[Memy]

Hello guys,

going through other stereochemistry problems i got stuck on these three molecules ( that i'm attaching here) :

1)2)
for the first two molecules that are apparently similar except that one contains nitrogen and the other phosphorous , I though that in the first case there are no possible enantiomers since the molecule is fluxional ( acording to my tutor) and therefore the lone pair of nitrogen, the phenyl and methyl groups keep on changing place with each other.
However, in the second case, the molecule containing phosphorous , is not fluxional (because of two lone pairs) and therefore is possible to draw diastereoisomers for that molecule, which can exist as two enantiomers.

3)
for the third molecule i don't have a clue on how to discuss the stereochemistry, other than putting the aldehyde function up and down.
ps: are the two rigid rings perpendicular to each other in order to avoid steric clash?

Thank you very much

[movies]

You have the right idea for 1 and 2 except that phosphorus and nitrogen have the same valence electron count, so there should only be one lone pair of the P atoms.  You can actually make P atoms invert the way that nitrogen atoms do, but it requires elevated temperatures (~120 degrees C, I have heard).

For number 3 you have to consider a different type of chirality.  First off, the easiest way to tell if a molecule has an enantiomer is to draw its mirror image and try to superpose it with the original.  In this case it may be useful to build a model.