Chemistry - STEREO ISOMERISM Question with Solution | TestHub
Identify the chair or boat six-membered rings in the following structures and say why that particular shape is adopted.
Options:
Answer:
Solution:
The first three are relatively simple :
1st structure : a simple chair with all substituents equatorial - the chair is strain-free and the substituents choose equatorial positions.
2nd structure : a boat locked in the cage molecule (norbornane) with no choice - a chair is impossible because the one-atom bridge holds the two bridgehead carbons close together.
3rd structure : a chair inside a cage molecule with some choice - the starred carbon can move, so the six-membered ring adopts the chair.
The fourth molecule is adamantane - a tiny fragment of the diamond lattice; all its rings are chairs. The rings do not all look very chair-like in flat diagrams; making a model is the only way to appreciate this beautiful, symmetrical structure and to see all four chairs.
The fifth, the bicyclo[2.2.2] cage, has three boat rings and no choice : a chair would require the bridgehead carbons to move apart, which the three two-carbon bridges forbid.
The sixth molecule, bicyclo[3.3.1]nonane, has two chair rings sharing two carbons; with three-carbon bridges both rings can adopt the strain-free chair.
