Polarimeter

1. Polarimeter before after Concentration: pure liquid in g/mL; solution in g per 100 mL of solvent

2. Optical Activity Optically Active compounds rotate plane polarized light. Chiral compounds (compounds not superimposable on their mirror objects) are expected to be optically active. Optically Inactive compounds do not rotate plane polarized light. Achiral compounds are optically inactive.

3. Problems… If the specific rotation of pure R 2-bromobutane is 48 degrees what is the specific rotation of the pure S enantiomer? The pure S enantiomer has a specific rotation of -48 degrees. Equal but opposite!!

4. Mixtures of Enantiomers These are high school mixture problems. If you know the specific rotation of the pure enantiomers and the composition of a mixture then the specific rotation of the mixture may be predicted. And conversely the specific rotation of the mixture may be used to calculate the composition of the mixture. Specific rotation of mixture = (fraction which is R)(specific rotation of R) + (fraction which is S)(specific rotation of S)

5. Example Mixture of 30% R and 70% S enantiomer. The pure R enantiomer has a specific rotation of -40 degrees. What is the specific rotation of the mixture? Contribution from R Contribution from S

6. Using the specific rotation to obtain the composition of the mixture. For the same two enantiomers ([a] of R = -40) , suppose the specific rotation of a mixture is 8. degrees what is the composition? Specific rotation of mixture = (fraction which is R)( specific rotation of R) + (fraction which is S)( specific rotation of S) -40. 8. + (1. – fraction which is R) 40. Fraction which is R = 40%; fraction which is S is 60%.

7. Racemic Mixtures, Racemates The racemic mixture (racemate) is a 50:50 mixture of the two enantiomers. The specific rotation is zero. The racemic mixture may have different physical properties (m.p., b.p., etc.) than the enantiomers.

8. Optical Purity, Enantiomeric Excess Consider a mixture which is 80% R (and 20% S). Assume the specific rotation of the pure R enantiomer is 50 degrees. As before Specific rotation of mix = 0.80 x 50. + .20 x (-50.) = 30. R R R R R R R R S S Now, recall that a racemic mixture is 50% R and 50% S. Mixture is 60% R and 40% racemic. Specific rotation of mix = 0.60 x 50. + .40 x (0.) = 30. The optical purity (or enantiomeric excess) is 60%.

9. Fischer Projection Cl to Ethyl to Methyl Reposition to Look from this point of view. Standard Fischer projection orientation: vertical bonds recede horizontal bonds come forward H,low priority substituent, is closer so CCW is R. R and S designations may be assigned in Fischer Projection diagrams. Frequently there is an H horizontal making R CCW and S CW. Standard short notation:

10. Manipulating Fischer Projections Even number of swaps yields same structure; odd number yields enantiomer. 1 swap or R or Etc. S All of these represent the same structure, the enantiomer (different views)!!

11. Manipulating Fischer Projections Even number of swaps yields same structure; odd number yields enantiomer. 2 swaps or R or Etc. R All of these represent the same structure, the original (different views)!!

12. Rotation of Entire Fischer Diagrams Rotate diagram by 180 deg Same Structure simply rotated: H & Br still forward; CH3 &C2H5 in back. This simple rotation is an example of “proper rotation”. Rotation by 90 (or 270) degrees. Enantiomers. Non superimposable structures! Not only has rotation taken place but reflection as well (back to front). For example, the H is now towards the rear and ethyl is brought forward. This combination of a simple rotation and reflection is called an “improper rotation”.

13. Multiple Chiral Centers S Do a single swap on each chiral center to get the enantiomeric molecule. R S R Each S configuration has changed to R. Now do a single swap on only one chiral center to get a diastereomeric molecule (stereoisomers but not mirror objects). R S S R

14. Multiple Chiral Centers Enantiomers Enantiomers S R S R R S S R

15. Multiple Chiral Centers S R Diastereomers S R R S Diastereomers S R

16. Diastereomers Everyday example: shaking hands. Right and Left hands are “mirror objects” R --- R is enantiomer of L --- L and have equivalent “fit” to each other. R --- L and L --- R are enantiomeric, have equivalent “fit”, but “fit” differently than R --- R or L – L.

17. Diastereomers Require the presence of two or more chiral centers. Have different physical and chemical properties. May be separated by physical and chemical techniques.

