Substituents compete for equatorial positions. Consider the following disubstituted cycloalkanes: Equal energies Equal energies es for other ubstituted cydlohe m 4-5 Larger Cycloalkanes 4-5 Polycyclic Alkanes ordangestortem fusedorbridged pared to the disubsttute cngond2eaahioyapenaanbecomparuo Bicydic ring systems can be either cis-or trans-fused c p 电 4 4 Newman projections more clearly show the unfavorable 1,3- diaxial interactions: Energy differences for other monosubstituted cyclohexanes: Substituents compete for equatorial positions. Consider the following disubstituted cycloalkanes: Equal energies Equal energies Diequatorial more stable Large group equitorial more stable. 4-5 Larger Cycloalkanes Rings larger that cyclohexane have more strain. •Bond angle distortion •Partial eclipsing of hydrogens •Transannular steric repulsions Medium sized rings adopt several conformations that are very close in energies, such as cyclodecane: Strain energy of 14 kcal mol-1. Large-sized cycloalkanes such as cyclotetradecane are able to adopt staggered and all-anti conformations similar to straight chain alkanes and are essentially strain free. Attachment of substituents, however, usually introduces some strain. Strain energy of 14 kcal mol-1. 4-5 Polycyclic Alkanes Polycyclic alkanes may contain fused or bridged rings. The fused system, decalin, can be compared to the disubstituted molecule, 1,2-diethylcyclohexane. Decalin is an example of a fused bicyclic ring system. The shared carbon atoms are called ring-fusion carbons. Groups attached to the ring-fusion carbons are called ring-fusion substituents. A second example of ring fusion, norborane, can be compared to the compound, cis-1,3-dimethylcyclopentane: Norborane is an example of a bridged bicyclic ring system. Two non-adjacent carbon atoms belong to both rings and are called bridgehead carbon atoms. Bicyclic ring systems can be either cis- or trans-fused: