1. 5.4 Optical Activity
Because the structures of enantiomers are so similar, many of their properties are identical.
If you have a sample of a chiral compound, how can its enantiomeric purity be assessed?
In other words, how can one enantiomer be distinguished from another?
Copyright 2012 John Wiley & Sons, Inc.

2. 5.4 Optical Activity
Enantiomers have opposite configurations (R vs. S), so they will rotate plane-polarized light in opposite directions
What is plane-polarized light?
Copyright 2012 John Wiley & Sons, Inc.

3. 5.4 Optical Activity
To get light waves that travel in only one plane, light travels through a filter
When plane polarized light is directed through a sample of a pure chiral compound, the plane that the light travels on will rotate.
Compounds that can rotate the plane or plane-polarized light are called optically active
Copyright 2012 John Wiley & Sons, Inc.

4. 5.4 Optical Activity
Copyright 2012 John Wiley & Sons, Inc.

5. 5.4 Optical Activity
Enantiomers will rotate the plane of the light to equal degrees but in opposite directions
The degree to which light is rotated depends on the sample concentration and the pathlength of the light
Standard optical rotation measurements are taken with 1 gram of compound dissolved in 1 mL of solution, and with a pathlength of 1 dm for the light
Temperature and the wavelength of light can also affect rotation and must be reported with measurements that are taken
Copyright 2012 John Wiley & Sons, Inc.

6. 5.4 Optical Activity Consider the enantiomers of 2-bromobutane
R and S refer to the configuration of the chirality center
(+) and (-) signs refer to the direction that the plane of light is rotated
The optical activity was measured at 589 nm, which is the Sodium D line wavelength
Copyright 2012 John Wiley & Sons, Inc.

7. 5.4 Optical Activity
There is no relationship between the R/S configuration and the direction of light rotation (+/-)
Should the chirality center below be designated R or S?
As long as its bonds are not rearranged, its configuration CANNOT change
It is levorotatory (-) at 20°C, while at 100°C, it is dextrorotatory (+)
Copyright 2012 John Wiley & Sons, Inc.

8. 5.4 Optical Activity
The magnitude and direction of optical rotation can not be predicted from a chiral molecule’s structure or configuration. It can ONLY be determined experimentally
Predict the optical rotation for a racemic mixture (a sample with equal amounts of two enantiomers)
Predict the optical rotation for a sample of 2-methylbutane
Practice with SkillBuilder 5.5
Copyright 2012 John Wiley & Sons, Inc.

9. 5.4 Optical Activity
For unequal amounts of enantiomers, the enantiomeric excess (% ee) can be determined from the optical rotation
For a mixture of 70% (R) and 30% (S), what is the % ee?
Copyright 2012 John Wiley & Sons, Inc.

10. 5.4 Optical Activity
If the mixture has an optical rotation of +4.6, use the formula to calculate the % ee and the ratio of R/S
Practice with SkillBuilder 5.5
Copyright 2012 John Wiley & Sons, Inc.

11. 5.5 Stereoisomeric Relationships
Categories of isomers
Are cis/trans isomers enantiomers or diastereomers?
Copyright 2012 John Wiley & Sons, Inc.

12. 5.5 Stereoisomeric Relationships
Draw each of the four possible stereoisomers for the following compound. It might be helpful to also make a handheld model for each isomer
Pair up the isomers in every possible combination and label the pairs as either enantiomers or diastereomers
Copyright 2012 John Wiley & Sons, Inc.

13. 5.5 Stereoisomeric Relationships
Consider a cyclohexane with three substituents
What patterns do you notice?
Copyright 2012 John Wiley & Sons, Inc.

14. 5.5 Stereoisomeric Relationships
The number of possible stereoisomers for a compound depends on the number of chirality centers (n) in the compound
What is the maximum number of possible cholesterol isomers?
Practice with SkillBuilder 5.7
Copyright 2012 John Wiley & Sons, Inc.

