Main Question or Discussion Point
I wonder why the number of chiral centres in sorrbital is 4? Why isn't it 2? Is it because the H-C-OH bond cannot be rotated? Otherwise isn't the two chiral centres above identical to the 2 at the bottom?
Uhhm, I don't understand what you are saying.Are you saying that the whole chain has to rotate and not just a part of it?If the atoms on the carbons at the end are switched around, the carbon will be able to rotate to bring back the orignial configuration. If the other carbons have the atoms they are attached to switched around, they will not be able to rotate any which way to find the original configuration. Does that sound about right?
If so, shouldn't there be only two chiral centres? Why 4? Since if they can rotate aren't the top 2 same as the bottom 2?each of the carbon atoms can rotate individually. If you rotate the carbon at either end you will see that all the different configurations are equivalent. That is, if you switch the OH with one of the H's, it will look different but it will be the same molecule because the carbon atom can just rotate back. Don't forget that the bonds can always rotate and they are constantly rotating when there is any energy present at all.
Ok thanks. The third and fourth carbon from the top are also chiral centres right? Even if you rotate them, their numberings just change, that's all.Ok I think I understand your question. I think you're forgetting about the tetrahedral configuration of each of the carbons. The third carbon from the top can rotate but that does not mean it becomes equivalent to the ones around it. Remember that in this representation of the molecule the atoms that come out to the sides are actually intended to be coming out of the paper towards you. If it rotates around it will be going into the paper away from you. Get out a plastic model kit or visualize the tetrahedral structures in your head.