3 Heats of Combustion/CH2 Alkane O2 CO2 H2O Stability of cycloalkanes and Ring strains Long chain alkanes (CH2) 4 Cycloalkane Stability
5- and 6-membered rings most stable
Bond angle closest to 109.5
Angle (Baeyer) strain
Measured by heats of combustion per -CH2 -
Cyclopropane has ring strain energy of 27.6 kcal/mol (115 kJ/mol).
Cyclobutane has ring strain energy of 26.3 kcal/mol (110 kJ/mol).
5-membered ring 6.5 kcal/mol (27 kJ/mol).
6-membered ring 0.0 kcal/mol
7-membered ring 6.4 kcal/mol (27 kJ/mol).
8-membered ring 9.7 kcal/mol (41 kJ/mol).
Large ring strain due to angle compression
Very reactive weak bonds
Eclipsed Hydrogens 6 Cyclobutane
Angle strain due to compression
Torsional strain partially relieved by ring-puckering
If planar angles would
be 108 but all hydrogens
would be eclipsed.
reduces torsional strain.
Combustion data shows its unstrained.
Angles would be 120 if planar.
The chair conformer has 109.5 bond angles and all hydrogens are staggered.
No angle strain and no torsional strain.
Chair Conformer 8 Boat Conformer 9 Conformational Energy gt 10 Axial and Equatorial Positions Monosubstituted Cyclohexanes 11 13-Diaxial Interactions Conformational equilibrium of disubstituted cyclohexane 12 Cis-Trans Isomers
Bonds that are cis alternate axial-equatorial around the ring.
Groups like t-butyl cause a large energy difference between conformer.
Most stable conformer
puts t-butyl equatorial
13 Bicyclic Alkanes
Fused rings share two adjacent carbons.
Bridged rings share two nonadjacent Cs.
14 Cis- and Trans-Decalin
Fused cyclohexane chair conformers
Bridgehead Hs cis structure more flexible
Bridgehead Hs trans no ring flip possible.
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