I search a web which contain theory and question, test of stereochemistry (level university). help me, please. Thanks alot

 

Here, i have some questions of stereochemistry. Which is the stable conformation of these composes (substitute: which is axial bond? which is equatorial bond?)

 

These questions can be tricky. In general you want most groups equatorial rather than axial. This is due to a small 1, 3 axial clash. However it is more important to have bigger groups equatorial as the clash can then be quite serious.

 

In the first one, the NH2 should be the only group axial.

 

In the second, the OMe should go equatorial as it is the most likely to clash, but if it does both the Me and Cl must go axial. This is probably a close call but OMe axial is probably favoured allowing Me and Cl to go equatorial

 

In the third it is best to have the OMe and Cl that are next to each other axial and the other 2 OMe and Me equatorial. The other conformer would have three groups axial with a horrendous 1,3 OMe, OMe clash

I might be wrong about this, but isn't there also supposed to be some stabilization from pi-anti-bond interaction with lone pairs electrons from the methoxy? Or is that only with heterocyclic compounds?

Evergreen - I think you may be getting at the anomeric effect which occurs when there is an oxygen in the ring adjacent to the OMe