Câu hỏi tuần 2

[bluemonster]

Question 1:Decalin is a polycyclic compound, it is formed by two cyclohexane fused. The two rings can be fused cis and trans.
Draw the chair conformation for trans- and cis-decalin, then compare their flexibilities, thermal stablily.
Question 2:Consider statement “A three- and a six- membered ring cannot be fused trans.” Are you agree? Explain!

Have fun!!!

[La Tan]

1/ Decanlin is a fused ring .Its nomenclature is Bicyclo[4.4.0]. It is formed by fused two cyclohexanes and has two geomergic isomers : cis- and trans- decanlin.Their structures are drawn below:




As we see,in the cis-decalin,one H is at an equatorial, the other is at an axial position on the cyclohexane. So two bondings can change each other .While in the trans- formula, two Hs are equatorial subtituents, they can not change each other. For this reason, the cis- is more flexible than trans-
But trans- has both large substituents in the equatorial positions, so it is more stable than
cis-

2/ “A three- and a six- membered ring cannot be fused trans.”. This statement is true because two carbons is too far apart to close the small ring

[bluemonster]

Keep posting!

Nguyên văn bởi La Tan

While in the trans- formula, two Hs are equatorial subtituents

they are axial substituents! ok!
Have fun!

[bluemonster]

Question 1:Decalin is a polycyclic compound, it is formed by two cyclohexane fused. The two rings can be fused cis and trans.
Draw the chair conformation for trans- and cis-decalin, then compare their flexibilities, thermal stablily

For each ring can be viewed as 1,2-substituents (bridgehead). For the trans-isomer only the rigid (ee) conformation is possible. Fusing to the (aa) orientation is impossible because the diaxial bonds point 180o away from each other and cannot be bridged by only four C’s to complete the second ring. Mainwhile, cis fusion is equilibrating between (ea) and (ae) conformers. So the cis isomer is flexible. The (ee) conformation of trans-decalin is stable, because it overcome eclipse strain, so it is the more thermal stability.

Question 2:Consider statement “A three- and a six- membered ring cannot be fused trans.” Are you agree? Explain!

Yes, a three- and a six- membered ring cannot be fused trans. Now, we will study about structure of cyclohexane conformation.


we have considering that the equatorial bonds (ee) of C 5 and 6 are further than apart of the axial and the equatorial (ae or ea). Fusing to the (aa) orientation is impossible because the diaxial bonds point 180o away from each other and cannot be bridged by only single C’s to complete the second ring. So the trans bonds on the potential bridgehead C’s are too far apart to be bridged by a single carbon bridge.
@La Tan: Your answer is exactly, keep posting!!!
The third question is posted, more active!!!
Have fun!!!