which is more stable cis or trans decalin, and why?
trans (by about 8.4 kJ/mol) because cis has one of the C-C bonds next to the fused edge in an axial position which creates steric strain
The cis and trans isomers of decalin differ in the relative orientation of their two fused cyclohexane rings. The trans-decalin has the two cyclohexane rings arranged in a straight line, while the cis-decalin has the two rings arranged in a more angular fashion. Due to the strain in the molecule, cis isomers are often less stable than trans isomers in cyclic compounds.
In the case of decalin, the trans isomer is more stable than the cis isomer. This is because, in the cis isomer, the adjacent hydrogens on the same carbon atom are in an eclipsed conformation, creating steric strain. This strain is relieved in the trans isomer, as the adjacent hydrogens are farther apart and adopt a staggered conformation.
This principle is known as the “trans-cis” isomerism rule and is commonly observed in cyclic compounds. Therefore, the trans-decalin isomer is favored over the cis-decalin isomer.
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