Unformatted text preview: jections, 1. Compare the two structures. 2. Make successive changes unt il the formulas are ident ical. 3. If an odd number of changes are made, the two original formulas are enantio mers. If an even number of changes are made, the two original formulas are ident ical. For example: (X) (Y) Are these structures enantiomers or the same? int rch
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CH an Cl int
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CH and H (Y) (X) · Since two changes were made to convert Y into X, the two structures are the same. Examples: 1. Determine if the structures shown below are enantiomers or ident ical. (A) (B ) 8 Chemistry 52 Chapter 26 RACEMIC MIXTURES
· A mixture containing equal amounts of a pair of enant io mers is known as a racemic mixture.
· Such a mixture is optically inactive and shows no rotation of polarized light.
· Each enant iomer rotates the plane o f polarized light by the same amount, but in opposite direct ions. Thus, the rotation by each isomer is canceled.
· Many pharmaceut ical co mpounds are prepared as racemic mixtures, since organic synthesis react ion are often not stereospecific. Unfortunately, often only one enant io mer of these mixtures has bio logical act ivit y because many bio logical mo lecules are stereospecific.
· Furthermore, the inactive enantio mer may cause adverse affects. The most famous example of such effect is the racemic mixt ure of the drug thalido mide. While one iso mer is effective at suppressing immune responses, the other iso mer causes birth defects. Thalidomide 9 Chemistry 52 Chapter 26 DIASTEREOMERS & MESO COMPOUNDS
· The number of stereoiso mers increases as the number of chiral atoms in a mo lecule increases. The maximum number of stereoiso mers for a given co mpound is given by n
the formula 2 , where n is the number of chiral carbons in a mo lecule. n
2 = Maximum number of stereoisomers for a given chiral compound. n = Number of chiral carbon atoms in a molecule. · As an example, 2bromo3chlorobutane has 2 chiral carbons and therefore four possible stereoiso mers (structures IIV shown belo w). CH3 H CH3 CH3 CH3 Br Br H H Br Br H H Cl Cl H Cl H H Cl CH3 CH3 (I) CH3 CH3 (II) (III) (IV) enantiomers enantiomers · Note that iso mes I and II are enant io mers and iso mers III and IV are also enant io mers. All four compounds are optically active, however properties of I and II differ from properties of III and IV, because they are not mirror images of each another.
· Stereoiso mers that are not enantiomers (mirror images of each other) are called diastereomers. Examples: 1. How many stereoiso mers can exist for the compound shown below? Draw their structures and label any pairs of enant io mers and diastereomers. CH3 H Br H Br CH2CH3 10 Chemistry 52 Chapter 26 DIASTEREOMERS & MESO COMPOUNDS
· Tartaric acid has two chiral carbons and can therefore have four stereoiso mers shown below. COOH HO COOH H H H COOH COOH OH OH HO H H OH HO H H OH HO H COOH COOH COOH (A) (B) (C) COOH (D) · Isomers A and B are nonsuperimposable mirror images, and are therefore enantiomers. Close examinat ion of isomers C and D indicates that they are superimposable, and therefore are the same co mpound.
· Stereoisomers that contain chiral carbons and are superimposable on their mirror images are called meso comounds. Due to the plane of symmetry of these mo lecules, the rotation of light in one direct ion by half of the mo lecule is cancelled by rotation in the opposite direct ion by the other half o f the mo lecule. Therefore, all meso compounds are optically inact ive. COOH H OH H Plane of symmetry OH COOH Examples: 1. How many stereoiso mers can exist for the compound shown below? Draw their structures and label any pairs of enant io mers and meso compounds. CH2Br Cl H H Cl CH2Br 11 Chemistry 52 Chapter 26 SUMMARY OF ISOMERISM different structures Structural Isomers not interconvertible by bond rotation Geometric Isomers (cis/trans) Isomers same structures contain chiral carbon nonsuperimposable mirror image Stereoisomers contain chiral carbon superimposable mirror image 12 Enantiomers Diastereomers...
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