It is sometimes convenient to have a two-dimensional representation for three-di
ID: 708985 • Letter: I
Question
It is sometimes convenient to have a two-dimensional representation for three-dimensional molecules, particularly when we are studying stereoisomerism. One common convention, devised by the German organic chemist Emil Fischer and named after him, is described in this section
Procedure: Construct a model of an asymmetric carbon atom [with a black, a blue, a green, and a red ball attached] that corresponds to the following three-dimensional drawing:
Set the model on the desk as shown. With your left hand, tip the model to the left so that only the red and green balls rest on the table. Viewed from the top, the black and blue balls project toward you (left and right, respectively).
The convention for Fischer projection formulas is as follows: The asymmetric carbon lies in the plane of the page or paper, horizontal groups come out of the plane of the paper toward the viewer, and vertical groups recede behind the paper away from the viewer.
Save the model you have just constructed for comparison with other models that you will construct.
Let us first consider the effect of interchanging any two groups in a Fischer projection formula. Your model corresponds to formula A below:
Consider formula A', with groups blue and green interchanged. Construct a model corresponding to A'.
35. Is A' identical to A, or is it the enantiomer of A?
a. Identical
b. Enantiomer
Now construct a model corresponding to A" (like A, but with green and red interchanged).
36. Is A" identical to A, or is it the enantiomer of A?
a. Identical
b. Enantiomer
37. What generalization can you make about the interchange of any two groups in a Fischer projection formula?
a. The new molecule is an enantiomer of the first.
b. The molecule is identical to the first.
38. If you were to make an even number of group interchanges in a Fischer projection formula, would you obtain the original molecule or its enantiomer?
a. Original molecule
b. Enantiomer of the original model
39. Do the following Fischer projections represent the same molecule or enantiomers?
a. Same molecule
b. Enantiomers
Rotation of a Fischer projection formula in the plane of the paper also affects the structure it represents.
.
Consider formula B, which corresponds to the Fischer projection formula of A rotated 90
Explanation / Answer
Two of the above answer are incorrect:
40: b (1)
43: a (0)
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