Chapter 17

Stereoisomerism

©2005 Mark S. Davis

Goals• Distinguish between Constitutional and

Stereoisomers

• Distinguish between Chiral and Achiral molecules

• Identify tetrahedral stereocenters

• Describe optical activity of chiral compounds

• Draw Enantiomers

©2005 Mark S. Davis

Biological Importance

• Biological systems can only use one stereoisomer

• Perception in light, Box 12.2

• Insect pheromones, Box 12.3

©2005 Mark S. Davis

Review of Isomerism

• Constitutional isomers (Structural Isomers)

– Same formula– Different connectivity

1.

2. Different

3. Different

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Stereoisomers• Same connectivity, different

• Enantiomers–

• Diastereomers– – cis- and trans- isomers

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Enantiomers• Must meet two requirements

1. The molecules are

2. They are not

• If the molecule meets the two conditions, it is said to be

– Hand,

• This is called

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Ultimately

• A pair of enantiomers is possible only when a compound contains a

©2005 Mark S. Davis

Ways to draw• Wedge-bond

• Fischer projection - rules– Horizontal =

– Vertical =

©2005 Mark S. Davis

Naming Enantiomers• Use prefix, either

– These are confirmed with x-ray crystallography

• RULES – must be followed1. The tetrahedral stereocenter must have

1.

2. A

3.

©2005 Mark S. Davis

Naming Continued2. Fischer projections with

1. R groups pointed

2. R1, the most substituted Carbon (least # of H) is pointed

3. R2 is pointed

3. The D form has the heteroatom on the

4. The L form has the heteroatom on the

©2005 Mark S. Davis

Properties of Enantiomers

• Chiral compounds are different than achiral compounds in – –

• A pair of enantiomers have the same physical properties, mp, bp, solubility

• They differ in

©2005 Mark S. Davis

Optical Activity• Ability to rotate

• Use a polarimeter and a solution of the desired enantiomer

• Light is either rotated

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Optical Activity

• Note that designations are based on Fischer drawings and NOT on the rotation of light.

• Some

• A mixture of equal concentrations of

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Chiral Recognition

• Enantiomers have the same reactivity with

• Enantiomers have large

– See figure 17.12, p. 509– This is called

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Chirality in Organisms

• Humans and most organisms use and synthesize only

• Reactions in living cells produce

• Reactions in laboratory produce a

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Chirality

• Different enantiomers can produce different effects in the body. See box 17.1 page 511

• Box 17.2 page 512 – Dopamine, L-Dopa and D-Dopa

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Review…• Enantiomers are

• The only difference in physical properties is

• No reactivity difference to achiral compounds

• Large differences in reactivity to chiral compounds or chiral enzymes

©2005 Mark S. Davis

Two or more Stereocenters• Increases the number of stereoisomers

• If there are

• D-Glucose has

• Starch (many glucose connected) has hundreds of stereocenters

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Consider…

• 2,3-pentanediol–

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Nutrasweet (Aspartame)

• Two stereocenters,

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Consider…• 2,3-butanediol

–

©2005 Mark S. Davis

Cyclic stereoisomers• Cyclic compounds can be enantiomers

– Carvone from box 17.1 p 511

• A ring carbon is a tetrahedral stereocenter if:–