Slide 1

Formulae

Students sometimes use incorrect ways to represent organic compounds on exams.For example, 1-bromopropane cant be written as:

On exams these structures will be strictly interpreted according to chemical notation despite your intention.

Im fairly certain that anyone who writes C-C-C-Br does in fact mean 1-bromopropane BUT you have to use the conventions. You are welcome to use pencil on exams, so if youve drawn a bond to nowhere, make sure you erase it afterward. 1Chapter 5Stereochemistry

5.1 ChiralityChiral objects-lack a symmetry planearent superposable on their mirror imagesChiral means handedness

5.1 Chiral ObjectsChinese wisteria twines clockwise

Japanese wisteria twines counterclockwise

5.2 Stereoisomers

same molecular formulasame connectivitydifferent spatial arrangementdifferent connectivitystereoisomersthat arent enantiomersnonsuperimposablemirror imagescis/trans or E/Z55.3 Molecular ChiralityThe most common source of chirality is a C with 4 different groups.Are the following molecules chiral?

5.3 Molecular ChiralityWhich of the following is chiral?

Look for a chirality center or a symmetry plane.5.6 Symmetry PlanesSymmetry planes bisect a molecule into two halves that are mirror images.A molecule isnt chiral if it has a plane of symmetry.

4 identicalgroups3 identicalgroups

2 groupsidenticalno groupsidenticalno symmetryplane

W85.7 How to Draw EnantiomersTwo ways, use one or the other but not both:1.Invert all wedges and dashes.A.Draw the structures mirror image.

If you perform both (1) and (A), youll draw the same compound. 5.7 Enantiomer NomenclatureIUPAC rules use R and S to designate chirality centers.The four groups on C are assigned priorities (a d) based on atomic number.With the lowest priority group (d) oriented away from the viewer the higher priority groups a c are connected in a circle. R = right, S = left

SR

W105.7 When d is not in the backIf the d group is in the front (wedged bond), connect a c and reverse the assignment.

appears Ranswer = S

The orientation of the d group away from the viewer is crucial.115.7 When d is not in the backSwap positions of d and the group in the back and then reverse the assignment.

SR

I used a dotted arrow because this is not a reaction. I use an asterisk to remind myself that Ive drawn the enantiomer of the compound Im actually interested in.125.7 When d is not in the backRotate the molecule to put the d group in the back.This draws the same compound.

RR135.7 Assigning PriorityHigher atomic number = higher priority

The lowest priority atom must face awayNow connect groups a - cS(S)-bromochlorofluoroiodomethane

abcdW145.7 Assigning PriorityIf 2+ groups have the same atom, look at the next set of atoms until a point of difference is found.

dS

CBr, Br, BrbCI, H, HaCH, H, Hc

(S)-1,1,1-tribromo-3-iodo-2-methylpropane155.7 Assigning PriorityGroups with multiple bonds are treated as if both atoms were doubled or tripled.

double bondtriple bondW16a5.7 Assigning PriorityGroups with multiple bonds are treated as if both atoms were doubled or tripled.

bcd

R(R)-2,3-dimethylpent-1-en-4-yne

175.7 Menthol Provide the full IUPAC name of menthol (oil of peppermint) including R/S designations.

(1R,2S,5R)-2-isopropyl-5-methylcyclohexanol

5.8 Optical RotationEnantiomers have all identical physical properties (bp, mp, solubility, density, etc.) except optical activity.Optical activity the ability to rotate plane-polarized light

[a] = +61[a] = -61195.8A Plane-Polarized LightLight emanates in all directions from its source. Polarizing filters remove all but parallel sheets of light.Plane-polarized light is a real phenomenon. Polarized sunglasses and camera filters are every day examples.

planepolarization

without polarizing filter with polarizing filter W205.8B PolarimeterPolarimeters measure optical activity.

+ + polarimeter blank sample +W215.8C Specific RotationOne enantiomer rotates plane-polarized light to the left (-), the other to the right (+), and the magnitude of [a] is the same.R/S and +/- dont correlate.

