Quiz:OrganicP010: Tutorial on Stereochemistry: Principles of Chirality
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Based on an exercise developed by Dr. Linda Jacob, Connecticut, USA. Work through this exercise with a molecular modelling kit.
This tutorial involves the use of models, so please have a model kit available when you work through this. Also review what is meant by the following terms: plane of symmetry, and chiral atom. You will need to identify these in the models you construct.
Have you ever tried to put your right foot in your left shoe? You know immediately, even in the dark, that something is wrong. It's a foot, and shoes are supposed to be designed to fit on feet, but there is a significant difference between your two feet that makes it possible for that left shoe to feel comfortable only when it is placed upon your left foot. The property that this illustrates is known as chirality, and a structure or atom with this property is said to be chiral. Since an sp3 carbon is tetrahedral in shape, it can be asymmetrical (lacking symmetry). This makes it possible for it to be a chiral centre or stereogenic centre. If the structure contains no asymmetry, it is said to be achiral.
In this laboratory, you will explore the requirements necessary for a compound to be chiral and learn the conventions for representing and designating the absolute configuration of a molecule.
Exploring the requirements necessary for a carbon to be a chiral or stereogenic centre
Construct a model that has two green and two white groups attached to it.
1. Does the molecule have at least one plane of symmetry? See answer
Once you have checked your answer, replace one of the white balls with a blue one.
2. Does the molecule have at least one plane of symmetry? See answer
Once you have checked your answer, replace one of the green balls with a red ball.
3. Does the molecule have at least one plane of symmetry? See answer
4. Write a statement that describes the requirements necessary for a carbon to be considered a stereogenic centre.
5. Draw the following compounds using either structural formulas or skeletal formulas with wedged-dashed lines. Indicate any stereogenic centres with an asterisk.
Once you have worked these out, look over the answers
Stereogenic centres and enantiomers
When a compound has a stereogenic centre it has the potential to be a chiral compound. To be chiral, a compound must possess a nonsuperimposable mirror image. To understand what this means, examine your own hands. Hold them right in front of you with the palms facing away. One hand is the reflection of the other. That means if you were to place a mirror where one of the hands is so that it reflected the remaining hand, the image in the mirror would look like the hand you had removed (assuming that there is no distinguishing feature on either hand). Your hands are mirror images of each other. Notice that when you put one of them on top of the other with both palms facing down, that the fingers don't line up exactly the same. One thumb is on the right, and the other is on the left. No amount of twisting or turning is going to allow you to line up all of your fingers. Your hands are chiral and they represent a pair of nonsuperimposable mirror images.
In chemistry, a pair of compounds that are nonsuperimposable mirror images are said to be enantiomers: Both compounds possess the property of chirality or "handedness". Enantiomers differ in only two properties. They rotate plane polarised light in opposite directions and they can interact with other chiral molecules differently. All other properties (e.g., melting point, density, refractive index, etc.) will be the same for both enantiomers. The ability to rotate plane polarised light is referred to as optical activity. It was the observation of this property that enabled Pasteur to formulate his theories about chirality and structure. The different interaction with other chiral species is an important property and has powerful ramifications for living organisms, since many of the compounds living species are made of are chiral. Often one enantiomer has the desired properties, while the other enantiomer either lacks that property or perhaps even has the opposite effect. The sweetener aspartame is a chiral compound. One enantiomer is sweet, but the other enantiomer is very bitter. Another famous example, one that highlights just how important chirality is, is that of Thalidomide. This drug, as you might have guessed, exists as a mixture of isomers (or a racemic mixture), and whilst only one of the enantiomers had a positive pharmacological effect, the other did not; in fact, it did the opposite and actually damaged unborn babies when taken during pregnancy! However, optical isomerism has also saved many lives. Penicillin breaks down D-amino acids (one type of enantiomer), moreover, peptide links involving D-alanine. These exist in bacterial cell walls of bacteria but not in humans (who only have L-enantiomers), so penicillin kills bacteria but has no such effect on us!
If you find that a molecule has a mirror image form that is superimposable, then this molecule probably has a plane of symmetry and it is achiral (not chiral). If you make models, you will find that the mirror image form is identical to the original molecule, just like the way the mirror image of an achiral object such as a sofa is identical to the original sofa. Construct a model in the following manner: 1) Take a black tetrahedral piece. This represents the stereogenic carbon centre; 2) Attach a white ball. Place this pointing up; 3) Look down on the white ball and place a green ball, a red ball, and a blue ball so that they progress in the indicated order as you go clockwise. This means that as you are looking down at the white ball, if you place the green ball at the 12 o'clock position, the red ball will be at the 4 o'clock position and the blue one will be at the 8 o'clock position. Now construct a second model that is the mirror image of the first one.
6. If you line the white and green balls up in the same position as the first model, where, respectively, are the red and blue balls?
7. In which direction must you proceed in order to go from the green to the red to the blue ball in the first model? (clockwise or anticlockwise/counterclockwise)
8. In which direction must you proceed in order to go from the green to the red to the blue ball in the second model?
9. Describe the relationship of the two models using the terms introduced in this section.
Diastereomers and Meso Compounds
When a compound has more than one stereogenic centre, there may be stereoisomers that are not superimposable mirror images. These are diastereomers. Diastereomers can differ in all physical properties including solubility. In fact, they can differ in properties so much that one might think they were constitutional isomers. This is not the case, however, because their connectivities are identical. It is not possible to have diastereomers in single bonded species unless you have at least two stereogenic centres.
Construct two identical models. Use red, blue, green, and white pieces on your central atom. Now remove the white ball from one model and the white ball from the second model. Connect the two models together using the bond that the first white ball had been connected to. This will be called model A.
10. How many stereogenic centres does this model have?
11. What groups are attached to each centre?
12. Does each stereogenic centre have the same groups attached?
13. Does the model have a plane or centre of symmetry?
14. Is the model chiral or achiral?
Now construct a model that is the mirror image of model A. This will be model B. (Do not destroy A. If you don't have enough pieces, have a partner construct this model so that you both can look at model A and model B.
15. What term describes the relationship between the two models?
16. Is each model chiral or achiral?
17. Would the compound model B represents be optically active?
Now interchange the red and green pieces on one of the carbons on model B. This is model C.
18. Are models A and C superimposable? .
19. Are they mirror images?
20. Are these constitutional isomers or stereoisomers?
21. What term describes the relationship between models A and C? Carefully examine model C for the following questions.
22. Does the model have a plane of symmetry?
23. Is it chiral or achiral?
24. Would a model that is the mirror image of model C be superimposable?
25. Would a compound represented by model C be optically active?
The last model you studied, model C, is an example of a meso compound. While such compounds possess two stereogenic centres, they are identically substituted and the mirror image of each other. An analogous example can be made by considering a pair of gloves. Each glove is chiral. But when the two of them are stapled together at the thumb so that the palm of each glove is pointing down, the single unit now contains a plane of symmetry that passes through the staple. Therefore, the pair of gloves attached in this way is achiral and optically inactive!
Tartaric acid is a molecule that possesses a meso form for one of its stereoisomers. Two of the stereoisomers are optically active enantiomers (models A and B). The third stereoisomer (model C) is the meso form, which is the diastereomer of the other two.
Quick-fire questions (a great way to end the lesson and encourage a bit of healthy competition!):
1. What is a chiral centre?
2. Why does the amino acid glycine not exhibit optical isomerism?
3. Which of the following molecules exhibit optical isomerism?