Names:
Balo, Marielle
Date Submitted: 31 March 2017
Deseo, Maria Rogemae Gerolaga, Winston Jake Group: Group:
4
EXPERIMENT NO. 6 STEREOCHEMISTRY
I.
Introduction
Stereochemistry is concerned with the shapes of molecules and how the differences in shapes can affect the properties and reactions of compounds. Subtle differences in molecular shape have far-reaching consequences and it is probably the field of molecular biology that a full awareness of molecular shapes has emerged. Knowledge of the molecular shapes of vital compounds such as sugars, amino acids, and proteins is essential in understanding their reactions and properties. Molecular structures are so frequently represented in two dimensions that we often experience considerable considerabl e difficulty in thinking about molecules in three dimensions. The purpose of this exercise is to persuade you to think in three dimensions.
II.
Chirality, enantiomerism, diastereomerism and optical activity. Objects which are IDENTICAL IDENTICAL are SUPERIMPOSABLE – – that is, when we imagine the two objects are “used” together, al l the corresponding parts match exactly. Thus, a sphere is superimposable on another sphere of the same diameter. A sphere, however, is not superimposable on a cube nor on a sphere of a different diameter.
1.
Determine
whether the following pairs of objects are superimposable or not
superimposable with each other. a. Two forks Answer: SUPERIMPOSABLE. b. Your left left hand and your right hand Answer: 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? Answer: NO. b. Is your right hand the mirror image of your left hand? Answer: NO.
3.
Using a mirror, decide whether the following objects are superimposable or not superimposable on their mirror images. a. A fork Answer: SUPERIMPOSABLE. b. A pentagon Answer: SUPERIMPOSABLE. c. Your left hand Answer: NOT SUPERIMPOSABLE.
A PLANE OF SYMMETRY is an imaginary plane or sheet that divides the objects so that one half is exactly a reflection of the other half.
4.
Yes or No. Tell whether the following objects contain at least one plane of symmetry.
a. A sphere Answer: YES. Answer: YES. b. A regular tetrahedron Answer: YES. Answer: YES. c. A shoe Answer: NO.
5.
Determine whether the objects in No. 4 are superimposable or not superimposable on their mirror images. a. A sphere Answer: SUPERIMPOSABLE. b. A regular tetrahedron Answer: SUPERIMPOSABLE.
c. A shoe Answer: SUPERIMPOSABLE.
6.
Describe the relationship relationshi p between the presence of a plane of symmetry in an object and the object’s superimposability on its mirror image in your own words. Answer: If there is a presence of a plane of symmetry in a molecule, then the molecule will be superimposable on its mirror image.
Objects which are not superimposable on their mirror images are CHIRAL objects, while those which are superimposable on their mirror images are ACHIRAL.
7.
Look for the model of CH 4. Is it chiral or achiral? Why?
Answer: Methane (CH4) is achiral since there is a plane of symmetry and it is superimposable on its mirror image.
8.
Look for the model of CBrFClH. Is it chiral or achiral? Why?
Answer: CBrFClH is chiral since the mirror image of the molecule is not superimposable.
Non-superimposable mirror images are called ENANTIOMERS.
9. a. Are enantiomers chiral? Answer: YES. Answer: YES.
b. Do enantiomers have the same formula? Answer: YES. Answer: YES. c. Does one structural formula represent both members of a pair of enantiomers? Answer: NO.
10. Are enantiomers stereoisomers? stereoisomer s? Answer: Y es because their only difference is their three-dimensional arrangement.
One of the most important property of chiral molecules is their effect on plane – polarized – polarized light. Both enantiomers rotate rotat e the plane of polarized light. Substances with
optically active activ e. Chiral substances containing molecules this property are said to be optically which are not superimposable on their mirror images are optically active. Enantiomers are also known as optical isomers.
11. Determine whether each of the following are optically active or optically inactive . a. CH3CH(NH2)COOH Answer: OPTICALLY ACTIVE. b. CH3CH2CHClCH3 Answer: OPTICALLY ACTIVE. c. CH3CH2CH2CH3 Answer: OPTICALLY INACTIVE.
The rotation of the plane – plane – polarized polarized light is used to observe experimentally one of the main differences between a pair of enantiomers. One enantiomer rotates plane – – polarized light clockwise (as seen by the observer), the other enantiomer, counterclockwise by the same amount. Conventions used to indicate the direction of rotation of the plane of polarization are:
Clockwise (+): dextrorotatory, d Counter-clockwise ( - ): levorotatory, l
12. Which enantiomer of of alanine rotates the plane-polarized plane-polari zed light clockwise? Answer: (+)-alanine
13. Which of the following aqueous solutions are likely to be optically inactive? a. A solution of ethanol, CH 3CH2OH Answer: OPTICALLY ACTIVE. b. A solution of (+)-alanine Answer: OPTICALLY ACTIVE. c. A solution of (-)-alanine Answer: OPTICALLY ACTIVE. d. A solution containing equimolar quantities of (+)-alanine (+)-alani ne and (-)-alanine (-)-a lanine Answer: OPTICALLY INACTIVE.
A mixture containing containing equimolar quantities quantities of a pair of enantiomers enantiomers is called a RACEMIC MIXTURE.
14. How is the racemic mixture of alanine represented? Answer: (±)-alanine
15. Examine the models for the presence of a plane symmetry. Which of them has at least one plane of symmetry?
(a)
(b)
Answer: A and B. B.
