EXPERIMENT 6 INTRODUCTION TO STEREOCHEMISTRY Chirality, Enantiomerism, Diastereomerism and Optical Activity 1. Determine whether the following pairs of objects are supe rimposable or not superimposable with each other. a) two forks Superimposable b) your left hand and your right hand Not superimposable 2. Yes or No. View your right hand in front of the mirror a) Is the reflection of your right hand superimposable on your left hand? b) Is your right hand the mirror image of your left hand?
Yes Yes
3. Using a mirror, decide whether the following objects are superimposable or not superimposable on their mirror images. a) a fork Superimposable b) a pentagon Superimposable c) your left hand Not superimposable 4. Yes or No. Tell whether the following objects contain at least one plane of symmetry. a) a sphere Yes b) a regular tetrahedron Yes c) a shoe No 5. Determine whether the objects in No. 4 are superimposable or not superimposable on their mirror images. a) a sphere Superimposable b) a regular tetrahedron Superimposable c) a shoe Not superimposable 6. Describe the relationship between the presence of a plane of symmetry in an object and the object’s superimposability on its mirror m irror image in your own words. Answer: The presence of a plane of symmetry means that the object is superimposable because one half of the object is exactly e xactly a reflection of the other half. Thus, the mirror image is align with all parts of the object. 7. Look for the model of CH4. Is it chiral or achiral? Why? Answer: CH4 is achiral because it is superimposable on its mirror image. 8. Look for the model of CBrFClH. Is it chiral or achiral? Why? Anwser: CBrFClH is chiral because it is non-superimposable on its mirror image.
9.
a) Are enantiomers chiral? Yes b) Do enantiomers have the same molecular formula? Yes c) Does one structural formula represent both members No of a pair of enantiomers?
10. Are enantiomers stereoisomers? Answer: Yes. 11. Determine whether each of the following are optically active or optically inactive. a) CH3CH(NH2)COOH optically active b) CH3CH2CHClCH3 optically active c) CH3CH2CH2CH3 optically inactive 12. Which enantiomer of alanine rotates the plane – plane – polarized polarized light clockwise? Answer: (+)-alanine 13. Which of the following aqueous solutions are likely to be optically inactive? a) a solution of ethanol, CH3CH2OH optically active b) a solution of (+) – (+) – alanine alanine optically active c) a solution of (-) – (-) – alanine alanine optically active d) a solution containing equimolar quantities of (+) – (+) – optically inactive alanine and (-) – (-) – alanine alanine 14. How is the racemic mixture of alanine represented? Answer: A solution containing equimolar quantities of (+) – (+) – alanine alanine and (-) – (-) – alanine. alanine. This is represented as (+/-) – (+/-) – alanine. alanine.
15. Examine the models for the presence of a plane of symmetry. Which of them has at least one plane of symmetry?
Answer: a and b 16. Which are superimposable on their mirror image? Answer: a and b 17. Which model represents a chiral molecule? Answer: c and d
18. How many different atoms or groups of o f atoms must be attached to the central carbon atom in order to confer chirality to the molecule? Answer: 4 different atoms or group of atoms Consider the following structural formula for Nos. 19-20
19. Which is a chiral molecule? Answer: a 20. Which can exist as a pair of enantiomers? Draw the pair of enantiomers.
COOH
CH3
CH3
Now we will consider molecules with more than one chiral center. Consider 2,3dihydroxybutanoic acid.
The flying – flying – wedge wedge representation of 2,3-dihydroxybutanoic acid is shown below.
COOH OH
22. a) Draw the flying flying wedge representation of its mirror image (II) (II)
OH CH3
b) Are I and II chiral? Answer: Yes
A compound which contains two different chiral centers can exist as four optically active stereoisomers. The flying wedge representation of the four stereoisomeric 2,3-dihydroxybutanoic acid are given below.
23. What is the relationship between? a) I and II b) III and IV c) I and IV d) II and IV
24. What is the relationship between? a) I and III b) II and III
Diastereomers Diastereomers
OPEN CHAIN COMPOUNDS 25. Look for the model of ethane. Rotate the C – C – C C single bond. a) Is there a change in the th e relative positions of the different atoms as the C-C bond is rotated? b) Does the ethane molecule have more than one conformation?
Yes Yes
26. In which extreme conformation are the th e hydrogens farthest apart from each other? Answer: Staggered conformation 27. Draw the Newman and Sawhorse representation of ethane with the highest potential energy.
28. Draw the Newman and Sawhorse representation of ethane with the lowest potential energy.
29. Which is the preferred conformation of ethane ? Answer: Staggered conformation 30. Look at the model of chloroethane. a) Do all chloroethane molecules spend all of their time in this preferred conformation? b) What is the preferred conformation called?
No Staggered conformation
RING COMPOUNDS 31. Look at the model of cyclohexane. Rotate the carbon atoms and examine the different conformations a) Are there any conformations in which all carbon atoms of the ring are in one plane? Answer: No. Cyclohexane has no flat form. 32. Does flipping of carbon 4 result in a change in conformation? Answer: Yes 33. In which conformations are the h ydrogens farthest apart? Answer: Chair (staggered) conformation 34. Which is the preferred conformation? Answer: Chair (staggered) conformation Compare your models with the Newman Projections below by looking through the C2 – C2 – C3 and C5 – C5 – C6 C6 bonds at the same time.
35. Which is the eclipsed conformation? Answer: II 36. Which is the staggered conformation? Answer: I 37. Mark the axial bonds. Hold carbon atoms 1, 2, 3, 4, 5, and 6 firmly and flip up carbon 4. What is the resulting conformation? Answer: Chair conformation 38. Then holding carbons 2, 3, 4, 5, and 6 firmly, flip down carbon 1. What is now the resulting conformation? Answer: Chair conformation 39. Did you break any bonds? Answer: No 40. What is the orientation of the marked bonds now? Answer: Equatorial bonds 41. Make a sketch of the final conformation showing the marked bonds.
42. Represent Represent the process using the symbol ( in dynamic equilibrium.
) to indicate that the two two chair conformations are
43. Construct a model of methylcyclohexane with the methyl attached axially. Make the ring undergo inversion and complete the following: Answer:
CH3
44. a) In which chair conformation (a or b) is the methyl group farthest from the neighboring hydrogen atom? Answer: b b) In which chair conformation (a or b) does the methyl group have greater room? Answer: b c) Which is the preferred conformation of methylcyclohexane (a or b)? Answer:b
45. Make a general statement about the preferred conformation of monosubstituted cyclohexanes. Answer: Conformations that maximize those repulsive forces between each atom are what monosubstituted cyclohexanes prefer. This type of conformations are the stable ones, therefore less energy is needed in each atom. This is called the staggered conformation.
