torsional strain

[joonkimdds]

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The species with the most torsional strain is
a) cyclopropane
b) the planar conformation of cyclopentane
c) the 'bent' conformation of cyclopentane
d) the chair conformation of cyclohexane
e) the planar conformation of cycloehexane

the answer is e) but I don't know why.

could someone explain this to me?

it would be even better if someone tell me the lowest torsional strain species as well so i can perfectly master this one.

Thanks.

 

[blue_moon01]

because the electron clouds are too close to another resulting in repulsion. It would be hard to explain this without having a model or a drawing to explain.

 

[cdmOMR]

Well you can rule out C and D because those conformations occur to relieve some torsional strain compared to the planar model. To decipher between cyclopropane, cyclopentane, and cyclohexane (planar)...Im guessing because cyclohexane has the most electron clouds confined to a cyclic molecule the electron clouds would want to repel more than a molecule with less electron clouds confined to a similar space. Thats just a guess. Now if the question which would have more ANGLE strain, that would obviously be cyclopropane.

PS. am I the only one who likes organic chem more than gen chem??

 

[joonkimdds]

Well you can rule out C and D because those conformations occur to relieve some torsional strain compared to the planar model. To decipher between cyclopropane, cyclopentane, and cyclohexane (planar)...Im guessing because cyclohexane has the most electron clouds confined to a cyclic molecule the electron clouds would want to repel more than a molecule with less electron clouds confined to a similar space. Thats just a guess. Now if the question which would have more ANGLE strain, that would obviously be cyclopropane.

PS. am I the only one who likes organic chem more than gen chem??

I think i understand about 50% of your explanation(sorry about that).

could you check my conclusion i wrote below and see if i m right?

the planar picture make internal angle bigger and so its angle strain becomes less but the torsional strain increases since all C-H bonds would be eclipsed(am i right?)

is that why we get rid of C and D?

and since E has more C-H bonds, its eclips will have bigger torsional strain than the B?

what about A? it says cyclopropane. Does this mean planar form of cyclopropane?

what's difference between bent and chair? aren't they the same thing?

 

[star7126]

Actually, cycloalkanes, with the exception of cyclopropane, are not planar. The reason why cyclopentane rids itself of strain is because of this envelope puckered conformation it takes. This reduces eclipsing and torsional strain. Cyclohexane forms the chair conformation which eliminates both angle and torsional strain. Cyclopropane is really unstable and with a little bit of heat will just open up.

 

[denteng]

could someone explain this to me?

Here's a link to a good tutorial: http://en.wikipedia.org/wiki/Torsional_strain

 

[cdmOMR]

Well you can rule out C and D because those conformations occur to relieve some torsional strain compared to the planar model. To decipher between cyclopropane, cyclopentane, and cyclohexane (planar)...Im guessing because cyclohexane has the most electron clouds confined to a cyclic molecule the electron clouds would want to repel more than a molecule with less electron clouds confined to a similar space. Thats just a guess. Now if the question which would have more ANGLE strain, that would obviously be cyclopropane.

PS. am I the only one who likes organic chem more than gen chem??

I think i understand about 50% of your explanation(sorry about that).

could you check my conclusion i wrote below and see if i m right?

the planar picture make internal angle bigger and so its angle strain becomes less but the torsional strain increases since all C-H bonds would be eclipsed(am i right?)

is that why we get rid of C and D?

and since E has more C-H bonds, its eclips will have bigger torsional strain than the B?

what about A? it says cyclopropane. Does this mean planar form of cyclopropane?

what's difference between bent and chair? aren't they the same thing?

Like the other guy said, its not the internal angle...thats angle strain, torsional strain is repulsion of nearing electron clouds. The "bent" conformation of cyclopentane occurs to relieve the torsional strain that a planar cyclopentane would theoretically have. Same thing for the chair conformation of cyclohexane...it exists to relieve the torsional strain that the planar cyclohexane would theoretically have (I say theoretical because I dont even know if they can exist as planar in real life). Thats why we rule out C + D.

So now we have the choice between 3 planar molecules in cyclopropane, cyclopentane, and cyclobutane. My guess was that because cyclohexane has the electron clouds confined to a cyclic molecule that it would have the most torsional strain. This might not be 100% the reason, but seems most logical. IF the question was angle strain, then it would be cyclopropane.