Organic Chemistry 1 - CHM 2001
Fall 2010
4.3 Stability of Cycloalkanes: Ring Strain
Baeyer (1885): since carbon prefers to have bond angles of approximately
109°, ring sizes other than five and six may be too strained to exist
Rings from 3 to 30 C’s do exist but are strained due to bond bending
distortions and steric interactions (ideal value of 109°tetrahedral )
4.3 Heat of Combustion
Measure strain on cyclic compounds = total energy of the compound –
energy of the strain-free reference compounds
Difference between two equals extra energy in the molecule due to strain
heat of combustion (heat released when the compound burns completely
with O2
(CH2) n + 3n/2 O2 nCO2 + nH2O + heat
3952 kJ/mol / 658.7 kJ/mol = 6  CH2
658.7 kJ/mol * 3 (CH2) = 1976.1
2091 kJ/mol – 1976.1 kJ/mol = 114.9 kJ/mol
Professor Parikh
1
Organic Chemistry 1 - CHM 2001
Fall 2010
Baeyer’s Theory: miscalculated
Angle strain - expansion or compression of bond angles away from most
stable (3 and 4 membered rings)
Most rings (7 and larger) strain arises from:
Torsional strain - eclipsing of bonds on neighboring atoms
Steric strain - repulsive interactions between nonbonded atoms in close
proximity
4.4 Cyclopropane: An Orbital View
3-membered ring must have planar structure
Symmetrical with C–C–C bond angles of 60°
Requires that sp3 based bonds are bent (and weakened)
Angle strain is caused by bond angles different from 109.5o
Torsional strain is caused by eclipsing C-H bonds on adjacent carbons
Cyclopropane has both high angle and torsional strain
Professor Parikh
2
Organic Chemistry 1 - CHM 2001
Fall 2010
4.4 Conformations of Cyclobutane
Cyclobutane has less angle strain than cyclopropane but more torsional
strain because of its larger number of ring hydrogens
Cyclobutane is slightly bent out of plane - one carbon atom is about 25°
above
The bend increases angle strain but decreases torsional strain
4.4 Conformations of Cyclopentane
Planar cyclopentane would have no angle strain but very high torsional
strain
Actual conformations of cyclopentane are nonplanar, reducing torsional
strain
Four carbon atoms are in a plane
The fifth carbon atom is above or below the plane – looks like an envelope
Professor Parikh
3