Chapter 06 Chirality: The Handedness of Molecules

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1 tereoisomers A A B D D tereoisomers: Isomers (different compounds) that have all the same number and kind of atoms that are all connected the same, but differ in their arrangement in three dimensions. hapter 06 hirality: The andedness of Molecules B 2 IME ymmetric Molecules IME onstitutional tereoisomers (bonded differently) (nly different in 3-dimensional arrangement) Enantiomers Diastereomers (mirror images) (not mirror images) onfigurational (same number and kind of atoms, but are different molecules) cis/trans 3 4 hirality hirality - tereoisomers 5 Enantiomers - molecules that are not superimposable on their mirror image. Molecules that can exist as s are called hiral Molecules. Molecules that have a plane of symmetry are not (a). tereogenic arbons are those that have four different groups attached. 6

2 ymmetry - Assymmetry Biomolecules are hiral arbohydrates / ugars DA Amino Acids / Proteins Glucose hirality in mell and Taste Mirror image molecules (s) interact with receptors in our nose differently. ()-(-)-arvone spearmint oil ()-()-arvone caraway seed oil ()-()-Limonene orange citrus hiral Molecules in Enzymes ()-(-)-Limonene turpentine/lemon Me 9 10 The ase of Thalidomide stereogenic carbon ()-thalidomide beneficial sedative ()-thalidomide teratogen causes birth defects ptical Activity 11 Enantiomers have identical physical properties mp, bp, spectroscopic, absorptions, etc. BUT they are different when interacting with plane-polarized light hristiaan uygens - ( ) Dutch astronomer, mathematician, and physicist, discovers plane polarized light 12

3 ptical Activity Light is polarized and oscillates in a single dimension arl arl Wilhelm cheele ( ) In 1815, he notes that certain when the polarized light passes through a, it is bent out of the original plane I ote that the plane will bend thepolarized light in the opposite direction light h, ho drin pharma to me. B my care discover natural organic compounds (liquids or solutions) rotate Inplane 1769polarized he discovers Tartaric Acid light (ptical Activity). In 1815 he discovers that certain natural organic compounds rotate plane polarized light. light travels in waves but oscillates in all three dimensions along the path of travel istorica Jean Baptiste Biot ( ) Jean-Baptiste Biot ( ) molecule e molecul e cule mole molecule cul moleculemole from tarter - the potassium salt of tartaric acid that crystallizes out of wine. UT Tartaric Acid istorical perspective tube containing a liquid organic compound or solution In 1769, he discovers T Acid from tartar (the p salt of tartaric acid, de on barrels and corks du fermentation of grape j plane polarized light 7 polarizing filter Tartaric Acid Louis Pasteur - ( ) In1849 he separated mirror image crystals of sodium ammonium tartrate. a 4 Louis Pasteur I found...that the tartrate was hemihedral..., but strange to say the paratartrate was hemihedral also. nly the hemihedral faces which in the tartrate were all turned the same way, were, in the paratartrates inclined sometimes to the right and sometimes to the left... I carefully separated the crystals that were hemihedral to the right from those hemihedral to the left, and examined their solutions separately in the polarizing apparatus. I then saw with no less surprise than pleasure that the crystals hemihedral to the right deviated the plane of polarization to the right, and that those hemihedral to the left deviated it to the left; and when I took an equal weight of each of the two kinds of crystals, the mixed solution was indifferent towards the light in consequence of the neutralization of the two equal and opposite individual deviations Amount of rotation depends on how many molecules the light interacts with. Light at 589 nm (odium D line) acemates...when I took an equal weight of each of the two kinds of crystals, the mixed solution was indifferent towards the light in consequence of the neutralization of the two equal and opposite individual deviations. tandardized for unit cell and concentration [α]d = ptical Activity observed rotation ( ) cell length (dm) x conc. (g/ml) acemic Mixture - 50/50 mixture of s. hows optical activity. light travels in waves but oscillates in all three dimensions along the path of travel Light is polarized and oscillates in a single dimension when the polarized light passes through a, it is bent out of the original plane ote that the will bend the light in the opposite direction polarizing filter 17 18

4 IP ules for & onfiguration & onfiguration ectus - ight, inister - Left 1 l 4 4 l 1 2 sphingosine 2 3 ()-2-chlorobutane 3 2 ()-2-chlorobutane & onfiguration & onfiguration 2 3 alanine proline glucose Diastereomers Diastereomers - stereoisomers that are not mirror images TEEIME cis/trans Diastereomers Although not optically active (not ), cis/ trans isomers are a form of diastereomer. They differ only in their arrangement in 3-dimensions but are not mirror images. DIATEEME EATIME 23 24

5 configurational Diastereomers EATIME ME compounds Meso ompounds are molecules that contain stereogenic carbons, but are not. Their mirror images are identical. This occurs when there is a plane of symmetry in the molecule. DIATEEME DIATEEME 25 IDETIAL 26 Fischer Projections Fischer Projections Diasteromers IME Diastereomers have different physical properties molecule config optical rotation (,) 12 (,) -12 mp density IME (same number and kind of atoms, but are different molecules) onstitutional (bonded differently) Enantiomers (mirror images) tereoisomers (nly different in 3-dimensional arrangement) Diastereomers (not mirror images) (,) meso onfigurational cis/trans 29 30

6 eactions can produce stereogenic centers hirality in eactions acemic mixtures will always result unless there is other ity to influence a difference in the reaction path. 2 50: hirality in eactions hirality in eactions cis-2-butene 2 s 2 identical trans-2-butene ME Diastereoselectivity An existing stereogenic center can influence the creation of another. esolution of Enantiomers The separation of s from a racemic mixture is called a resolution Major Product 3 Major Product 3 3 a

7 hemical esolution Enantiomers have identical physical properties, so we need to prepare diastereomers. hemical esolution We can also make covalent bonds. racemic mixture 2 one diastereomeric salts acemic Mixture l one Ph Ph diastereomeric products Indication of mixed stereochemistry 2 acemic Mixture 39