(R)-(-)-bromobutane [a] = 23.1

(S)-(+)-bromobutane [a] = +23.1

(R)-(+)-1-phenylethanol[a] = +40.0

(S)-(-)-1-phenylethanol[a] = -40.0 W225.8C Specific Rotation[a] =specific rotation (units: degreesmL/dmg, ssdsdbut everyone just uses degrees)a =observed rotation (from polarimeter)c =concentration in g/mLl = length of sample tube (dm)

This equation will be provided to you on exams.W235.9A Racemic MixturesA racemic mixture contains equal amounts of both enantiomers and has no optical activity.

racemic alanine[a] = 0W245.9B Enantiomeric Excess (%ee)%ee is the purity of a mixture of enantiomers. Its calculated based on moles or [a]:

Also %ee = %R - %S (or vice versa)A racemic mixture has a %ee of zero.

W2510 g of optically pure fructose were diluted to 500 mL with water and placed in a polarimeter tube 20 cm long. If the measured rotation was -5.20, calculate the specific rotation of fructose.If this solution were mixed with 5 g of racemic fructose, what would be the specific rotation of the resulting mixture?

5.9B Enantiomeric Excess (%ee)

-5.20 (10 g/500 mL)(2 dm)

[a] = -130 % ee = mass enantiomer 1 mass enantiomer 2 (100%) mass both enantiomers% ee = 12.5 2.5 (100%) = 67% 15

67% = [a] mixture (100%) [a] mixture = -87 -130W265.12 >1 Chirality CenterA molecule with n chirality centers has a maximum of 2n stereoisomers.There may be fewer than 2n.

23 = 8 stereoisomers max.RSR (pictured)RRRRSSRRSSSSSRSSSRSRRW275.12 >1 Chirality CenterHow many stereoisomers does glucose have?

25 = 32 stereoisomersAll 32 are known.Allose, altrose, galactose, galose, glucose, idose, mannose, talose (8); D, L (2); alpha, beta anomers (2)285.12A Meso CompoundsMeso compounds achiral molecules with Cs with 4 different groupshave an internal symmetry planedont have enantiomers

achiral[a] = 0W295.12A Meso Compounds

superimposablemirror imagesSuperimposable mirror images = the same compoundNonsuperimposable mirror images = enantiomersW305.13 Fischer Projections Represent chirality centers without wedges and dashes.Useful for linear molecules with 2 chirality centers.Commonly used for sugars (Chapter 22).

D-glucoseW315.13 Fischer Projections Structures are drawn vertically.Horizontal bonds are up and all eclipsed.

W325.12 DiastereomersDiastereomers are stereoisomers that arent enantiomers.A pair of enantiomers have the same physical properties (except optical rotation).Diastereomers have all different physical properties.

melting pointspecific rotationsolubility(2R,3R)-(+)-tartaric acid171+11.98139(2S,3S)-()-tartaric acid17111.98139(2R,3S)-tartaric acid146012533Enantiomers all chirality centers opposite Diastereomers 1 CC same, 1 CC opposite

RSR (pictured)RRRRSSRRSSSSSRSSSRSRR5.12 DiastereomersRSRsameRRRdiastereomerRSSdiastereomerRRSdiastereomerSSSdiastereomerSRSSSRdiastereomerSRRdiastereomerenantiomerRSRW345.13 Fischer Projections

Fischer projections can be rotated in the plane of the paper, but only by certain angles.180 rotation same compound90 rotation enantiomer

SRSRW355.14 Cyclic MoleculesIn general, cyclic molecules with one group are achiral.

achiral achiral chiral

W365.14 Cyclic MoleculesFor cyclic molecules with 2+ groups:If the groups are the same, there will be at least one meso compound.There may be more than one meso compound; look for symmetry planes

How many stereoisomers?W375.14 Cyclic MoleculesHow many stereoisomers does 1,2-dichlorocyclopropane have?

trans isomerschiralcis isomerachiralW385.18 Atropisomers

Skip.

W39ReviewWhat is the relationship between each of these pairs?

same compound

same compound

diastereomers

same compound(2S,3R)-2,3-dibromopentaneand(2R,3S)-2,3-dibromopentaneenantiomersReview

What is the relationship between: A B

C DA and BenantiomersA and CdiastereomersA and DdiastereomersB and CdiastereomersB and DdiastereomersC and Dsame compound

W41