Structure in Chemistry
SC10. Diastereomers and Physical Properties
Threitol is a metabolite closely related to carbohydrates. D-threitol is the enantiomer of L-threitol. The two are non-identical, but they are mirror images of each other. Both compounds can be crystallized, forming needle-like crystals. Both have melting points of 88-89oC. D-threose has an optical rotation [a]D= - 4.0 (c, 7 in H2O, but L-threose has an optical rotation [a]D= + 4.6 (c, 6 in H2O). You might notice that these two numbers are not exactly opposites, but if you have ever tried measuring an optical rotation yourself, you know that this is pretty good.
Figure SC10.1. D-threitol, a reduced form of threose.
Figure SC10.2. A ball-and-stick model of D-threitol.
Go to Animation SC10.1. A three-dimensional model of D-threitol.
Here is the other anantiomer. Once again, the Fischer projection makes it really easy to see the difference, but navigating back and forth between Fischer and wedge-dash can be tricky. The ball-and-stick model is rotated a little compared to the model of D-threose; see if can confirm that this is really the enantiomer.
Figure SC10.3. L-threitol, the enantiomer of D-threitol.
Figure SC10.4. A ball-and-stick model of L-threitol.
Go to Animation SC10.2. A three-dimensional model of L-threitol.
Erythritol is a diastereomer to both L-threose and D-threose. It has one identical chiral center and one opposite one. Erythritol is a solid with a melting point of 121oC and no optical rotation. Erythritol has properties that are different from threitol.
Figure SC10.5. Erythritol, the diastereomer of threitol.
Figure SC10.6. A ball-and-stick model of D-threitol.
Go to Animation SC10.3. A three-dimensional model of erythritol.
The fact that erythritol contains chiral centers but has no optical rotation is unusual. Erythritol is chiral but not optically active. It is a special case called a meso compound.
It may be easier to see the mirror symmetry within erythritol if it is in a less-stable conformation. In the following model, the carbon backbone is held in a curled-up, eclipsed conformation rather than the usual zig-zag, staggered conformer. You might notice in the animation that this is really what we are looking at in a Fischer projection: the backbone is always curling away from us, with attached groups projecting towards us on either side. Here, the top half of the molecule is just a reflection of the bottom half.
Figure SC10.7. A ball-and-stick model of erythritol in an eclipsed conformation.
Go to Animation SC10.4. Another three-dimensional model of erythritol.
What are the absolute configurations of each chiral carbon in:
In problem SC10.1., you should have numbered the carbons corresponding to each chiral center when you were denoting the absolute configuration. How did you know which end of the chain to start counting from?
From the following group of molecules, select:
a) a pair that are the same compound.
b) a pair that would have the same physical properties but opposite optical activities.
c) a pair that have different physical properties.
d) a compound that contains chiral centers but has no optical activity.
What is the isomeric relationship between the following pairs of molecules (diastereomers, enantiomers, constitutional isomers, identical)?
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Structure & Reactivity in Organic, Biological and Inorganic Chemistry by Chris Schaller is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License.
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