18. Meso Compounds Must have same set of substituents on corresponding chiral carbons. S R R S As we had before here are the four structures produced by systematically varying the configuration at each chiral carbon. S R S R

19. Meso Compounds What are the stereochemical relationships? S R Enantiomers Mirror images, not superimposable. R S Diastereomers. S R S R Mirror images! But superimposable via a 180 degree rotation. Same compound. Meso

20. Meso Compounds: Characteristics Has at least two chiral carbons. Corresponding carbons are of opposite configuration. Can be superimposed on mirror object, optically inactive. Can demonstrate mirror plane of symmetry Molecule is achiral. Optically inactive. Specific rotation is zero. R S S R Meso Can be superimposed by 180 deg rotation.

21. Meso Compounds: Recognizing What of this structure? It has chiral carbons. Is it optically active? Is it meso instead? Assign configurations. Looks meso. But no mirror plane. R S Rearrange by doing even number of swaps on upper carbon. Now have mirror plane. R Original structure was meso compound. In checking to see if meso you must attempt to establish the plane of symmetry. S Meso

22. Cycloalkanes Vertical reflection plane. Horizontal reflection plane. Look for reflection planes! There are other reflection planes as well. Do you see them? Based on these planar ring diagrams we observe reflection plane and expect optical inactivity…. But the actual molecule is not planar!! Examine cyclohexane. This plane of symmetry (and two similar ones) are still present. Achiral. Optically inactive. The planar diagrams predicted correctly.

23. Substituted cyclohexanes The planar diagram predicts achiral and optically inactive. But again we know the structure is not planar. cis Mirror objects!! This is a chiral structure and would be expected to be optically active!! But recall the chair interconversion…. Earlier we showed that the two structures have the same energy. Rapid interconversion. 50:50 mixture. Racemic mixture. Optically Inactive. Planar structure predicted correctly

24. More… trans No mirror planes. Predicted to be chiral, optically active. Enantiomer. Ring Flips?????? R,R R,R trans 1,2 dimethylcyclohexane Each structure is chiral. Not mirror images! Not the same! Present in different amounts. Optically active! Other isomers for you… 1,3 cis and trans, 1,4 cis and trans.

25. Resolution of mixture into separate enantiomers. Mixtures of enantiomers are difficult to separate because the enantiomers have the same boiling point, etc. The technique is to convert the pair of enantiomers into a pair of diastereomers and to utilize the different physical characteristics of diastereomers. Formation of diastereomeric salts. Racemic mixture of anions allowed to form salts with pure cation enantiomer. Racemic mixture reacted with chiral enzyme. One enantiomer is selectively reacted. Racemic mixture is put through column packed with chiral material. One enantiomer passes through more quickly.

26. Chirality in the Biological World A schematic diagram of an enzyme surface capable of binding with (R)-glyceraldehyde but not with (S)-glyceraldehyde. All three substituents match up with sites on the enzyme. If two are matched up then the third will fai!

27. Acids and Bases

28. Different Definitions of Acids and Bases Arrhenius definitions for aqueous solutions. acid: a substance that produces H+ (H3O+) ions aqueous solution base: a substance that produces OH- ions in aqueous solution • Bronsted-Lowry definitions for aqueous and non-aqueous solutions. • Conjugate acid – base pair: molecules or ions interconverted by transfer of a proton. • acid: transfers the proton. • base: receives the proton.

29. Lewis Acids and Bases • Focuses on the electrons not the H+. • An acid receives electrons from the base making a new bond. • Acid electron receptor. • Base electron donor. Types of electrons: Energy lone pairs pi bonding electrons sigma bonding electrons Basicity

30. Acid – Base Eqilibria The position of the equilibrium is obtained by comparing the pKa values of the two acids. Equivalently, compare the pKb values of the two bases.

31. Acid – Base Eqilibria Same equilibrium with electron pushing (curved arrows).

32. Lone Pair acting as Base. Note the change in formal charges. As reactant oxygen had complete ownership of lone pair. In product it is shared. Oxygen more positive by 1. Similarly, B has gained half of a bonding pair; more negative by 1.

33. An example: pi electrons as bases Bronsted Lowry Base Bronsted Lowry Acid For the moment, just note that there are two possible carbocations formed. The carbocations are conjugate acids of the alkenes.

