CHEM:341 Organic Chemistry I Week 6 Notes
CHEM:341 Organic Chemistry I Week 6 Notes CHEM341
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This 8 page Class Notes was uploaded by Raffasarru on Saturday October 1, 2016. The Class Notes belongs to CHEM341 at Colorado State University taught by Anna Elizabeth Allen in Fall 2016. Since its upload, it has received 5 views.
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Date Created: 10/01/16
CHEM:341 Modern Organic Chemistry I Notes Week 6 o Nomenclature of Chiral Molecules Pair of enantiomers are two different compounds so we need to distinguish them by name New Convention: enantiomers designated either R or S to indicate configuration of chiral center Find stereogenic carbon Assign priorities to all atoms directly bonded to chiral carbon (highest atom number gets highest priority) If two atoms are same, assign priorities to all atoms bonded to these atoms (one atom of higher atomic number determines priority) Orient molecule so lowest priority group is pointed toward back Draw curve from highest priority to lowest priority o Clockwise rotation (going right) is R o Counterclockwise (going left) is S May need to rotate carbons to get fourth priority to the back of the molecule 1 Note: Alkynes take precedence over alkenes; alkenes take precedence over alkanes as a general rule (only if higher atomic number isn’t involved) o How to Recognize and Draw Enantiomers Remember two things: Non-superimposible mirror images Have opposite R/S designations o Two ways to draw an enantiomer of compound Draw mirror image Switch two groups on chiral carbon Do either one or the other, not both o Molecules with more than One Chiral Carbon Molecules with one chiral carbon exist as a pair of enantiomers (relationship between two molecules) Two chiral carbons? Multiple isomers 2 o Enantiomers: SS v. RR and SR v. RS (basically the S/R switches for the other enantiomer Maximum stereoisomers (most of the time): never be more than 2 where ???? is the number of stereogenic carbons Diasetereomers: stereoisomers that are not mirror images of each other (also alkenes that are Z and E) Naming molecules with more than one chiral carbons Add carbon number to R or S Example: (2R,3R)-2-bromo-3-chloropentane Summary: o Identical compounds Can all be superimposed All same configurations (R/S) o Enantiomers Non-superimposable mirror image All configurations opposite (R/S) o Diastereomers Stereoisomers that are not mirror images (not superimposable) At least one R/S the same and at least one R/S different 3 E/Z o Compounds with Two Chiral Carbons: Meso Compounds Meso compound: achiral molecule that contains stereogenic carbons Achiral molecule that has chiral diastereomers Don’t have an enantiomer For every meso we can draw, we lose one stereoisomer Identifying Meso compounds: Must have one R and one S Must have plane of symmetry o Physical Properties of Chiral Molecules Pair of enantiomers have identical physical and chemical properties (boiling point, melting point, solubility, etc.) in the absence of chiral environment (when they aren’t interacting with something else that is chiral) The only exception is the way they interact with polarized light 4 Chiral compounds are optically active: they rotate with polarized light Two enantiomers rotate polarized light an equal amount but in opposite directions Rotation of light clockwise (to the right) is detroroatory, designated by (+) or d Rotation of light counterclockwise (to the left) is levorotatory, designated by (-) or l Do not confuse: experimental optical rotation is not related to structural configuration (R,S) Sample of compound composed of single enantiomer is enantiomerically pure (rotate light) Only enantiomers have equal and opposite optical rotation, all others, we don’t know without experimentation Sample of compound composed of equal mixtures of both enantiomers (50%/50%) is called a racemate or racemic mixture and has no observed rotation (see no rotation, rotation cancels) No achiral molecules rotate light o Importance of Chiral Compounds in Biological Systems Human body is filled with chiral molecules (proteins, carbohydrates, etc.) Enantiomers of chiral molecules will interact differently with these molecules Ibuprofen: pain reliever, sold as racemate S is active, R is inactive Thalidomide: 1950’s made as racemic mixture 5 S: teratogen in humans (birth-defects), R: effective sedative/anti- nausea for morning sickness Carvone: S smells like caraway, R sounds like spearmint Chapter 5: Rings Consider cyclopropane: o o 6 eclipsing interactions (3 per side at 1 ???????????????? each) = 6 ???????????????? o Total strain: 26.4 ???????????????? o Strain comes from: Ideal bond: 109.5° for ???????? (which is what each ???? is) Actual bond angle: 60° Angle Strain: increase in energy caused by deviation of bond angles from their optimum angle 6 Cycloalkanes o Cycloalkanes with more than three carbons aren’t flat Puckering: relieves eclipsing interactions (reduce torsional strain) reduces internal bond angles (reduces angle strain in 6-membered rings or greater) Cyclohexane Rings o If flat: Angles are 120°, so there’s angle strain o With puckering: Angles are then about 109.5° so there’s no angle strain Also no eclipsing hydrogens so no torsional strain Called the chair Lowest energy cycloalkane 6 axial hydrogens (either straight above or below molecule): 3 axial up, 3 axial down 7 6 equatorial hydrogens (slightly above or below molecule): 3 equatorial up, 3 equatorial down On the carbons with axial up, the other hydrogen will be equatorial down, and vice versa Wedges up, dashes down o Like acyclic alkanes, cyclohexane rings do not remain in single conformation o Ring flip: o Ring flip mechanism: o All equatorial become axial and all axial become equatorial 8
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