pmt optical isomerism 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* (star on C)

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7
Q

Give two examples of chiral molecules.

A

All alpha amino acids, except glycine.

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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 xo 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

Are racemic mixtures formed in nature? Why?

A

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

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15
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.

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16
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.
17
Q

Examples of optically active drugs?

A

Ibuprofen, Thalidomide

18
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