5.7 Fischer Projections Fischer projections can also be used to represent molecules with chirality centers Horizontal lines represent attachments coming out of the page Vertical lines represent attachments going back into the page Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 5-1

5.7 Fischer Projections Fischer projections can be used to quickly draw molecules with multiple chirality centers Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 5-2

5.7 Fischer Projections Fischer projections can also be used to quickly assess stereoisomeric relationships Practice with SkillBuilder 5.9 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 5-3

5.8 Interconverting Enantiomers Molecules can rotate around single bonds. Recall the gauche rotational conformation of butane Is the gauche conformation of butane chiral? Draw its mirror image. Is it superimposable on its mirror image? Why is butane’s optical rotation equal to zero? To be chiral, a compound cannot be a rotational conformer of its mirror image Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 5-4

5.8 Interconverting Enantiomers Compare the (cis)-1,2-dimethylcyclohexane chair with the Haworth projection The Haworth image can be used to quickly identify the compound as an achiral meso compound. However, a plane of symmetry can NOT be found in the chair conformation Which conformation better represents the molecule’s actual structure? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 5-5

5.8 Interconverting Enantiomers Should the (cis)-1,2-dimethylcyclohexane chair conformation be chiral or achiral? – It is not superimposable on its chair mirror image – Is has two chirality centers and no plane of symmetry Recall that chairs can flip their conformation by rotating single bonds and interconvert readily at room temperature If the chair could interconvert with its mirror image, would it be chiral? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 5-6

5.8 Interconverting Enantiomers The freely interconverting mirror images cancel out their optical rotation, so it is achiral This analysis is much easier to do with a handheld models than in your mind If the Haworth image has a mirror plane, then the chair will be able to interconvert with its enantiomer, and it will be achiral. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 5-7

5.9 Resolution of Enantiomers Most methods of separating compounds from one another take advantages of the compounds’ different physical properties Distillation – separates compounds with different boiling points Recrystallization – separates compounds with different solubilities Can you think of more methods of separation or purification? Such methods often don’t work to separate one enantiomer from its partner. WHY? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 5-8

5.9 Resolution of Enantiomers In 1847, Pasteur performed the first resolution of enantiomers from a racemic mixture of tartaric acid salts The different enantiomers formed different shaped crystals that were separated by hand using tweezers This method doesn’t work for most pairs of enantiomers. WHY? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 5-9

5.9 Resolution of Enantiomers Another method is to use a chiral resolving agent The differing physical properties of diastereomers allow them to be more easily separated Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 5-10

5.9 Resolution of Enantiomers Affinity chromatography is often used to separate compounds For example, a glass column or tube can be packed with particles of a polar solid substance If a mixture of two compounds with different polarities are passed through the column, what will happen? A pair of enantiomers will not have different polarities, so how might such chromatography be modified to resolve the enantiomers in a racemic mixture? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 5-11