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