15. 5.6 Symmetry and Chirality
Any compound with only ONE chirality center will be chiral and have an optical rotation
However, compounds with an even number (2,4,6,etc.) of chirality centers may or may not be chiral
Identify each chirality center for both molecules below
Copyright 2012 John Wiley & Sons, Inc.

16. 5.6 Symmetry and Chirality
Both molecules have 2 chirality centers, but only one is a chiral molecule, and the other is achiral
If a molecule has a plane of symmetry, it will be achiral
Half of the molecule reflects the other half
Its optical activity will be canceled out within the molecule, similar to how a pair or mirror image enantiomers cancel out each others optical rotation
Copyright 2012 John Wiley & Sons, Inc.

17. 5.6 Symmetry and Chirality
Molecules with an even number of chirality centers that have a plane of symmetry are called meso compounds
Another way to test if a compound is a meso compound is to see if it is identical to its mirror image
Draw the mirror image of the cis isomer and show that it can be superimposed on its mirror image
When a compound is identical to its mirror image, it is NOT chiral. It is achiral
Copyright 2012 John Wiley & Sons, Inc.

18. 5.6 Symmetry and Chirality
In another example, the plane of symmetry identifies it as a meso compound
meso compounds also have less than the predicted number of stereoisomers based on the 2(n) formula
Draw all four expected isomers and show how two of them are identical. A handheld model might be helpful
Practice with SkillBuilder 5.8
Copyright 2012 John Wiley & Sons, Inc.

19. 5.6 Symmetry and Chirality
Determine the relationship between the two compounds below
Analyze both molecules to see if either of them are meso compounds
Copyright 2012 John Wiley & Sons, Inc.

20. 5.6 Symmetry and Chirality
Some compounds do NOT have a plane of symmetry and yet are still achiral
The molecule to the right has chirality centers, yet because it is identical to its mirror image, it is not chiral
Draw the mirror image of the molecule above and show that it can be superimposed on its mirror image
Having handheld models to work with can simplify this challenging task
Practice with conceptual checkpoint 5.23
Copyright 2012 John Wiley & Sons, Inc.

21. 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 2012 John Wiley & Sons, Inc.

22. 5.7 Fischer Projections
Fischer projections can be used to quickly draw molecules with multiple chirality centers
Copyright 2012 John Wiley & Sons, Inc.

23. 5.7 Fischer Projections
Fischer projections can also be used to quickly assess stereoisomeric relationships
Practice with SkillBuilder 5.9
Copyright 2012 John Wiley & Sons, Inc.

24. 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 must be optically active and unable to interconvert with their mirror image
Copyright 2012 John Wiley & Sons, Inc.

25. 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 2012 John Wiley & Sons, Inc.

26. 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 2012 John Wiley & Sons, Inc.

27. 5.8 Interconverting Enantiomers
The freely interconverting mirror images cancel out their optical rotation making it 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 2012 John Wiley & Sons, Inc.

28. 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 2012 John Wiley & Sons, Inc.

29. 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 2012 John Wiley & Sons, Inc.

30. 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 2012 John Wiley & Sons, Inc.

31. 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 2012 John Wiley & Sons, Inc.

32. 6.1 Enthalpy
Enthalpy (ΔH or q) is the heat energy exchange between the reaction and its surroundings at constant pressure
Breaking a bond requires energy. WHY?
Show on figure 6.1 how much energy is needed
Copyright 2012 John Wiley & Sons, Inc.

33. 6.1 Enthalpy Bonds can break homolytically or heterolytically
Bond dissociation energy (BDE) or ΔH for bond breaking generally represents the energy associated with HOMOlytic cleavage
Copyright 2012 John Wiley & Sons, Inc.

34. 6.1 Bond Dissociation Energies
More BDEs in table 6.1
Copyright 2012 John Wiley & Sons, Inc.

35. 6.1 BDEs
How heat energy be exchanged between the reaction (system) and the solution (surroundings) for each scenario below?
H• and F• free radicals come together to form bonds
A C–Br bond is broken
A strong bond is broken and a weak bond is formed
A weak bond is broken and a strong bond is formed
Copyright 2012 John Wiley & Sons, Inc.

