Why when a ring has 8 carbon's and contains a double bond the trans version is very unstable to exist, for instance a molecule like cholesterol?
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Are you asking why a trans configuration of a double bond in an eight membered cyclic system is less stable than the cis configuration? – Martin - マーチン Jun 05 '15 at 11:22
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Yes, i was asked a question whether a molecule of cholesterol displays cis-trans isomerism at the C=C bond. – Abmon98 Jun 05 '15 at 11:50
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Because you're stuck in the ring structure.
The trans isomer has branches that, in their natural state, would want to rest antiparallel to one another. But in double bond 6 membered ring structure (like in cholesterol), these bonds would need to be twisted a lot so that they could sit in the octagonal ring structure. Given how a double bond is rigid and will already resist rotation, this theoretical trans isomer will have such a high energy requirement for it to exist that it will simply refuse to do so.
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