ORGANIC CHEMISTRY有机化学英文影印版第八版PPT

Chapter 5Stereochemistry: Chiral Molecules

ORGANIC CHEMISTRY有机化学英文影印版第八版PPT

Examples of Constitutional Isomers

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Examples of Stereoisomers

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Subdivision of Isomers

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Enantiomers and Chiral Molecules Enantiomers: stereomers that are nonsuperposable mirror images of each other

Chiral Molecule: a molecule that is not superposable on its mirror image. Chiral molecules have handedness and are capable of existing as a pair of enantiomers Achiral molecule: a molecule that is superposable on its mirror image Chirality: the property of having handedness

Hands, ears, shoes

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Examples of Enantiomersstereocenter carbon has 4 non-equivalent groups attachedHandedness center Handedness carbon

Its mirror image is non-superposable

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Tests for Chirality In Molecules Has no internal plane of symmetry Has at least one stereocenter carbon

Its mirror image is non-superposable (not identical) Achiral molecule: the molecule can be superposable on its mirror image (it has planar symmetry, at least two identical groups may attached to the center carbon)

2-butanol Chiral molecule

Achiral molecule Symmetric plane

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unsymmetrical operation — compare molecular geometry Interchange any two groups of the stereocenter carbon, will produce its enantiomer (unsymmetrical operation) exchange

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Examples of Achiral Molecules2-Propanol

All molecules with a plane of symmetry are achiral.

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Requirements For Enantiomeric Pairs Non-superimposable mirror images of one another (exhibit chirality) Must have only one stereocenter carbon Must not have a plane of symmetry The two rotate plane polarized light in opposite directions but to the same degree (optically active) Two will have opposite configurations

ORGANIC CHEMISTRY有机化学英文影印版第八版PPT

Biological Importance of Chirality Amino Acids-19/20 are chiral, all left handed Carbohydrates are chiral and right handed DNA is chiral and right handed Seashells coil always right handed Honeysuckle winds left handed Bindweed winds right handed

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Physiological Differences Between Enantiomeric Pairs Limonene– One enantiomer(R) = odor of oranges – One enantiomer(S) = odor of Lemons

Carvone– One enantiomer = odor of caroway seeds – One enantiomer = odor of spearmint Thalidomide– One enantiomer (R) cures morning sickness – One enantiomer (S) causes birth defects

Conversion of one enantiomer in vivo is slow but certain

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Nomenclature of Enantiomers: — the (R-S) system Locate stereocenter carbon Establish Priority of each atom directly bonded to stereocenter using Cahn-Ingold-Prelog rules Project atom of lowest priority to back of molecule

Trace through other three atoms beginning with highest to lowest If trace rotates clockwise (to the right) then configuration is “R” ; If trace rotates counter clockwise (left) then configuration is “S” For 3-D structures if lowest priority is in front, determine configuration as above and then reverse it.

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Cahn-Ingold-Pre

log Priority Rules Atoms directly bonded to stereocenter will have priority based on atomic number If two atoms have same atomic number (isotopes) then mass number decides If two atoms are same (in A.N. and mass number) then atoms directly attached to them will decide by totaling atomic number of all atoms For atoms with multiple bonding multiply atom with single bonds:

-CH=CH2 is higher Than —CH(CH3)2 Assigning Configuration of 3-D formulas Determining whether two molecules are identical or entiomers (Problems 5.13 and 5.14)

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Example of nomenclature by R-S System

(R)

? Do the problem 5.9, p197 ? Do the problem 5.12(c), p198

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Compare between two compounds — sample problemApproach 3: Naming by R-S

identical ?

Approach 1: Hold the molecule by one group, rotate the other groups until at least one group is in the same place

Approach 2: invert the configuration by exchange of two groups