36. 6.1 BDEs
Most reactions involve multiple bonds breaking and forming. The energy associated with each bond that breaks and forms must be considered
If during a chemical reaction the temperature of the reaction solution DECREASES, what can be said about the relative potential energies, stabilities, and bond strengths for the reactants and products
If during a chemical reaction, the temperature of the reaction solution INCREASES, what can be said about the relative potential energies, stabilities, and bond strengths for the reactants and products
Copyright 2012 John Wiley & Sons, Inc.

37. 6.1 Enthalpy ΔH
Match the reaction coordinate diagrams below with the statements below
The reaction causes the surrounding temp. to DECREASE
The reaction causes the surrounding temp. to INCREASE
Copyright 2012 John Wiley & Sons, Inc.

38. 6.1 Enthalpy ΔH
For a chemical reaction, why is the sign (+/-) for ΔH important?
If you were performing a chemical reaction in a lab, what experimental considerations might you make if you knew ΔH = +
If you were performing a chemical reaction in a lab, what experimental considerations might you make if you knew ΔH = -
Practice with SkillBuilder 6.1
Copyright 2012 John Wiley & Sons, Inc.

39. Study Guide for sections 5.4-5.9, 6.1
DAY 11, Terms to know:
Sections , 6.1 plane-polarized light, optical activity, specific rotation, configuration vs conformation, levorotatory, dextrorotatory, racemic mixture, enantiomeric excess, meso compound, Fischer projection, chiral resolving agent, affinity chromatography, Enthalpy, homolytic, heterolytic, cleavage, bond dissociation energy, the system, the surroundings, reaction coordinate diagram, exothermic, endothermic
DAY 11, Specific outcomes and skills that may be tested on exam 2:
Sections , 6.1
Be able to explain the (+) and (-) notation and how they refer to the rotation of plane-polarized light and NOT the R or S configuration specifically.
Given percentages of R and S in a mixture, be able to determine the enantiomeric excess
Given the enantiomeric excess and the optical rotation observed for a pure enantiomer, be able to determine the %R and %S in a mixture.
Be able to determine if a molecule with chiral centers is chiral overall or whether it is achiral and a meso compound.
Be able to describe the methods of determining whether a molecule is chiral or achiral including locating chiral centers, looking for a plane a symmetry, measuring its optial rotation, and seeing if it is superimposable on its mirror image.
Be able to convert back and forth between wedge/dash structure and Fischer projections for molecules with 1 chiral carbon.
Be able to use Fischer projections to determine whether pairs of molecules are enantiomers or diastereomers.
Be able to recognize in a molecule that if only one rotational conformation of the molecule is achiral, then the molecule overall is achiral
Be able to describe how chiral resolving agents work and how chiral affinity columns work to separate pairs of enantiomers.
Be able to explain how and why the potential energy is changed when bonds break and how that affects the temperature of the surroundings.
Be able to explain how and why the potential energy is changed when bonds form and how that affects the temperature of the surroundings.
Be able to explain how and why the potential energy is changed when weak bonds break and are replaced by strong bonds and how that affects the temperature of the surroundings.
Be able to explain how and why the potential energy is changed when strong bonds break and are replaced by weak bonds and how that affects the temperature of the surroundings.
Be able to describe how the temperature of the surroundings is affected for both exothermic and endothermic reactions.
Klein, Organic Chemistry 2e

40. Practice Problems for sections 5.4-5.9, 6.1
Complete these problems outside of class until you are confident you have learned the SKILLS in this section outlined on the study guide and we will review some of them next class period
Klein, Organic Chemistry 2e

41. Prep for Day 12 Must Watch videos: Other helpful videos:
(entropy)
(predicting the sign for deltaS)
(free energy and equilibria)
(kinetics basics)
(kinetic vs thermodynamic control)
Other helpful videos:
(kinetics vs thermodynamics)
(lectures 18-20)
(Hammond Postulate)
Read sections
Klein, Organic Chemistry 2e