16. Which are superimposable on their mirror image? Answer: A, B, and C.
(c)
(d)
17. Which model represents a chiral molecule? Answer: D only.
18. How many different atoms of groups of atoms must be attached to the central carbon atom in order to confer chirality to the molecule? Answer: There should be four different types of atoms or groups of atoms attached to the central atom in order to confer the chirality of the molecule.
Consider the following structural formulas for items 19 and 20.
(a)
(b)
19. Which is a chiral molecule? Answer: A only.
20. Which can exist as pair of enantiomers? Draw the pair of enantiomers. Answer: A only.
HOOC
CH3 HO
Now, we will consider molecules with more than one chiral center. Consider 2,3dihydroxybutanoic acid.
The flying-wedge flying-wedg e representation of 2,3-dihydroxybutanoic 2,3-dihydroxybuta noic acid is shown below.
MIRROR IMAGE (I)
(II)
21. No question. 22. a. Draw the flying wedge representation of its mirror image (II).
(I)
(II)
b. Are I and II chiral? Answer: NO.
A compound which contains contains two different different chiral centers can can exist as four four optically inactive stereoisomers. The flying-wedge representations of the four stereoisomeric 2,3dihydroxybutanoic acid are given below.
Some are related as enantiomers while others, called DIASTEREOISOMERS DIASTEREOISOMERS are are not related as mirror images at all.
23. What is the relationship between a. I and II Answer: ENANTIOMERS. b. III and IV Answer: ENANTIOMERS. c. I and IV Answer: DIASTEREOMERS. d. II and IV Answer: DIASTEREOMERS.
24. What is the relationship between a. I and III Answer: DIASTEREOMERS. b. II and III Answer: DIASTEREOMERS.
CONFORMATIONS. CONFORMATIONS . Atoms within the molecules can vibrate and are free to rotate about the single bond. This rotation may result in the atom taking different positions relative to the rest of the molecule. Molecular structures that are interconvertible by simple bond rotations are called conformations.
OPEN CHAIN COMPOUNDS
25. Look for the model of ethane. Rotate the C – C – C single bond. a. Is there a change in the relative positions of the different atoms as the C – C bond is rotated? Answer: YES. Answer: YES. b. Does the ethane molecule have more than one conformation? Answer: YES. Answer: YES.
Ethane can adopt an infinite number of conformations but the two extreme
tag g ered. conformations are eclipsed and s tagg NEWMAN PROJECTION
Eclipsed
Staggered
SAWHORSE PROJECTION
Eclipsed
Staggered
26. In which extreme conformation are the 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? 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? Answer: YES. Answer: YES. b. What is the preferred conformation called? Answer: 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 of the ring are in one plane? Answer: NO.
The two extreme conformations of cyclohexane are referred to as chair and boat .
Put your model carefully in the chair conformation. Look at the relative positions of the hydrogen atoms along the axes of C2 – C3 – C3 and C5 – C5 – C6. C6. Hold carbons 2, 3, 5, and 6 firmly in one hand, then without moving carbon 1, flip carbon 4 up (or down) without breaking any bond. Again look look at the relative relative positions positions of the the hydrogens hydrogens along the C2 – C3 – C3 and C5 – C5 – C6 C6 bonds.
32. Does flipping of carbon 4 result in a change in conformation? Answer: YES. Answer: YES.
33. In which conformations are the hydrogens farthest apart? Answer: Chair conformation.
34.
Which is the preferred conformation? Answer: Chair conformation.
Compare your models with the Newman projections below by looking through the C2 – C2 – C3 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
In the chair conformation, there are two conformations of the hydrogen atoms. Bonds that are oriented vertically are called axial bonds while the rest is known as equatorial bonds.
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 projection.
38.
Then holding carbons 2, 3, 4, 5, and 6 firmly, flip down carbon 1. What is now the resulting conformation? Answer: Boat conformation.
39.
Did you break any bonds? Answer: NO.
40.
What is the orientation of the marked bonds now? Answer: Equatorial bond.
41.
Make a sketch of the final conformation showing the marked bonds. bonds.
Now with the equatorial bonds marked, reverse the process and rotate the bonds to reform the chair conformation with the axial bonds marked. This overall process of one chair conformation changing to another chair conformation is called RING INVERSION. INVERSION.
42.
Represent the process using the symbol are in dynamic equilibrium.
to indicate that the to chair conformations
43.
Construct a model of of methylcyclohexane with the methyl attached axially. Make the ring undergo inversion and complete the following.
(a)
(b)
44. a. In which chair conformation (a or b) is the methyl group farthest from the neighboring hydrogen atom? Answer: Conformation A. b. In which chair conformation (a or b) does the methyl group have greater room? Answer: Conformation B. c. Which is the preferred conformation? Answer: Conformation A.
45.
Make a general statement about the preferred conformation of monosubstituted cyclohexanes.
A substituent substituent on the cyclohexane cyclohexane ring can occupy the axial axial or equatorial equatorial position. Interconversion between cyclohexane with methyl in the equatorial position and cyclohexane with methyl in the axial position takes place via a ring flip. The conformation where methyl is in the equatorial position is more stable by around 1.7 kcal/mol therefore, it is the most preferred conformation of a monosubtituted cyclohexane.
