 2-hydroxypropanoic acid

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8
Q

How is light polarised?

A

By passing it through a polaroid filter, so

oscillations are only in one plane.

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9
Q

What effect does the racemic mixture have on plane

polarised light?

A

None, as the rotation by each enantiomer

cancels out to nothing

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10
Q

What effect does the + isomer have on plane polarised light?

A

Rotates plane of polarisation by x0 clockwise

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11
Q

What effect does the - isomer have on plane

polarised light?

A

Rotates plane of polarisation by x0 anti clockwise

same angle, opposite direction

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12
Q

What is the structure of a polarimeter?

A

Light source (unpolarised light) → polarising filter (polarised light) → polarised light passes through compartment containing sample → detector determines the angle of rotation of the plane polarised light

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13
Q

What are polarimeters used for?

A

To identify which enantiomer is present, the purity of the sample, the concentration of the sample etc.

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14
Q

What is the first stage of the synthesis of lactic acid(2-hydroxypropanoic acid) from ethanal? (Equation).

A

(Reagents are KCN and HCl but it is acceptable
to write HCN in the balanced equation as this is
the H+ from the HCl and -CN from KCN)
CH3CHO + HCN → CH3CH(OH)CN

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15
Q

Why is the CH3CH(OH)CN molecule formed chiral?

A

H, CH3, OH and CN groups attached to the central chiral carbon atom - 4 substituents

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16
Q

What is the second stage of the synthesis of lactic

acid (2-hydroxypropanoic acid) from ethanal?

A

Hydrolysis: CH3CH(OH)CN + HCl +2H2O →

CH3CH(OH)COOH + NH4Cl

17
Q

How does the second stage of the synthesis of lactic acid affect the chirality?

A

Does not affect it - still racemic as chirality not

affected by this stage.

18
Q

Are racemic mixtures formed in nature? Why?

A

Not often, as enzyme mechanisms are 3D so only form one enantiomer

19
Q

Why is optical isomerism a problem for the drug industry?

A

Sometimes, only one enantiomer is effective due to enzyme’s active site/cell receptors being 3D.

20
Q

What are the options to resolve the issue of only one

enantiomer being effective?

A
  1. Separate enantiomers - difficult and expensive as have very similar properties
  2. Sell racemate - wasteful as half is inactive
  3. Design alternative synthesis to only produce one enantiomer.
21
Q

Examples of optically active drugs?

A

Ibuprofen, Thalidomide

22
Q

Why is ibuprofen able to be sold as a racemate,
even though the + isomer is needed to treat
inflammation?

A

Sold as 50% racemate.

But body converts 60% of R- isomer to S+ isomer → end up with 80% S+ isomer