Chemistry - 3.3.7: Optical Isomerism

What shape do you draw enantiomers as?

Tetrahedral

Enantiomers

A pair of optical isomers

How can you measure optical rotation?

A polarimeter

How does a polarimeter work? (2)

Polarised light passed through two solutions of the same concentration, each containing a different optical isomer of the same substance - one rotates it clockwise [(+)-isomer], the other, anticlockwise [(-)-isomer]

Give another name for a racemate

Racemic mixture

How does an enzyme bind to only one enantiomer?

The enzyme has an active site that is complementary to only that enantiomer.

What is optical isomerism a type of?

Steroisomerism

Name the two types of stereoisomerism

Optical isomerism and E-Z isomerism

When does optical isomerism occur?

When there are 4 different substituents bonded to one carbon atom, producing two isomers that are non-superimposable mirror images of one another

Optical isomers

Pairs of molecules that are non-superimposable mirror images of each other

What does a + isomer do?

Rotate plane-polarised light clockwise

What does a - isomer do?

Rotate plane-polarised light anticlockwise

How do you indicate the chiral centre?

An *

Racemate

An equal mixture of two enantiomers. It is optically inactive.

Give an example of a reaction where a racemate is produced

The nucleophilic addition of -CN to aldehydes/ketones

What is a problem associated with the use of optical isomers as drugs? How do you overcome this? (3)

Both are complementary to the enzyme's active site, but only one isomer may be an effective drug, while the other is inactive.

Can overcome by separating the two isomers (difficult, expensive), selling mixture as a drug (wasteful, half inactive) or design alternative synthesis of drug so only required isomer is produced

Chiral centre

An atom (the carbon atom) to which 4 different atoms or groups are bonded, causing the parent molecule to exist as a pair of non-superimposable mirror images.

Why might a racemic mixture form?

The x group / carbocation is planar, so there is an equal chance of attack from either side, above or below.

Why is a racemic mixture not optically active?

The effects of the two isomers cancel out