What is the phenomenon of chirality as it applies to carbon chemistry? How did it arise? And what are the consequences of such chirality?
2 Answers
A carbon with 4 different substituents has the possibility of handedness, in that one configuration cannot be superimposed upon another. Biology and biochemistry are replete with such handedness.
Explanation:
How did such isomerism arise? I certainly cannot give any reasonable account for the origin of biological isomerism; yet it exists. In the 19th Century, Pasteur laboriously separated left and right-handed crystals of tartaric acid (distinguished on the basis of their macroscopic geometry). He also found a way to distinguish them in separate solutions (by their rotation of plane polarized light). He had no way of knowing whether this macroscopic isomerism extended to the molecular level (it did with tartaric acid, it might not with other chiral crystals); nor whether it was a general condition.
It turned out that Pasteur had demonstrated, and found convincing evidence for a fundamental principle, and one with which biochemistry is replete, and for which inorganic chemistry can also demonstrate and utilize in assymmetric catalyses. Sugars, proteins, enzymatic catalysts, all feature optical isomerism both in their structure and their function.
The next time you shake someone's hand, try shaking their right hand with your left hand. Will it work? No, of course it won't; and biology and chemistry have so many examples of such handedness.
Just to give a practical example, consider the following diagram....
Given the molecule on the left, or at least, a model of the molecule, the interchange of ANY two substituents on the left, will give the mirror image, i.e. the molecule on the right. Interchange again, and you gets the mirror image of a mirror image, i.e. the enantiomer of an enantiomer, i.e. the original molecule. With me?
Optical isomers arise whenever a carbon atom has 4 different substituents.
Explanation:
The models below have four different groups, and they are mirror images of each other.
They are optical isomers because there is no way to superimpose one model over the other so that all four colours coincide.
A carbon atom that has four different groups is said to be chiral.
We look for chiral carbons to decide whether a molecule can have optical isomers.
For example, the formula of butan-2-ol is
Hence, butan-2-ol can exist as a pair of optical isomers.