34. Sigma bonding electrons as bases. Much more unusual!! A very, very electronegative F!! A very positive S!! The OH becomes very acidic because that would put a negative charge adjacent to the S. Super acid

35. Trends for Relative Acid Strengths Totally unionized in aqueous solution Aqueous Solution Totally ionized in aqueous solution.

36. Example pKa = 15.9 Weaker acid pKa = 9.95 Stronger acid H2O + PhOH H3O+ + PhO- H2O + EtOH H3O+ + EtO- Ka = [H3O+][EtO-]/[EtOH] = 10-15.9 Ka = [H3O+][PhO-]/[PhOH] = 10-9.95 Ethanol, EtOH, is a weaker acid than phenol, PhOH. It follows that ethoxide, EtO-, is a stronger base than phenolate, PhO-. For reaction PhOH + EtO- PhO- + EtOH where does equilibrium lie? Weaker base. Stronger base K = 10-9.95 /10-15.9 = 106.0 Query: What makes for strong (or weak) acids?

37. What affects acidity? Increasing basicity of anion. Increasing basicity of anion. 1. Electronegativity of the atom holding the negative charge. Increasing electronegativity of atom bearing negative charge. Increasing stability of anion. Increasing acidity. 2. Size of the atom bearing the negative charge in the anion. Increasing acidity. Increasing size of atom holding negative charge. Increasing stability of anion.

38. What affects acidity? - 2 3. Resonance stabilization, usually of the anion. Increasing resonance stabilization. Increased anion stability. Increasing basicity of the anion. Acidity No resonance structures!! Note that phenol itself enjoys resonance but charges are generated, costing energy, making the resonance less important. The more important resonance in the anion shifts the equilibrium to the right making phenol more acidic.

39. Two different bases or two sites in the same molecule may compete to be protonated (be the base). An example: competitive Bases & Resonance Acetic acid can be protonated at two sites. Pi bonding electrons converted to non-bonding. Which conjugate acid is favored? The more stable one! Which is that? Recall resonance provides additional stability by moving pi or non-bonding electrons. No valid resonance structures for this cation. Non-bonding electrons converted to pi bonding.

40. An example: competitive Bases & Resonance Comments on the importance of the resonance structures. All atoms obey octet rule! The carbon is electron deficient – 6 electrons, not 8. Lesser importance All atoms obey octet rule!

41. What affects acidity? - 3 4. Inductive and Electrostatic Stabilization. Increasing anion stability. Increasing anion basicity. Acidity. d+ d+ Due to electronegativity of F small positive charges build up on C resulting in stabilization of the anion. Effect drops off with distance. EtOH pKa = 15.9

42. What affects acidity? - 4 Note. The NH2- is more basic than the RCC- ion. 5. Hybridization of the atom bearing the charge. H-A  H+ + A:-. sp3 sp2 sp More s character, more stability, more “electronegative”, H-A more acidic, A:- less basic. Increasing Acidity of HA Increasing Basicity of A- Know this order.

43. Example of hybridization Effect.

44. What affects acidity? - 5 6. Stabilization of ions by solvents (solvation). Solvation provides stabilization. Comparison of alcohol acidities. 17 18 pKa = 15.9 Crowding inhibiting solvation (CH3)3CO -, crowded Solvation, stability of anion, acidity

45. Example Para nitrophenol is more acidic than phenol. Offer an explanation The lower lies further to the right. Why? Could be due to destabilization of the unionized form, A, or stabilization of the ionized form, B. B A

46. Examine the equilibrium for p-nitrophenol. How does the nitro group increase the acidity? Examine both sides of equilibrium. What does the nitro group do? First the unionized acid. Note carefully that in these resonance structures charge is created: + on the O and – in the ring or on an oxygen. This decreases the importance of the resonance. Structure D occurs only due to the nitro group. The stability it provides will slightly decrease acidity. Resonance structures A, B and C are comparable to those in the phenol itself and thus would not be expected to affect acidity. But note the + to – attraction here

47. Now look at the anion. What does the nitro group do? Remember we are interested to compare with the phenol phenolate equilibrium. In these resonance structures charge is not created. Thus these structures are important and increase acidity. They account for the acidity of all phenols. Structure D occurs only due to the nitro group. It increases acidity. The greater amount of significant resonance in the anion accounts for the nitro increasing the acidity. Resonance structures A, B and C are comparable to those in the phenolate anion itself and thus would not be expected to affect acidity. But note the + to – attraction here

48. Sample Problem