cyclic conformation help.

[hellomister]

why is the energy difference between conformations of ethylcyclohexane is about the same for methylcyclohexane even though the ethyl group is larger than the methyl group?

 

thanks!

[hermanntrude]

OK what is it about the cycloalkane conformations that makes one higher than the other?

[Zellinger]

why is the energy difference between conformations of ethylcyclohexane is about the same for methylcyclohexane even though the ethyl group is larger than the methyl group?

 

thanks!

 

Recommended reading:

 

http://wetche.cmbi.ru.nl/organic/cyclohexane/jm/exer1.html

 

or "Organic chemistry" by Stanley H. Pine

Edited November 23, 2008 by Zellinger

[hermanntrude]

zellinger, this question is almost certainly a homework question. This forum's policy on homework is not to give out the answer but provide sources and clues so that a student can learn for themselves

[Zellinger]

zellinger, this question is almost certainly a homework question. This forum's policy on homework is not to give out the answer but provide sources and clues so that a student can learn for themselves

 

Sorry I wasn't aware of it. Question did sound a bit too basic. You should move this thread to appropriate section then.

[hermanntrude]

we're currently allowing people to ask homework questions either in homework help OR in the respective science category. It does cause confusion i'll admit.

 

Have a quick read of the rules. it really helps get to know how a forum works if you've at least skimmed the rules

[hellomister]

is it because both groups are in the equatorial position most of the time? so there are no 1,3 diaxial interactions with either group?

[Zellinger]

Well no. They are in the equatorial position most of the time to avoid these repulsions but you can't say it's why they have similar energy barriers between 2 conformations. The fact that they are almost exactly the same amount of time in equatorial position is the result of the almost the same energy barriers. To demonstrate: by your reasoning you could say that tert-butyl group has the same energy barrier as the two mentioned groups between conformations because it's all the time in equatorial position. So you must realize that the fact that they are in equatorial position is the result of energy barrier between 2 conformations not vice-versa. You must concentrate on the spatial dimension of the problem. Ok?

 

Try drawing the most stable position (with drawn hydrogen atoms; remember free rotation around single bonds if u wonder what I mean by most stable) for both equatorial and axial position of both ethyl and methyl group, then draw the same for let us say tert-butyl group and you should figure it out. I can't help you more then this without actually telling you the answer. Hint: all of these groups have similar energy levels at equatorial position as I remember since there are no axial repulsions.

 

p.s. I hope I helped you if even a bit.

[hellomister]

Oh i think I understand, thank you.

 

Um sorry it is indeed a textbook question but it is not a homework assignment, our teacher said to do it for extra practice, and I couldn't figure it out. Thanks and sorry if it was in the wrong spot.

[hermanntrude]

it wasn't in the wrong spot at all. I'm glad you got the response you were looking for